(Redirected from X-ray astronomy)
This is a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, to September 6, 2001. Credit: David Chenette, Joseph B. Gurman, Loren W. Acton.

X-ray astronomy is a physical subfield of astronomy, more specifically radiation astronomy, that uses a variety of X-ray detectors fashioned into X-ray telescopes to observe natural sources that emit, reflect, transmit, or fluoresce X-rays. X-rays can only penetrate so far into a planetary atmosphere such as that surrounding the crustal and oceanic surface of the Earth. This limitation requires that these detectors and telescopes be lofted above nearly all of the atmosphere to function. Another alternative is to place them on astronomical bodies such as the Moon or in orbit.

Like the learning resource on Earth-based astronomy, this resource starts out at a secondary level, proceeds through a university undergraduate level, and engages the learner with the state of the art.

This lecture serves as both an introduction to the field of X-ray astronomy and to connect together both observational and theoretical X-ray astronomy.

The objective of this lecture is to give students and others an opportunity learn about X-rays and the radiation astronomy that benefits from their detection and observation. At the conclusion of reading and studying this lecture students and others should have a well-rounded understanding of the radiation astronomy of X-rays; i.e. X radiation astronomy.

Astronomy

Main source: Astronomy

X-ray astronomy consists of three fundamental parts:

1. logical laws with respect to incoming X-rays, or X-radiation,
2. natural X-ray sources, and
3. the sky and associated realms with respect to X-rays.

X-rays

Def. an action or process of throwing or sending out a traveling X-ray in a line, beam, or stream of small cross section is called X-radiation.

Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.

From 0.1 nm to 0.01 nm (about 12 to 120 keV) are hard X-rays.

From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft x-rays.

Super soft X-ray sources (SSXSs) are in most cases only detected below 0.5 keV.

Ultra-soft x-rays are also known as grenz-rays (GRs).[1]

"For AM Her's or intermediate polars, there is also a very soft black-body like component, often detected in ultra-soft X-rays. Often only the soft component is detected."[2]

Optical "properties in a sample of 53 AGNs exhibiting ultrasoft X-ray excess has led to [a] comparison of these Ultrasoft Survey (USS) H-beta FWHM distribution with other X-ray selected samples [confirming] that the permitted lines of the USS AGNs are biased to narrow widths, due either to observation of a face-on broad-line region, or to that region's lying farther away from the central source than for other AGNs."[3]

Planetary sciences

Bright X-ray arcs of low energy (0.1 - 10 keV) are generated during auroral activity. The images are superimposed on a simulated image of the Earth. The color code represents brightness, maximum in red. Distance from the North pole to the black circle is 3,340 km (2,080 mi). Observation dates: 10 pointings between December 16, 2003 and April 13, 2004. Instrument: HRC. Credit: NASA/MSFC/CXC/A.Bhardwaj & R.Elsner, et al.; Earth model: NASA/GSFC/L.Perkins & G.Shirah.

X-ray observations offer the possibility to detect (X-ray dark) planets as they eclipse part of the corona of their parent star while in transit. "Such methods are particularly promising for low-mass stars as a Jupiter-like planet could eclipse a rather significant coronal area."[4]

"Auroras are produced by solar storms that eject clouds of energetic charged particles. These particles are deflected when they encounter the Earth’s magnetic field, but in the process large electric voltages are created. Electrons trapped in the Earth’s magnetic field are accelerated by these voltages and spiral along the magnetic field into the polar regions. There they collide with atoms high in the atmosphere and emit X-rays".[5]

Def. a theory of the science of the biological, chemical, physical, and logical laws (or principles) with respect to any natural X-ray source in the sky especially at night is called theoretical X-ray astronomy.

An individual science such as physics (astrophysics) is theoretical X-ray astrophysics.

"Theoretical X-ray astronomy is a branch of theoretical astronomy that deals with the theoretical astrophysics and theoretical astrochemistry of X-ray generation, emission, and detection as applied to astronomical objects."[6]

"Like theoretical astrophysics, theoretical X-ray astronomy uses a wide variety of tools which include analytical models to approximate the behavior of a possible X-ray source and computational numerical simulations to approximate the observational data. Once potential observational consequences are available they can be compared with experimental observations. Observers can look for data that refutes a model or helps in choosing between several alternate or conflicting models."[6]

"Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model."[6]

"Most of the topics in astrophysics, astrochemistry, astrometry, and other fields that are branches of astronomy studied by theoreticians involve X-rays and X-ray sources. Many of the beginnings for a theory can be found in an Earth-based laboratory where an X-ray source is built and studied."[6]

"From the observed X-ray spectrum, combined with spectral emission results for other wavelength ranges, an astronomical model addressing the likely source of X-ray emission can be constructed. For example, with Scorpius X-1 the X-ray spectrum steeply drops off as X-ray energy increases up to 20 keV, which is likely for a thermal-plasma mechanism.[7] In addition, there is no radio emission, and the visible continuum is roughly what would be expected from a hot plasma fitting the observed X-ray flux.[7] The plasma could be a coronal cloud of a central object or a transient plasma, where the energy source is unknown, but could be related to the idea of a close binary.[7]"[6]

"In the Crab Nebula X-ray spectrum there are three features that differ greatly from Scorpius X-1: its spectrum is much harder, its source diameter is in light-years (ly)s, not astronomical units (AU), and its radio and optical synchrotron emission are strong.[7] Its overall X-ray luminosity rivals the optical emission and could be that of a nonthermal plasma. However, the Crab Nebula appears as an X-ray source that is a central freely expanding ball of dilute plasma, where the energy content is 100 times the total energy content of the large visible and radio portion, obtained from the unknown source.[7]"[6]

"The "Dividing Line" as giant stars evolve to become red giants also coincides with the Wind and Coronal Dividing Lines.[8] To explain the drop in X-ray emission across these dividing lines, a number of models have been proposed:

1. low transition region densities, leading to low emission in coronae,
2. high-density wind extinction of coronal emission,
3. only cool coronal loops become stable,
4. changes in a magnetic field structure to that of an open topology, leading to a decrease of magnetically confined plasma, or
5. changes in the magnetic dynamo character, leading to the disappearance of stellar fields leaving only small-scale, turbulence-generated fields among red giants.[8]"[6]

Entities

Astronomical X-ray entities are often discriminated further into sources or objects when more information becomes available, including that from other radiation astronomies.

A researcher who turns on an X-ray generator to study the X-ray emissions in a laboratory so as to understand an apparent astronomical X-ray source is an astronomical X-ray entity. So is one who writes an article about such efforts or a computer simulation to possibly represent such a source.

Dominant groups

"The X-ray luminosity of the dominant group is an order of magnitude fainter than that of the X-ray jet."[9]

Sources

Def. a natural source usually of X-rays (X-radiation) in the sky especially at night is called an astronomical X-ray source.

The apparent source may be reflecting, generating and emitting, transmitting, or fluorescing X-rays which may be detectable.

"Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus, the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere."[10]

Serpens X-1

Main source: Serpens X-1

Serpens X-1 is an X-ray source with an error circle fixed for all time on the celestial sphere. It is also an X-ray entity in the sense that it has an "independent, separate, or self-contained astronomical existence." from theoretical astronomy. It has a history, a spatial extent, and a spectral extent.

Coronal clouds

This image shows the Sun as viewed by the Soft X-Ray Telescope (SXT) onboard the orbiting Yohkoh satellite. Credit: NASA Goddard Laboratory for Atmospheres.

Although a coronal cloud (as part or all of a stellar or galactic corona) is usually "filled with high-temperature plasma at temperatures of T ≈ 1–2 (MK), ... [h]ot active regions and postflare loops have plasma temperatures of T ≈ 2–40 MK."[11]

In the image at right, the photosphere of the Sun is dark in X-rays. However, apparently associated with the Sun is a high-temperature plasma that radiates in X-rays at temperatures 1,000 times as hot as the photosphere.

Super soft X-ray sources

A super soft X-ray source (SSXS, or SSS) is an astronomical source of very low energy X-rays. Soft X-rays have energies in the 0.09 to 2.5 keV range, whereas hard X-rays are in the 1-20 keV range.[12]

Super soft X-ray sources (SSXSs) are in most cases only detected below 0.5 keV, so that within our own galaxy they are usually hidden by interstellar absorption in the galactic disk.[13] They are readily evident in external galaxies, with ~10 found in the Magellanic Clouds and at least 15 seen in M31.[13]

Ultraluminous X-ray sources

This is a composite image (X-ray - red, optical - blue & white) of the spiral galaxy M74 with an ultraluminous X-ray source (ULX) indicated inside the box. Image is 9 arcmin per side at RA 01h 36m 41.70s Dec +15° 46' 59.0" in Pisces. Observation dates: June 19, 2001; October 19, 2001. Aka: NGC 628, ULX: CXOU J013651.1+154547. Credit: X-ray; J. Liu (U.Mich.) et al., CXC, NASA - Optical; Todd Boroson/NOAO/AURA/NSF.

