Radiation astronomy/Neutrals

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This is an artist's rendering of the Interstellar Boundary Explorer (IBEX) satellite. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab.

Neutrals astronomy is the astronomy of observing neutral atoms or molecules, their sources and apparent entities or objects of origin.

Strong forces[edit]

A "new type of neutron star model (Q stars) [is such that] high-density, electrically neutral baryonic matter is a coherent classical solution to an effective field theory of strong forces and is bound in the absence of gravity. [...] allows massive compact objects, [...] and has no macroscopic minimum mass."[1]

"Compact objects in astronomy are usually analyzed in terms of theoretical characteristics of neutron stars or black holes that are based upon calculations of equations of state for matter at very high densities. At such high densities, the effects of strong forces cannot be neglected. There are several conventional approaches to describing nuclear forces, all of which find that for a baryon number greater than ~250, a nucleus will become energetically unbound. High-density hadronic matter is not stable in these theories until there are enough baryons for gravitational binding to form a neutron star, typically with a minimum mass ≳ 0.1 M and maximum mass ≲ 3 M."[1]

"Another possibility [called "baryon matter"] is that in the absence of gravity high-density baryonic matter is bound by purely strong forces. [...] nongravitationally bound bulk hadronic matter is consistent with nuclear physics data [...] and low-energy strong interaction data [...] The effective field theory approach has many successes in nuclear physics [...] suggesting that bulk hadronic matter is just as likely to be a correct description of matter at high densities as conventional, unbound hadronic matter."[1]

"The idea behind baryon matter is that a macroscopic state may exist in which a smaller effective baryon mass inside some region makes the state energetically favored over free particles. [...] This state will appear in the limit of large baryon number as an electrically neutral coherent bound state of neutrons, protons, and electrons in β-decay equilibrium."[1]

Electroweak interactions[edit]

"Weak neutral current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the Z boson. The discovery of weak neutral currents was a significant step toward the unification of electromagnetism and the weak force into the electroweak force, and led to the discovery of the W and Z bosons."[2]


The image is a schematic view of the Mount Norikura solar neutron telescope. Credit: Y. Muraki, K. Murakami, M. Miyazaki, K. Mitsui. S. Shibata, S. Sakakibara, T. Sakai, T. Takahashi, T. Yamada, and K. Yamaguchi.

A "new detector to observe solar neutrons [has been in operation] since 1990 October 17 [...] at the Mount Norikura Cosmic Ray Laboratory (CRL) of [the] Institute for cosmic Ray Research, the University of Tokyo."[3]

"The solar neutron telescope [image at right] consists of 10 blocks of scintillator [...] and several lead plates which are used to place kinetic energies Tn of incoming particles into three bands (50-360 MeV, 280-500 MeV, and ≥ 390 MeV)."[3] The telescope is inclined to the direction of the Sun by 15°.[3] The plane area of the detector is 1.0 m2 and protected by lead plates (Pb) to eliminate gamma-ray and muon background from the side of the detector.[3] The anti-coincident counter (A) is used to reject the muons and gamma rays, coming from the side of the detector and the top scintillators.[3] (P) and (G) are used to identify the proton events and gamma rays.[3] The central scintillator blocks are optically separated into 10 units.[3]

"The horizontal scintillator just above the 10 vertical scintillators distinguishes neutral particles (neutrons) from the charged particles (mainly muons, protons and electrons)."[3]

The Neutron Monitor aboard Ulysses was used to measure cosmic rays as well as neutrons.


Notation: let the symbol GZK represent Greisen-Zatsepin-Kuzmin.