Ultraluminous X-ray sources (ULXs) are pointlike, nonnuclear X-ray sources with luminosities above the Eddington limit of 3 × 1039 ergs s−1 for a 20 Mʘ black hole.[14] Many ULXs show strong variability and may be black hole binaries. To fall into the class of intermediate-mass black holes (IMBHs), their luminosities, thermal disk emissions, variation timescales, and surrounding emission-line nebulae must suggest this.[14] However, when the emission is beamed or exceeds the Eddington limit, the ULX may be a stellar-mass black hole.[14] The nearby spiral galaxy NGC 1313 has two compact ULXs, X-1 and X-2. For X-1 the X-ray luminosity increases to a maximum of 3 × 1040 ergs s−1, exceeding the Eddington limit, and enters a steep power-law state at high luminosities more indicative of a stellar-mass black hole, whereas X-2 has the opposite behavior and appears to be in the hard X-ray state of an IMBH.[14]

The X-ray/optical composite at right "highlights an ultraluminous X-ray source (ULX) shown in the box. ... The timing and regularity of these outbursts ... make the object one of the best candidates yet for a so-called intermediate-mass black hole. ... Chandra X-ray Observatory observations of this ULX have provided evidence that its X-radiation is produced by a disk of hot gas swirling around a black hole with a mass of about 10,000 suns."[15] "Chandra observed M74 twice: once in June 2001 and again in October 2001. The XMM-Newton satellite also (a European Space Agency mission) observed this object in February 2002 and January 2003."[15]

Objects

Many astronomical objects when studied with visual astronomy may not appear to also be X-ray objects.

The SIMBAD database "contains identifications, 'basic data', bibliography, and selected observational measurements for several million astronomical objects."[16] Among these are some 209,612 astronomical X-ray objects. This information is found by going to the SIMBAD cite listed under 'External links', clicking on "Criteria query" and entering into the box "otype='X'", without the quotes, for an 'object count', and clicking on 'submit query'.

Continua

X-ray continuum emission can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet active galactic nuclei (AGN) there is an excess of soft X-ray emission in addition to the power-law component.

X-ray line emission is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines.

Using X-rays to determine a crystal structure results in diffraction intensities that are represented in reciprocal space as peaks. These have a finite width due to a variety of defects away from a perfectly periodic lattice. There may be significant diffuse scattering, a continuum of scattered X-rays that fall between the Bragg peaks.

The X-ray continuum can arise from bremsstrahlung, black-body radiation, synchrotron radiation, or what is called inverse Compton scattering of lower-energy photons by relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.[7]"

Backgrounds

This graph shows the power density spectrum of the extragalactic or cosmic gamma-ray background (CGB). Credit: pkisscs@konkoly.hu.

The diffuse cosmic X-ray background is indicated in the figure at right with the notation CXB.

In addition to discrete sources which stand out against the sky, there is good evidence for a diffuse X-ray background.[7] During more than a decade of observations of X-ray emission from the Sun, evidence of the existence of an isotropic X-ray background flux was obtained in 1956.[17] This background flux is rather consistently observed over a wide range of energies.[7] The early high-energy end of the spectrum for this diffuse X-ray background was obtained by instruments on board Ranger 3 and Ranger 5.[7] The X-ray flux corresponds to a total energy density of about 5 x 10−4 eV/cm3.[7] The ROSAT soft X-ray diffuse background (SXRB) image shows the general increase in intensity from the Galactic plane to the poles. At the lowest energies, 0.1 - 0.3 keV, nearly all of the observed soft X-ray background (SXRB) is thermal emission from ~106 K plasma.

Meteors

The image contains a 27.70 g fragment of the Carancas meteorite fall. The scale cube is 1 cm3. Credit: Meteorite Recon.

On September 20, the X-Ray Laboratory at the Faculty of Geological Sciences, Mayor de San Andres University, La Paz, Bolivia, published a report of their analysis of a small sample of material recovered from the impact site. They detected iron, nickel, cobalt, and traces of iridium — elements characteristic of the elemental composition of meteorites. The quantitative proportions of silicon, aluminum, potassium, calcium, magnesium, and phosphorus are incompatible with rocks that are normally found at the surface of the Earth.[18]

In X-ray wavelengths, many scientists are investigating the scattering of X-rays by interstellar dust, and some have suggested that astronomical X-ray sources would possess diffuse haloes, due to the dust.[19]

Cosmic rays

The distribution of ²⁶Al in the Milky Way is shown. Credit: the COMPTEL Collaboration.

Some cosmic-ray observatories also look for high energy gamma rays and x-rays.

Aluminium-26, 26Al, is a radioactive isotope of the chemical element aluminium, decaying by either of the modes beta-plus or electron capture, both resulting in the stable nuclide magnesium-26. The half-life of 26Al is 7.17×105 years. This is far too short for the isotope to survive to the present, but a small amount of the nuclide is produced by collisions of argon atoms with cosmic ray protons.

Aluminium-26 also emits gamma rays and X-rays,[20] and is one of the few radionuclides to emit X-rays.

Neutrons

Circinus X-1 is imaged with the Chandra X-ray Observatory. Credit: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S.Heintz et al.

Circinus X-1 is an X-ray binary star system that includes a neutron star. Observation of Circinus X-1 in July 2007 revealed the presence of X-ray jets normally found in black hole systems; it is the first of the sort to be discovered that displays this similarity to black holes.

Protons

Following the discovery of the atomic nucleus by Ernest Rutherford in 1911, Antonius van den Broek proposed that the place of each element in the periodic table (its atomic number) is equal to its nuclear charge. This was confirmed experimentally by Henry Moseley in 1913 using X-ray spectra.

Electrons

X-rays remove electrons from atoms and ions, and those photoelectrons can provoke secondary ionizations. As the intensity is often low, this X-ray heating is only efficient in warm, less dense atomic medium (as the column density is small). For example in molecular clouds only hard x-rays can penetrate and x-ray heating can be ignored. This is assuming the region is not near an x-ray source such as a supernova remnant.

In an X-ray tube, electrons are accelerated in a vacuum by an electric field and shot into a piece of metal called the "target". X-rays are emitted as the electrons slow down (decelerate) in the metal. The output spectrum consists of a continuous spectrum of X-rays, with additional sharp peaks at certain energies characteristic of the elements of the target.

Positrons

X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor (usually a relatively normal star) to the other component, called the accretor, which is compact: a white dwarf, neutron star, or black hole. The infalling matter releases gravitational potential energy, up to several tenths of its rest mass, as X-rays. (Hydrogen fusion releases only about 0.7 percent of rest mass.) An estimated 1041 positrons escape per second from a typical hard low-mass X-ray binary.[21][22]

Gamma rays

"LSI+61°303 is a periodic, radio-emitting binary system that is also the gamma-ray source, CG135+01."[6]

Ultraviolets

Of some 87,216 astronomical ultraviolet sources in the SIMBAD database, at least 2,767 are known X-ray sources.

The image shows the gamma-ray detection of a young radio galaxy, PKS 1718-649, belonging to the class of Compact Symmetric Objects (CSOs), with the Large Area Telescope (LAT) on board the Fermi satellite. Credit: G. Migliori, A. Siemiginowska, M. Sobolewska, et. al.{{fairuse}}

Compact extragalactic radio sources "show radio features typically observed in large-scale radio galaxies (jets, lobes, hot spots), but contained within the central 1 kpc region of the host galaxy."[23]

"Compact Symmetric Objects (CSOs, a subclass of GigaHertz Peaked spectrum radio sources) are symmetric and not affected by beaming. Their linear radio size can be translated into a source age if one measures the expansion velocity of the radio structures."[23]

"Using the Chandra X-ray Observatory and XMM-Newton we observed a pilot sample of 16 CSOs in X-rays (6 for the first time). Our results show heterogeneous nature of the CSOs X-ray emission indicating a range of AGN luminosities and a complex environment."[23]

In "a sub-population of CSOs the radio jets may be confined by the dense X-ray obscuring medium."[23]

Highly "relativistic plasma contained within young radio lobes and shocks accompanying a powerful jet expansion are expected to generate high energy radiation."[24]

"PKS 1718-649 [a Compact Symmetric Object hosting the most compact (2 pc) and youngest (100 years) extragalactic radio jet known to date and the first robustly confirmed gamma-ray CSO emitter has been observed] for the first time in X-rays and found [to be] a low luminosity X-ray source, L(2-10 keV) ~ 6 x 1041 erg s-1, and [have an] X-ray spectrum [...] consistent with a mildly (intrinsically) absorbed power law (Gamma ~ 1.75, NH ~ 1021 cm-2)."[24]

PKS 1718-649 is plotted in the diagram on the right of gamma-ray luminosity, along with flat-spectrum radio quasars (FSRQs), Steep-spectrum radio quasars (SSRQs), radio galaxies of the radio-loud (RL) AGN population, lacking prominent extended radio emission (FR0), and FRI and FRII radio galaxies also representative of the radio-loud (RL) AGN population.

Superluminals

"Observations made by Chandra indicate the presence of loops and rings in the hot X-ray emitting gas that surrounds Messier 87."[6]

In 1994, a Galactic speed record was obtained with the discovery of a superluminal source in our own Galaxy, the cosmic x-ray source GRS 1915+105. The expansion occurred on a much shorter timescale. Several separate blobs were seen to expand in pairs within weeks by typically 0.5 arcsec.[25] Because of the analogy with quasars, this source was called a microquasar.

The frequency spectrum of Cherenkov radiation by a particle is given by the Frank–Tamm formula. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately proportional to the frequency. That is, higher frequencies (shorter wavelengths) are more intense in Cherenkov radiation. This is why visible Cherenkov radiation is observed to be brilliant blue. In fact, most Cherenkov radiation is in the ultraviolet spectrum—it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum.

There is a cut-off frequency above which the equation ${\displaystyle \cos \theta =1/(n\beta )}$ cannot be satisfied. Since the refractive index is a function of frequency (and hence wavelength), the intensity does not continue increasing at ever shorter wavelengths even for ultra-relativistic particles (where v/c approaches 1). At X-ray frequencies, the refractive index becomes less than unity (note that in media the phase velocity may exceed c without violating relativity) and hence no X-ray emission (or shorter wavelength emissions such as gamma rays) would be observed. However, X-rays can be generated at special frequencies just below those corresponding to core electronic transitions in a material, as the index of refraction is often greater than 1 just below a resonance frequency (see Kramers-Kronig relation and anomalous dispersion).

The phase velocity of an electromagnetic wave, when traveling through a medium, can routinely exceed c, the vacuum velocity of light. For example, this occurs in most glasses at X-ray frequencies.[26] However, the phase velocity of a wave corresponds to the propagation speed of a theoretical single-frequency (purely monochromatic) component of the wave at that frequency. Such a wave component must be infinite in extent and of constant amplitude (otherwise it is not truly monochromatic), and so cannot convey any information.[27] Thus a phase velocity above c does not imply the propagation of signals with a velocity above c.[28]

Plasma objects

Def. a cloud, or cloud-like, natural astronomical entity, composed of plasma at least hot enough to emit X-rays is called a coronal cloud.

Earth

This image is a composite of the first picture of the Earth in X-rays over a diagram of the Earth below. Credit: NASA, Ruth Netting.
The Chandra observations at right of the bright portion of the Moon detect X-rays from oxygen, magnesium, aluminum and silicon atoms. Credit: Optical: Robert Gendler; X-ray: NASA/CXC/SAO/J.Drake et al.

The Earth is a known astronomical object. It is usually not thought of as an X-ray source.

At right is a composite image which contains the first picture of the Earth in X-rays, taken in March, 1996, with the orbiting Polar satellite. The area of brightest X-ray emission is red.

Energetic charged particles from the Sun energize electrons in the Earth's magnetosphere. These electrons move along the Earth's magnetic field and eventually strike the ionosphere, causing the X-ray emission. Lightning strikes or bolts across the sky also emit X-rays.[29]

Close inspection of the Chandra X-ray image of the Moon shows a region of X-rays in the dark region (the shadow region) trending toward the lower left corner of the X-ray image at second right. These X-rays only appear to come from the Moon. Instead, they originate from radiation of the Earth's geocorona (an extended outer atmosphere) through which orbiting spacecraft such as the Chandra satellite move.

Jupiter

This image of Jupiter shows concentrations of auroral X-rays near the north and south magnetic poles. The Chandra X-ray Observatory accumulated X-ray counts from Jupiter for its entire 10-hour rotation on December 18, 2000. Credit: NASA/CXC/SWRI/G.R.Gladstone et al.
Jupiter shows intense X-ray emission associated with auroras in its polar regions (Chandra observatory X-ray image on the left). The accompanying schematic illustrates how Jupiter's unusually frequent and spectacular auroral activity is produced. Observation period: 17 hrs, February 24-26, 2003. Credit: X-ray: NASA/CXC/MSFC/R.Elsner et al.; Illustration: CXC/M.Weiss.

The "image of Jupiter [at right] shows concentrations of auroral X-rays near the north and south magnetic poles."[30] The Chandra X-ray Observatory accumulated X-ray counts from Jupiter for its entire 10-hour rotation on December 18, 2000.

In the second at right is a diagram describing interaction with the local magnetic field. Jupiter's strong, rapidly rotating magnetic field (light blue lines in the figure) generates strong electric fields in the space around the planet. Charged particles (white dots), "trapped in Jupiter's magnetic field, are continually being accelerated (gold particles) down into the atmosphere above the polar regions, so auroras are almost always active on Jupiter. Electric voltages of about 10 million volts, and currents of 10 million amps - a hundred times greater than the most powerful lightning bolts - are required to explain the auroras at Jupiter's poles, which are a thousand times more powerful than those on Earth. On Earth, auroras are triggered by solar storms of energetic particles, which disturb Earth's magnetic field. As shown by the swept-back appearance in the illustration, gusts of particles from the Sun also distort Jupiter's magnetic field, and on occasion produce auroras."[31]

A 0620-00

A0620-00, the first x-ray nova to also be visible on an optical telescope (designated V616 Mon), was seen and detected to flare.

Gaseous objects

Gaseous objects are astronomical objects with gases predominately detected and apparently constituting a surface.

Depending primarily upon gas temperature, the presence of gas may be used to determine the composition of the gas object observed, at least the outer layer.

Venus

This Chandra X-ray Observatory image is the first X-ray image ever made of Venus. Credit: NASA/MPE/K.Dennerl et al..

The first ever X-ray image of Venus is shown at right. The "half crescent is due to the relative orientation of the Sun, Earth and Venus. The X-rays from Venus are produced by fluorescent radiation from oxygen and other atoms in the atmosphere between 120 and 140 kilometers above the surface of the planet. In contrast, the optical light from Venus is caused by the reflection from clouds 50 to 70 kilometers above the surface. Solar X-rays bombard the atmosphere of Venus, knock electrons out of the inner parts of atoms, and excite the atoms to a higher energy level. The atoms almost immediately return to their lower energy state with the emission of a fluorescent X-ray. A similar process involving ultraviolet light produces the visible light from fluorescent lamps."[32]

"During the Soviet Venera program, the Venera 11 and Venera 12 probes detected a constant stream of lightning, and Venera 12 recorded a powerful clap of thunder soon after it landed. The European Space Agency's Venus Express recorded abundant lightning in the high atmosphere.[33]

Mars

This Chandra X-ray Observatory image is the first look at X-rays from Mars. Credit: NASA/CXC/MPE/K.Dennerl et al.

"In the sparse upper atmosphere of Mars, about 120 (75 miles) kilometers above its surface, the observed X-rays [shown in the image at right] are produced by fluorescent radiation from oxygen atoms."[34]

"X-radiation from the Sun impacts oxygen atoms, knock electrons out of the inner parts of their electron clouds, and excite the atoms to a higher energy level in the process. The atoms almost immediately return to their lower energy state and may emit a fluorescent X-ray in this process with an energy characteristic of the atom involved - oxygen in this case. A similar process involving ultraviolet light produces the visible light from fluorescent lamps."[34]

"The X-ray power detected from the Martian atmosphere is very small, amounting to only 4 megawatts, comparable to the X-ray power of about ten thousand medical X-ray machines. Chandra was scheduled to observe Mars when it was only 70 million kilometers from Earth, and also near the point in its orbit when it is closest to the Sun."[34]

"At the time of the Chandra observation, a huge dust storm developed on Mars that covered about one hemisphere, later to cover the entire planet. This hemisphere rotated out of view over the course of the 9-hour observation but no change was observed in the X-ray intensity, implying that the dust storm did not affect the upper atmosphere."[34]

"The astronomers also found evidence for a faint halo of X-rays that extends out to 7,000 kilometers above the surface of Mars. Scientists believe the X-rays are produced by collisions of ions racing away from the Sun (the solar wind) with oxygen and hydrogen atoms in the tenuous exosphere of Mars."[34]

Comets

Main source: Comets
X-ray emission from Hyakutake is seen by the ROSAT satellite. Credit: NASA.
Comet Lulin was passing through the constellation Libra when Swift imaged it on January 28, 2009. Credit: .

"One of the great surprises of Hyakutake's passage through the inner Solar System was the discovery that it was emitting X-rays [image at left], with observations made using the ROSAT satellite revealing very strong X-ray emission.[35] This was the first time a comet had been seen to do so, but astronomers soon found that almost every comet they looked at was emitting X-rays. The emission from Hyakutake was brightest in a crescent shape surrounding the nucleus with the ends of the crescent pointing away from the Sun.

The image at right of Comet Lulin merges data acquired by Swift's Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). At the time of the observation, the comet was 99.5 million miles from Earth and 115.3 million miles from the Sun.

NASA's Swift Gamma-ray Explorer satellite was monitoring Comet Lulin as it closed to 63 Gm of Earth. For the first time, astronomers can see simultaneous UV and X-ray images of a comet. "The solar wind—a fast-moving stream of particles from the sun—interacts with the comet's broader cloud of atoms. This causes the solar wind to light up with X-rays, and that's what Swift's XRT sees", said Stefan Immler, of the Goddard Space Flight Center. This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the Sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.[36]"[6]

"An image of comet Hale-Bopp (C/1995 O1) in soft x-rays reveals a central emission offset from the nucleus, as well as an extended emission feature that does not correlate with the dust jets seen at optical wavelengths."[37]

Jupiter

This image of Jupiter shows concentrations of auroral X-rays near the north and south magnetic poles. The Chandra X-ray Observatory accumulated X-ray counts from Jupiter for its entire 10-hour rotation on December 18, 2000. Credit: NASA/CXC/SWRI/G.R.Gladstone et al.

The Chandra X-ray Observatory accumulated X-ray counts from Jupiter for its entire 10-hour rotation on December 18, 2000. Note that X-rays from the entire globe of Jupiter are detected.

Saturn

An X-ray astronomy image of Saturn is compared here with the optical image in the visible. Credit: X-ray: NASA/U. Hamburg/J. Ness et al; Optical: NASA/STScI.
In this image the rings of Saturn sparkle in X-rays. Credit: NASA/CXC/SAO.

The X-ray astronomy image of Saturn is on the left in the composite at right. The Chandra X-ray Observatory "image of Saturn held some surprises for the observers. First, Saturn's 90 megawatts of X-radiation is concentrated near the equator. This is different from a similar gaseous giant planet, Jupiter, where the most intense X-rays are associated with the strong magnetic field near its poles. Saturn's X-ray spectrum, or the distribution of its X-rays according to energy, was found to be similar to that of X-rays from the Sun. This indicates that Saturn's X-radiation is due to the reflection of solar X-rays by Saturn's atmosphere. The intensity of these reflected X-rays was unexpectedly strong. ... The optical image of Saturn is also due to the reflection of light from the Sun - visible wavelength light in this case - but the optical and X-ray images obviously have dramatic differences. The optical image is much brighter, and shows the beautiful ring structures, which were not detected in X-rays. This is because the Sun emits about a million times more power in visible light than in X-rays, and X-rays reflect much less efficiently from Saturn's atmosphere and rings."[38]

"[T]he soft X-ray emissions of Jupiter (and Saturn) can largely be explained by scattering and fluorescence of solar X-rays."[39]

The second image at the right, "taken by the Chandra x-ray telescope, reveals that the rings of Saturn sparkle; in this x-ray/optical composite, they are visible as blue dots. This radiation’s source is likely fluorescence caused by solar x-rays as they strike oxygen atoms in the water molecules of the planet’s icy rings. As the image shows, most of the ring’s x-rays originate in the B ring—the bright white inner ring visible in this optical image—which is approximately 25,000 kilometers wide and 40,000 kilometers above the planet’s surface. X-rays may also be concentrated on Saturn’s left side, possibly because of their association with shadows in the planet’s rings that are known as spokes, or possibly as a result of the additional solar fluorescence caused by the transient ice clouds that produce spokes. Other Chandra observations of Saturn show that the x-ray brightness of the rings varies significantly from one week to the next."[40]

Rocky objects

Rocky objects are astronomical objects with solid surfaces.

Mercury

"Now, 205 measurements of Mercury's surface composition, made by the X-ray spectrometer onboard Messenger, reveal how much Mercury's surface differs from those of other planets in the solar system."[41]

"The surface is dominated by minerals high in magnesium and enriched in sulfur, making it similar to partially melted versions of an enstatite chondrite, a rare type of meteorite that formed at high temperatures in low-oxygen conditions in the inner solar system."[41]

""The similarity between the constituents of these meteorites and Mercury's surface leads us to believe that either Mercury formed via the accretion of materials somewhat like the enstatite chondrites, or that both enstatite chondrites and the Mercury precursors were built from common ancestors," [Shoshana] Weider [a planetary geologist at the Carnegie Institution of Washington] said."[41]

Moon

Main sources: Rocks/Rocky object/Moon and Moon
The Chandra observations at right of the bright portion of the Moon detect X-rays from oxygen, magnesium, aluminum and silicon atoms. Credit: Optical: Robert Gendler; X-ray: NASA/CXC/SAO/J.Drake et al.
This is an X-ray image of the Moon from observation by the ROSAT. Credit: J. Schmitt et al., ROSAT Mission, MPE, ESA.

Like the Earth, the Moon is generally not thought of as an astronomical X-ray source. But, as the image at right shows, the Chandra X-ray Observatory detects X-rays from the Moon. These X-rays are produced by fluorescence when solar X-rays bombard the Moon's surface.

The Chandra X-ray Observatory has detected X-rays from oxygen, magnesium, aluminum and silicon atoms on the Moon.[42]

With respect to the second image at right, "[t]his x-ray image of the Moon was made by the orbiting ROSAT (Röntgensatellit) Observatory [on June 29,] 1990. In this digital picture, pixel brightness corresponds to x-ray intensity. Consider the image in three parts: the bright hemisphere of the x-ray moon, the darker half of the moon, and the x-ray sky background. The bright lunar hemisphere shines in x-rays because it reflects x-rays emitted by the sun ... just as it shines at night by reflecting visible sunlight. The background sky has an x-ray glow in part due to the myriad of distant, powerful active galaxies, unresolved in the ROSAT picture but recently detected in Chandra Observatory x-ray images. But why isn't the dark half of the moon completely dark? It's true that the dark lunar face is in shadow and so is not reflecting solar x-rays. Still, the few x-ray photons which seem to come from the moon's dark half are currently thought to be caused by energetic particles in the solar wind bombarding the lunar surface."[43] The measured lunar X-ray luminosity of ~ 1.2 x 1012 erg/s makes the Moon one of the weakest known non-terrestrial X-ray source. The scale on the picture says "16 arcmin".

Mars

Main sources: Rocks/Rocky object/Mars and Mars
This is the first ever view of Martian soil in X-rays. Credit: NASA/JPL-Caltech/Ames.

The image at right is an X-ray diffraction pattern from Martian soil. The image is from "the Chemistry and Mineralogy (CheMin) experiment on NASA's Curiosity rover. The image reveals the presence of crystalline feldspar, pyroxenes and olivine mixed with some amorphous (non-crystalline) material. The soil sample, taken from a wind-blown deposit within Gale Crater, where the rover landed, is similar to volcanic soils in Hawaii."[44]

"Curiosity scooped the soil on Oct. 15, 2012, the 69th sol, or Martian day, of operations. It was delivered to CheMin for X-ray diffraction analysis on October 17, 2012, the 71st sol. By directing an X-ray beam at a sample and recording how X-rays are scattered by the sample at an atomic level, the instrument can definitively identify and quantify minerals on Mars for the first time. Each mineral has a unique pattern of rings, or "fingerprint," revealing its presence. The colors in the graphic represent the intensity of the X-rays, with red being the most intense."[44]

Eros

Main source: Eros

Evidence from NEAR Shoemaker's x-ray measurements of Eros indicate an ordinary chondrite composition despite a red-sloped, S-type spectrum, again suggesting that some process has altered the optical properties of the surface.

Astrochemistry

Main source: Astrochemistry
Typical energy dispersive XRF spectrum for a number of elements is shown. Credit: LinguisticDemographer.
This is a typical spectrum of a rhodium target tube operated at 60 kV, showing continuous spectrum and K lines. Credit: LinguisticDemographer.

"Each element has electronic orbitals of characteristic energy. Following removal of an inner electron by an energetic photon provided by a primary radiation source, an electron from an outer shell drops into its place. There are a limited number of ways in which this can happen ... The main transitions are given names: an L→K transition is traditionally called Kα, an M→K transition is called Kβ, an M→L transition is called Lα, and so on. Each of these transitions yields a fluorescent photon with a characteristic energy equal to the difference in energy of the initial and final orbital. The wavelength of this fluorescent radiation can be calculated from Planck's Law:

${\displaystyle \lambda =h\cdot c/E.}$

The second image at right "shows the typical form of the sharp fluorescent spectral lines obtained in the energy-dispersive method.

"[E]lemental abundances which cannot be determined from meteorites include several of the most important for interstellar X-ray absorption: H, He, C, N, O, Ne, and Ar."[45]

Hydrogens

Main sources: Chemicals/Hydrogens and Hydrogens

"Single ionization and double ionization of elements heavier than helium have only a small effect on the magnitude of X-ray absorption cross sections."[45]

For hydrogen, complete ionization "obviously reduces its cross section to zero, but ... the net effect of partial ionization of hydrogen on calculated absorption depends on whether or not observations of hydrogen [are] used to estimate the total gas. ... [A]t least 20 % of interstellar hydrogen at high galactic latitudes seems to be ionized".[45]

Calciums

Main sources: Chemicals/Calciums and Calciums

"The 2.98-3.07 Å [0.298-0.307 nm] range is centered on the Lα Ca XX lines and includes associated satellite line emission from Ca XIX."[46]

"The 3.14-3.24 Å [0.314-0.324 nm] region covers emission primarily from Ca XIX (He-like) and Ca XVIII."[46]

Irons

Main sources: Chemicals/Irons and Irons

"[I]n the X-ray region where high-temperature emission lines appear during solar flares ... The 1.82-1.97 Å [0.182-0.197 nm] range covers emission from Fe XXV (He-like) and similar transitions in lower degrees of ionization of iron."[46]

Locations on Earth

This image is a distant view (June 1946) of the V-2 launch complex at White Sands Proving Grounds in New Mexico prior to the launch on June 28, 1946. Credit: .
This scan of a photo is of the Missile Park Southern section at the Woomera Test Range in Southern Australia. Credit: .
NRL scientists J. D. Purcell, C. Y. Johnson, and Dr. F. S. Johnson among those recovering instruments from a V-2 used for upper atmospheric research above the New Mexico desert. This is V-2 number 54, launched January 18, 1951. Credit: photo by Dr. Richard Tousey, NRL.
The USS Point Defiance shown in this image is one of the first rocket-launching surface ships. Credit: .
This is an image of six of the eight Nike-Asp sounding rockets before launch. Credit: .
Initially, the RAE Skylark is a British ramp-launched, high-altitude research or sounding rocket developed by the Royal Aircraft Establishment at Farnborough. It has been used by many research organizations including NASA for X-ray astronomy research. Credit: ESA.

For possibly locating X-ray sources above the Earth's atmosphere, there are a number of reasons to consider probing from different geographical locations:

1. early visual observations of the solar corona are associated with eclipses of the Sun by the Moon,
2. if the Sun is an X-ray source, then perhaps other stars are, and only so many can be observed from one location,
3. laboratory measurements use a peak of intensity to background (possible unknown sources) technique which demands measuring an X-ray background noise, and
4. there may be X-ray scattering by the Earth's upper atmosphere.

Observatories on the Earth's surface do not seem like a useful place to conduct X-ray astronomy observations in view of the inability of X-rays to reach even the peaks of the highest mountains. From the earliest speculations about detecting X-rays above the Earth's atmosphere, the need to use an appropriate probe suggested a high altitude sounding rocket. The ending of World War II presented an opportunity to use a ballistic missile for just such a purpose. The White Sands Proving Grounds in New Mexico, at the time an army base, is the first location on land to test the concept. The image at the right shows the V-2 launch complex prior to the launch of V-2 number 6.

The first successful attempt to detect X-rays above the Earth's surface occurred at White Sands Proving Grounds on August 5, 1948, by lofting an X-ray detector with a V-2 rocket.

As with visual or optical astronomy observatories, there is a tendency to place them away from population centers. The photograph at right of the January 18, 1951, V-2 launch indicates one reason for doing so with X-ray observing. Rockets lofted upwards tend to return.

In the southern hemisphere at Woomera, South Australia, another X-ray observing location uses a famous and probably the most successful sounding rocket, the Skylark, to place X-ray detectors at suborbital altitudes. "[T]he first X-ray surveys of the sky in the Southern Hemisphere" are accomplished by Skylark launches.[47]

The NRL and NASA establish another rocket launching facility outside Natal, Brazil to detect X-ray sources in the southern hemisphere.[48] In addition to land-based surface launches of sounding rockets for X-ray detection, occasionally ocean surface ships served as stable platforms. The USS Point Defiance (LSD-31) is one of the first rocket-launching surface ships to support the 1958 IGY Solar Eclipse Expedition to the Danger Island region of the South Pacific. Launchers on deck fired eight Nike-Asp sounding rockets. Each rocket carried an X-ray detector to record X-ray emission from the Sun during the solar eclipse on October 12, 1958.

The importance of X-ray astronomy is exemplified in the use of an X-ray imager such as the one on GOES 14 for the early detection of solar flares, coronal mass ejections (CME)s and other X-ray generating phenomena that impact the Earth.

Astrohistory

Main source: Astrohistory
The first successful V-2 launch (V-2 number 2) at White Sands Proving Ground is on April 16, 1946. Credit: NRL.

In 1927, E.O. Hulburt of the US Naval Research Laboratory (NRL) and associates Gregory Breit and Merle Tuve of the Carnegie Institution of Washington considered the possibility of equipping Robert H. Goddard's rockets to explore the upper atmosphere.[49] "Two years later, he proposed an experimental program in which a rocket might be instrumented to explore the upper atmosphere, including detection of ultraviolet radiation and X-rays at high altitudes."[49]

In the late 1930s, "the presence of a very hot, tenuous gas surrounding the Sun ... was inferred indirectly from optical coronal lines of highly ionized species".[4] In the mid-1940s "radio observations revealed a radio corona" around the Sun.[4] "Of course, the sheer beauty of the solar corona has been admired in scattered visible light ever since humans first wondered about solar eclipses"[4].

The beginning of the search for X-ray sources above the Earth's atmosphere is August 5, 1948, at 12:07 GMT (Greenwich Mean Time).[50][51] As part of Project Hermes a US Army (formerly German) V-2 rocket number 43 is launched from White Sands Proving Grounds, launch complex (LC) 33, to an altitude of 166 km.[51] This is "the first detection of solar X-rays."[52] After detecting X-ray photons from the Sun in the course of the rocket flight, T.R. Burnight wrote, “The sun is assumed to be the source of this radiation although radiation of wave-length shorter than 4 angstroms would not be expected from theoretical estimates of black body radiation from the solar corona.”[4]

Astromathematics

For some plasma X-ray sources, "an exponential spectrum corresponding to a thermal bremsstrahlung source [may fit]":

${\displaystyle N(E)dE=E_{0}^{-1}*exp^{-E/kT}dE.}$
${\displaystyle dN/dE=(E_{0}/E)*exp^{-E/kT},}$

where a least squares fit to the X-ray detection data yields a kT.[53]

Def. any of many mathematical relationships in which something is related to something else by an equation of the form f(x) = a.xk is called a power law.

In terms of radiation detected, for example, f(x) = photons (cm2-sec-keV)-1 versus keV. As the photon flux decreases with increasing keV, the exponent (k) is negative. Observations of X-rays have sometimes found the spectrum to have an upper portion with k ~ -2.3 and the lower portion being steeper with k ~ -4.7.[54] This suggests a two stage acceleration process.[54]

Physics

Main source: Physics
This image shows absorption by wavelength. X-radiation spans 3 decades in wavelength ~(8 nm - 8 pm). The last being just off the left edge at 0.008 nm. Credit: .

X-rays are electromagnetic radiation from a portion of the wavelength spectrum of about 5 to 8 nanometers (nm)s down to approximately 5 to 8 picometers (pm)s. As the figure at the left indicates with respect to surface of the Earth measurements, they do not penetrate the atmosphere. Laboratory measurements with X-ray generating sources are used to determine atmospheric penetration.

Spatial distributions

This ROSAT image is an Aitoff-Hammer equal-area map in galactic coordinates with the Galactic center in the middle of the 0.25 keV diffuse X-ray background. Credit: NASA.

A spatial distribution is a spatial frequency of occurrence or extent of an existence or existences such as entities, sources, or objects. A space is a volume large enough to accommodate a thing.

There is an “extensive 1/4 keV emission in the Galactic halo”, an “observed 1/4 keV [X-ray emission originating] in a Local Hot Bubble (LHB) that surrounds the Sun. ... and an isotropic extragalactic component.”[55] In addition to this “distribution of emission responsible for the soft X-ray diffuse background (SXRB) ... there are the distinct enhancements of supernova remnants, superbubbles, and clusters of galaxies.”[55]

The ROSAT soft X-ray diffuse background (SXRB) image shows the general increase in intensity from the Galactic plane to the poles. At the lowest energies, 0.1 - 0.3 keV, nearly all of the observed soft X-ray background (SXRB) is thermal emission from ~106 K plasma.

Generally, a coronal cloud, a cloud composed of plasma, is usually associated with a star or other celestial or astronomical body, extending sometimes millions of kilometers into space, or thousands of light-years, depending on the associated body. The high temperature of the coronal cloud gives it unusual spectral features. These features have been traced to highly ionized atoms of elements such as iron which indicate a plasma's temperature in excess of 106 K (MK) and associated emission of X-rays.

Spectral distributions

The electromagnetic spectrum. The red line indicates the room temperature thermal energy. Credit: Opensource Handbook of Nanoscience and Nanotechnology.

A spectral distribution is often a plot or intensity, brightness, flux density, or other characteristic of a spectrum versus the spectral property such as wavelength, frequency, energy, particle speed, refractive or reflective index, for example.

The first three dozen or so astronomical X-ray objects detected other than the Sun "represent a brightness range of about a thousandfold from the most intense source, Sco XR-1, ca. 5 x 10-10 J m-2 s-1, to the weakest sources at a few times 10-13 J m-2 s-1."[56]

Temporal distributions

A temporal distribution is a distribution over time. Also known as a time distribution. A temporal distribution usually has the independent variable 'Time' on the abscissa and other variables viewed approximately orthogonal to it. The time distribution can move forward in time, for example, from the present into the future, or backward in time, from the present into the past. Usually, the abscissa is plotted forward in time with the earlier time at the intersection with the ordinate variable at left. Geologic time is often plotted on the abscissa versus phenomena on the ordinate or as a twenty-four hour clock analogy.

Supergiant fast X-ray transients (SFXTs)

There are a growing number of recurrent X-ray transients, characterized by short outbursts with very fast rise times (tens of minutes) and typical durations of a few hours that are associated with OB supergiants and hence define a new class of massive X-ray binaries: Supergiant Fast X-ray Transients (SFXTs).[57] XTE J1739–302 is one of these. Discovered in 1997, remaining active only one day, with an X-ray spectrum well fitted with a thermal bremsstrahlung (temperature of ∼20 keV), resembling the spectral properties of accreting pulsars, it was at first classified as a peculiar Be/X-ray transient with an unusually short outburst.[58] A new burst was observed on April 8, 2008 with Swift.[58]

Sciences

Main source: Sciences

An astronomical X-ray source may have one or more positional locations, plus associated error circles or boxes, from which incoming X-radiation (X-rays) has been detected. The location may be associated with a known astronomical object such as a source of electromagnetic radiation in another portion of the electromagnetic spectrum, for example, the visible or radio. An astronomical object previously detected say in the visible portion of the spectrum and later observed with an X-ray observatory in orbit around Earth is also an astronomical X-ray source. Striving to understand the generation of X-rays by the apparent source helps to understand the Sun, the universe as a whole, and how these affect us on Earth.

An astronomical X-ray source catalog or catalogue is a list or tabulation of astronomical objects that are X-ray sources, typically grouped together because they share a common type, morphology, origin, means of detection, or method of discovery. Astronomical X-ray source catalogs are usually the result of an astronomical survey of some kind, often performed using an X-ray astronomical observatory in orbit around Earth.

• "Distribution and Variability of Cosmic X-Ray Sources", published on April 1, 1967, describes 35 astronomical X-ray sources detected by sounding rocket launched with an X-ray detector on board by the X-ray astronomy group at the Naval Research Laboratory in the United States.[59]
• "Development of a Catalogue of Galactic X-ray Sources", published in June 1967, lists 17 sources in order of right ascension as of October 5, 1966.[60] It does not contain actual dates of initial observation.
• A Catalogue of Discrete Celestial X-ray Sources, contains 59 sources as of December 1, 1969, that at the least had an X-ray flux published in the literature.[61]
• "The fourth Uhuru catalog of X-ray sources", contains 339 sources observed over the entire active period of the satellite, but not necessarily the earlier designation.[62] It does not contain actual dates of observation for any sources. Sources detected during the final observation period from August 27, 1973, to January 12, 1974, are prefixed with "4U".
• "The Ariel V /3 A/ catalogue of X-ray sources. II - Sources at high galactic latitude |b| > 10°", contains sources with high galactic latitudes and includes some sources observed by HEAO 1, Einstein, OSO 7, SAS 3, Uhuru, and earlier, mainly rocket, observations.[63]

Technology

Main source: Technology

Many devices have been developed to improve X-ray astronomy.

Sounding rockets

Carried aloft on a Nike-Black Brant VC sounding rocket, the microcalorimeter arrays observed the diffuse soft X-ray emission from a large solid angle at high galactic latitude. Credit: NASA/Wallops.

"In astronomy, the interstellar medium (or ISM) is the gas and cosmic dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic medium. The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields.[64] The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field."[6]

"Of interest is the hot ionized medium (HIM) consisting of a coronal cloud ejection from star surfaces at 106-107 K which emits X-rays. The ISM is turbulent and full of structure on all spatial scales. Stars are born deep inside large complexes of molecular clouds, typically a few parsecs in size. During their lives and deaths, stars interact physically with the ISM. Stellar winds from young clusters of stars (often with giant or supergiant HII regions surrounding them) and shock waves created by supernovae inject enormous amounts of energy into their surroundings, which leads to hypersonic turbulence. The resultant structures are stellar wind bubbles and superbubbles of hot gas. The Sun is currently traveling through the Local Interstellar Cloud, a denser region in the low-density Local Bubble."[6]

"To measure the spectrum of the diffuse X-ray emission from the interstellar medium over the energy range 0.07 to 1 keV, NASA launched a Black Brant 9 from White Sands Missile Range, New Mexico on May 1, 2008.[65] The Principal Investigator for the mission is Dr. Dan McCammon of the University of Wisconsin."[6]

"The US Naval Research Laboratory group launched an Aerobee 150 during April, 1965 that was equipped with a pair of geiger counters.[66] This flight discovered seven candidate X-ray sources, including the first extragalactic X-ray source; designated Virgo X-1 as the first X-ray source detected in Virgo.[67] A later Aerobee rocket launched from White Sands Missile Range on July 7, 1967, yielded further evidence that the source Virgo X-1 was the radio galaxy Messier 87.[68] Subsequent X-ray observations by the HEAO 1 and Einstein Observatory showed a complex source that included the active galactic nucleus of Messier 87.[69] However, there is little central concentration of the X-ray emission.[70]"[71]

Balloons

Main sources: Astronomy/Balloons and Balloons
The MeV Auroral X-ray Imaging and Spectroscopy experiment (MAXIS) is carried aloft by a balloon. Credit: Michael McCarthy and NASA.

The MeV Auroral X-ray Imaging and Spectroscopy experiment (MAXIS) is carried aloft by a balloon for a 450 h flight from McMurdo Station, Antarctica. The MAXIS flight detected an auroral X-ray event possibly associated with the solar wind as it interacted with the upper atmosphere between January 22nd and 26th, 2000.[72]

Balloon flights can carry instruments to altitudes of up to 40 km above sea level, where they are above as much as 99.997% of the Earth's atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer. However, even at such altitudes, much of the X-ray spectrum is still absorbed. X-rays with energies less than 35 keV (5,600 aJ) cannot reach balloons.

On July 21, 1964, the Crab Nebula supernova remnant was discovered to be a hard X-ray (15 – 60 keV) source by a scintillation counter flown on a balloon launched from Palestine, Texas, USA. This was likely the first balloon-based detection of X-rays from a discrete cosmic X-ray source.[73]

High-energy focusing telescopes

The Crab Nebula is a remnant of an exploded star. This image shows the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. Each image is 6′ wide. Credit: .

"The high-energy focusing telescope (HEFT) is a balloon-borne experiment to image astrophysical sources in the hard X-ray (20–100 keV) band.[74] Its maiden flight took place in May 2005 from Fort Sumner, New Mexico, USA. The angular resolution of HEFT is ~1.5'. Rather than using a grazing-angle X-ray telescope, HEFT makes use of a novel tungsten-silicon multilayer coatings to extend the reflectivity of nested grazing-incidence mirrors beyond 10 keV. HEFT has an energy resolution of 1.0 keV full width at half maximum at 60 keV. HEFT was launched for a 25-hour balloon flight in May 2005. The instrument performed within specification and observed Tau X-1, the Crab Nebula."[6]

High-resolution gamma-ray and hard X-ray spectrometer (HIREGS)

One of the recent balloon-borne experiments is called the High-resolution gamma-ray and hard X-ray spectrometer (HIREGS).[75] It is launched from McMurdo Station, Antarctica in December 1991, steady winds carried the balloon on a circumpolar flight lasting about two weeks.

Aircraft assisted launches

The Array of Low Energy X-ray Imaging Sensors (ALEXIS) X-ray telescopes feature curved mirrors whose multilayer coatings reflect and focus low-energy X-rays or extreme ultraviolet light the way optical telescopes focus visible light. The Launch was provided by the United States Air Force Space Test Program on a Pegasus Booster on April 25, 1993.[76]

Orbital rocketry

This SXI image of the Sun was a first test of the imager taken on August 13, 2009 at 14:04:58 UTC. Credit: .
The Solar Heliospheric Observatory (SOHO) is launched atop an ATLAS-IIAS expendable launch vehicle. Credit: NASA.

Satellites in use today include the XMM-Newton observatory (low to mid energy X-rays 0.1-15 keV) and the INTEGRAL satellite (high energy X-rays 15-60 keV). Both were launched by the European Space Agency. NASA has launched the Rossi X-ray Timing Explorer (RXTE), and the Swift and Chandra observatories. One of the instruments on Swift is the Swift X-Ray Telescope (XRT).

The GOES 14 spacecraft carries on board a Solar X-ray Imager to monitor the Sun's X-rays for the early detection of solar flares, coronal mass ejections, and other phenomena that impact the geospace environment.[77] "It was launched into orbit on June 27, 2009 at 22:51 GMT from Space Launch Complex 37B at the Cape Canaveral Air Force Station."[78]

"On January 30, 2009, the Russian Federal Space Agency successfully launched the Koronas-Foton which carries several experiments to detect X-rays, including the TESIS telescope/spectrometer FIAN with SphinX soft X-ray spectrophotometer."[78]

The Italian Space Agency (ASI) gamma-ray observatory satellite Astro-rivelatore Gamma ad Imagini Leggero (AGILE) has on board the Super-AGILE 15-45 keV hard X-ray detector. It was launched on April 23, 2007 by the Indian PSLV-C8.[79]

"A soft X-ray solar imaging telescope is on board the GOES-13 weather satellite launched using a Delta IV from Cape Canaveral LC37B on May 24, 2006.[80] However, there have been no GOES 13 SXI images since December 2006."[78]

"Although the Suzaku X-ray spectrometer (the first micro-calorimeter in space) failed on August 8, 2005 after launch on July 10, 2005, the X-ray Imaging Spectrometer (XIS) and Hard X-ray Detector (HXD) are still functioning."[78]

The Solar Heliospheric Observatory (SOHO) is launched at top left atop an ATLAS-IIAS expendable launch vehicle. The early Atlas is a development (an Intercontinental Ballistic Missile, ICBM) for defense as part of the mutual assured destruction (MAD) effort which helped to end the Cold War.

The ASTRO-1 observatory's suite of four telescopes points skyward from the payload bay of Columbia, STS-35. Credit: NASA.

"The primary payload of mission STS-35 [December 1990] was ASTRO-1 ... The primary objectives were round-the-clock observations of the celestial sphere in ultraviolet and X-ray spectral wavelengths with the ASTRO-1 observatory. The Broad Band X-Ray Telescope (BBXRT) and its Two-Axis Pointing System (TAPS) rounded out the instrument complement in the aft payload bay.

"Spacelab 1 was the first Spacelab mission in orbit in the payload bay of the Space Shuttle (STS-9) between November 28 and December 8, 1983. An X-ray spectrometer, measuring 2-30 keV photons (although 2-80 keV was possible), was on the pallet. The primary science objective was to study detailed spectral features in cosmic sources and their temporal changes. The instrument was a gas scintillation proportional counter (GSPC) with ~ 180 cm2 area and energy resolution of 9% at 7 keV. The detector was collimated to a 4.5° (FWHM) FOV. There were 512 energy channels.

Spartan 1 was deployed from the Space Shuttle Discovery (STS-51G) on June 20, 1985, and retrieved 45.5 hours later. The X-ray detectors aboard the Spartan platform were sensitive to the energy range 1-12 keV. The instrument scanned its target with narrowly collimated (5' x 3°) GSPCs. There were 2 identical sets of counters, each having ~ 660 cm2 effective area. Counts were accumulated for 0.812 s into 128 energy channels. The energy resolution was 16% at 6 keV. During its 2 days of flight, Spartan-1 observed the Perseus cluster of galaxies and our galactic center region.

Heliocentric rocketry

A technician stands next to one of the twin Helios spacecraft during testing. Credit: NASA/Max Planck.
Shown is Helios 1 sitting atop the Titan IIIE / Centaur launch vehicle. Credit: NASA.
Trajectory of the Helio space probes is diagrammed. Credit: NASA.

Helios 1 and Helios 2 ... are a pair of probes launched into heliocentric orbit for the purpose of studying solar processes. ... The probes are notable for having set a maximum speed record among spacecraft at 252,792 km/h[81] (157,078 mi/h or 43.63 mi/s or 70.22 km/s or 0.000234c). Helios 2 flew three million kilometers closer to the Sun than Helios 1, achieving perihelion on 17 April 1976 at a record distance of 0.29 AU (or 43.432 million kilometers),[82] slightly inside the orbit of Mercury. Helios 2 was sent into orbit 13 months after the launch of Helios 1. The probes are no longer functional but still remain in their elliptical orbit around the Sun." On board, each probe carried an instrument for cosmic radiation investigation (the CRI) for measuring protons, electrons, and X-rays "to determine the distribution of cosmic rays.

Exploratory rocketry

Ulysses' second orbit: it arrived at Jupiter on February 8, 1992, for a swing-by maneuver that increased its inclination to the ecliptic by 80.2 degrees. Credit: NASA.
ISEE-3 is inserted into a "halo" orbit on June 10, 1982. Credit: NASA.

"Usually observational astronomy is considered to occur on Earth's surface (or beneath it in neutrino astronomy). The idea of limiting observation to Earth includes orbiting the Earth. As soon as the observer leaves the cozy confines of Earth, the observer becomes a deep space explorer.[83] Except for Explorer 1 and Explorer 3 and the earlier satellites in the series,[84] usually if a probe is going to be a deep space explorer it leaves the Earth or an orbit around the Earth."[6]

"For a satellite or space probe to qualify as a deep space X-ray astronomer/explorer or "astronobot"/explorer, all it needs to carry aboard is an XRT or X-ray detector and leave Earth orbit."[6]

"Ulysses is launched October 6, 1990, and reached Jupiter for its "gravitational slingshot" in February 1992. It passed the south solar pole in June 1994 and crossed the ecliptic equator in February 1995. The solar X-ray and cosmic gamma-ray burst experiment (GRB) had 3 main objectives: study and monitor solar flares, detect and localize cosmic gamma-ray bursts, and in-situ detection of Jovian aurorae. Ulysses was the first satellite carrying a gamma burst detector which went outside the orbit of Mars. The hard X-ray detectors operated in the range 15–150 keV. The detectors consisted of 23-mm thick × 51-mm diameter CsI(Tl) crystals mounted via plastic light tubes to photomultipliers. The hard detector changed its operating mode depending on (1) measured count rate, (2) ground command, or (3) change in spacecraft telemetry mode. The trigger level was generally set for 8-sigma above background and the sensitivity is 10−6 erg/cm2 (1 nJ/m2). When a burst trigger is recorded, the instrument switches to record high resolution data, recording it to a 32-kbit memory for a slow telemetry read out. Burst data consist of either 16 s of 8-ms resolution count rates or 64 s of 32-ms count rates from the sum of the 2 detectors. There were also 16 channel energy spectra from the sum of the 2 detectors (taken either in 1, 2, 4, 16, or 32 second integrations). During 'wait' mode, the data were taken either in 0.25 or 0.5 s integrations and 4 energy channels (with shortest integration time being 8 s). Again, the outputs of the 2 detectors were summed."[6]

"The Ulysses soft X-ray detectors consisted of 2.5-mm thick x 0.5 cm2 area Si surface barrier detectors. A 100 mg/cm2 beryllium foil front window rejected the low energy X-rays and defined a conical FOV of 75° (half-angle). These detectors were passively cooled and operate in the temperature range −35 to −55 °C. This detector had 6 energy channels, covering the range 5–20 keV."[6]

The International Cometary Explorer (ICE) spacecraft was originally known as the International Sun/Earth Explorer 3 (ISEE-3) satellite.

ISEE-3 was launched on August 12, 1978. It was inserted into a "halo" orbit about the libration point some 240 Earth radii upstream between the Earth and Sun. ISEE-3 was renamed ICE (International Cometary Explorer) when, after completing its original mission in 1982, it was gravitationally maneuvered to intercept the comet P/Giacobini-Zinner. On September 11, 1985, the veteran NASA spacecraft flew through the tail of the comet. The X-ray spectrometer aboard ISEE-3 was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV.

Telescopes

The XRT uses a grazing incidence Wolter 1 telescope to focus X-rays onto a state-of-the-art CCD. Credit: .

X-ray telescopes can use a variety of different designs to image X-rays. The most common methods used in X-ray telescopes are grazing incidence mirrors and coded apertures. The limitations of X-ray optics result in much narrower fields of view than visible or UV telescopes.

An extreme example of a reflecting telescope is demonstrated by the grazing incidence X-ray telescope (XRT) of the Swift satellite that focuses X-rays onto a state-of-the-art charge-coupled device (CCD), in red at the focal point of the grazing incidence mirrors (in black at the right).

A Wolter telescope is a telescope for X-rays using only grazing incidence optics. X-rays mirrors can be built, but only if the angle from the plane of reflection is very low (typically 10 arc-minutes to 2 degrees)[85]. These are called glancing (or grazing) incidence mirrors. In 1952, Hans Wolter outlined three ways a telescope could be built using only this kind of mirror.[86][87]. Not surprisingly, these are called Wolter telescopes of type I, II, and III. Each has different advantages and disadvantages.[88]

Mirrors

This is a diagram of Wolter telescopes of Types I, II, and III. Credit: .

"The mirrors can be made of ceramic or metal foil.[89] The most commonly used grazing angle incidence materials for X-ray mirrors are gold and iridium. The critical reflection angle is energy dependent. For gold at 1 keV, the critical reflection angle is 3.72 degrees. A limit for this technology in the early 2000s with Chandra and XMM-Newton was about 15 keV light.[90] Using new multi-layered coatings, computer aided manufacturing, and other techniques the [X-ray] mirror for the NuStar telescope pushed this up to 79 keV light.[90] To reflect at this level, glass layers were multi-coated with Tungsten (W)/Silicon (Si) or Platinum(Pt)/Siliconcarbite(SiC).[90]

Coded apertures

The coded aperture for the SIGMA instrument is at the top. Credit: .

Some X-ray telescopes use coded aperture imaging. This technique uses a flat aperture grille in front of the detector, which weighs much less than any kind of focusing X-ray lens, but requires considerably more post-processing to produce an image.

Coded Apertures or Coded-Aperture Masks are grids, gratings, or other patterns of materials opaque to various wavelengths of light. The wavelengths are usually high-energy radiation such as X-rays and gamma rays. By blocking and unblocking light in a known pattern, a coded "shadow" is cast upon a plane of detectors. Using computer algorithms, properties of the original light source can be deduced from the shadow on the detectors. Coded apertures are used in X- and gamma rays because their high energies pass through normal lenses and mirrors.

Modulation collimators

The diagram shows the principles of operation of the four-grid modulation collimator. Credit: H. Bradt, G. Garmire, M. Oda, G. Spada, and B.V. Sreekantan, P. Gorenstein and H. Gursky.

A modulation collimator consists of “two or more wire grids [diffraction gratings] placed in front of an X-ray sensitive Geiger tube or proportional counter.”[91] Relative to the path of incident X-rays (incoming X-rays) the wire grids are placed one beneath the other with a slight offset that produces a shadow of the upper grid over part of the lower grid.[92]

Use of wire grids

Each grid consists of only parallel wires (like a diffraction grating, not a network of crossing wires) of diameter d and a center-to-center spacing of 2d.[92] Let D be the distance between the grids for a bigrid, or the distance between the uppermost grid and the lower most grid (the grid immediately in front of the detector) in a multigrid system.

Incident parallel radiation from a distant point source "falls upon the first grid" so that "depending upon the angle of incidence, the portions of the beam ... transmitted by the first grid fall

1. solely on the wires of the second grid,
2. "solely [through] the open spaces, or
3. upon both wires and spaces of the second grid."[91]

The transmission of the grids in the first two cases is 0% and 50%, respectively.[92] In the third case, it varies linearly with incident angle.[92]

The planes of 50% transmittance, planes of maximum transmittance (PMT), through the bigrid or multiple grid system, intersect "the celestial sphere [to] form [two or] multiple great circles ('lines-of-position') upon one of which the [astronomical] X-ray source must lie."[91]

Net angular responses

"[T]he net angular response of [a] two-grid or bigrid modulation collimator to a parallel X-ray beam is cyclic and trangular in shape with a peak transmission of 50%".[91]

Def. the full width at half maximum (FWHM):

θr = d/D.[92]

is called "[t]he response angle" (θr).[91]

"The two-grid system unambiguously determines the angular size of an X-ray source with size between about θr/4 and 2θr, and clearly distinguishes sizes above and below this range."[91]

Grid enclosure

The collimating effects of the grid enclosure or external metal slats determine the envelope for the triangular transmission peaks.[91] The enclosure or slats, in general, slowly modulate the peak heights.[91]

Multigrid collimators

The multigrid collimator has the additional grid (third grid or more) inserted

1. at a specified intermediate position between the two grids,
2. aligned approximately parallel to them, and
3. "positioned and rotated so that each [third] wire lies in a plane defined by a wire in [the] outer grid and a wire in the [inner] grid."[91]

This positioning is such that every other triangular peak of the bigrid system is removed.[91] An additional grid would be placed midway between one of the initial grids "and the adjacent intermediate grid."[91]

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high frequency periodic waveform, called the carrier signal, with a modulating signal. This is done in a similar fashion as a musician modulating a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"). Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used.

Here with the 'modulation collimator' the amplitude (intensity) of the incoming X-rays is reduced by the presence of two or more 'diffraction gratings' of parallel wires that block or greatly reduce that portion of the signal incident upon the wires.

Collimators

This diagram shows how a lead (Söller) collimator filters a stream of rays. Credit: Pete Verdon.

A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction (i.e. collimated or parallel) or to cause the spatial cross section of the beam to become smaller.

The figure to the right illustrates how a Söller collimator is used in neutron and X-ray machines. The upper panel shows a situation where a collimator is not used, while the lower panel introduces a collimator. In both panels the source of radiation is to the right, and the image is recorded on the gray plate at the left of the panels.

For the modulation collimator, the collimating slats as represented in the diagram are replaced by wires (end on, ⊗←D→⊗, rather than a slat ▬).

Normal incidence optics

Like MSSTA, NIXT used normal incidence reflective multilayer optics.[93]

Detectors

This is an image of a real X-ray detector. The instrument is called the Proportional Counter Array and it is on the Rossi X-ray Timing Explorer (RXTE) satellite. Credit: NASA, Ruth Netting.

Detectors such as the X-ray detector at right collect individual X-rays (photons of X-ray light), count them, discern the energy or wavelength, or how fast they are detected. The detector and telescope system can be designed to yield temporal, spatial, or spectral information.

Energy-dispersive spectroscopy

The graph is an EDS spectrum of the mineral crust of Rimicaris exoculata Credit: L. Corbari , M.-A. Cambon-Bonavita , G. J. Long , F. Grandjean , M. Zbinden , F. Gaill , and P. Compere.
The photograph shows the X-Ray Spectrometer on the MESSENGER Spacecraft. Credit: NASA / JHU/APL.

The image at right is an EDS spectrum of the mineral crust of Rimicaris exoculata[94].

Energy-dispersive X-ray spectroscopy (EDS or EDX) is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on the investigation of an interaction of some source of X-ray excitation and a sample. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing unique set of peaks on its X-ray spectrum.[95] To stimulate the emission of characteristic X-rays from a specimen, a high-energy beam of charged particles such as electrons or protons (see PIXE), or a beam of X-rays, is focused into the sample being studied. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The number and energy of the X-rays emitted from a specimen can be measured by an energy-dispersive spectrometer. X-ray beam excitation is used in X-ray fluorescence (XRF) spectrometers.

"[M]odern [EDX] instruments [are] carried along, e.g., [on] the late NEAR Shoemaker ... SMART-1 ... Chandrayaan-1 ... Kaguya ... [and] ongoing MESSENGER ... mission[s]"[96]

The MESSENGER X-ray spectrometer (XRS) maps mineral composition within the top millimeter of the surface on Mercury by detecting X-ray spectral lines from magnesium, aluminum, sulphur, calcium, titanium, and iron, in the 1-10 keV range.[97][98]

Wavelength-dispersive spectroscopy

A technique called wavelength dispersive X-ray spectroscopy (WDS) "is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging X-ray and the crystal's lattice spacings are related by Bragg's law ... [where the detector] counts only [X]-rays of a single wavelength. Many elements emit or fluoresce specific wavelengths of X-rays which in turn allow their identification.

In order to interpret the data, the expected elemental wavelength peak locations need to be known. For the ESRO-2B WDS X-ray instruments, calculations of the expected solar spectrum had to be performed and were compared to peaks detected by rocket measurements.[99]

Hypotheses

Main source: Hypotheses
1. Many if not most X-ray sources are not stars.

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