Single π0 production occurs "in neutral current neutrino interactions with water by a 1.3 GeV wide band neutrino beam."[4]

"The Gamma-Ray Spectrometer (GRS) on [Solar Maximum Mission] SMM has detected [...] at least two of the flares have spectral properties >40 MeV that require gamma rays from the decay of neutral pions. [Pion] production can occur early in the impulsive phase as defined by hard X-rays near 100 keV."[5]

Gamma-ray "emission matches remarkably well both the position and shape of the inner [supernova remnant] SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a prominent peak near 1 GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay."[6]

"Neutral current single π0 production induced by neutrinos with a mean energy of 1.3GeV is measured at a 1000 ton water Cherenkov detector as a near detector of the K2K long baseline neutrino experiment."[4]

"The single π0 production rate by atmospheric neutrinos could be usable to distinguish between the νµ ↔ ντ and νµ ↔ νs oscillation hypotheses. The NC rate is attenuated in the case of transitions of νµ’s into sterile neutrinos, while it does not change in the νµ ↔ ντ scenario."[4]

"[B]ased on interactions between cosmic rays and the photons of the cosmic microwave background radiation (CMB) ... cosmic rays with energies over the threshold energy of 5x1019 eV ... interact with cosmic microwave background photons to produce pions via the resonance,"[7]


"Pions produced in this manner proceed to decay in the standard pion channels—ultimately to photons for neutral pions, and photons, positrons, and various neutrinos for positive pions. Neutrons decay also to similar products, so that ultimately the energy of any cosmic ray proton is drained off by production of high energy photons plus (in some cases) high energy electron/positron pairs and neutrino pairs."[7]

"The pion production process begins at a higher energy than ordinary electron-positron pair production (lepton production) from protons impacting the CMB, which starts at cosmic ray proton energies of only about 1017eV. However, pion production events drain 20% of the energy of a cosmic ray proton as compared with only 0.1% of its energy for electron positron pair production. This factor of 200 is from two sources: the pion has only about ~130 times the mass of the leptons, but the extra energy appears as different kinetic energies of the pion or leptons, and results in relatively more kinetic energy transferred to a heavier product pion, in order to conserve momentum. The much larger total energy losses from pion production result in the pion production process becoming the limiting one to high energy cosmic ray travel, rather than the lower-energy light-lepton production process."[7]

"The pion production process continues until the cosmic ray energy falls below the pion production threshold. Due to the mean path associated with this interaction, extragalactic cosmic rays traveling over distances larger than 50 Mpc (163 Mly) and with energies greater than this threshold should never be observed on Earth. This distance is also known as GZK horizon."[7]


"The positrons can annihilate in flight before being slowed to thermal energies, annihilate directly with electrons when both are at thermal energies, or form positronium at thermal energies (or at greater than thermal energies if positronium formation occurs via charge exchange with neutrals)."[8]

"Positrons entering a gaseous medium at [0.6 to 4.5 MeV] are quickly slowed by ionizing collisions with neutral atoms and by long-range Coulomb interactions with any ionized component."[8]


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: The Max Planck Institute for Extraterrestrial Physics, Snowden et al. 1995, ApJ, 454, 643; Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

"By comparing the soft X-ray background with the distribution of neutral hydrogen, it is generally agreed that within the Milky Way disk, super soft X-rays are absorbed by this neutral hydrogen."[9]

The ROSAT image at the right 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.


The Necklace Nebula glows brightly in this Nasa Hubble Space Telescope image. Credit: NASA.

"A giant cosmic necklace glows brightly in this Nasa Hubble Space Telescope image."[10]

"The object, aptly named the Necklace Nebula, is a recently discovered planetary nebula, the glowing remains of an ordinary, sun-like star."[10]

"The nebula consists of a bright ring, measuring 12trillion miles wide, dotted with dense, bright knots of gas that resemble diamonds in a necklace."[10]

"Newly discovered: The Necklace Nebula glows brightly in this composite image taken by the Hubble Space Telescope last month. The glow of hydrogen, oxygen, and nitrogen are shown by the colours blue, green and red respectively".[10]

"It is located 15,000 light-years away in the constellation Sagitta."[10]

"A pair of stars orbiting close together produced the nebula, also called PN G054.2-03.4."[10]

"About 10,000 years ago, one of the ageing stars ballooned to the point where it engulfed its companion star. The smaller star continued orbiting inside its larger companion, increasing the giant’s rotation rate. The bloated companion star spun so fast that a large part of its gaseous envelope expanded into space. Due to centrifugal force, most of the gas escaped along the star’s equator, producing a ring. The embedded bright knots are dense gas clumps in the ring. The pair is so close, only a few million miles apart, that they appear as one bright dot in the centre. The stars are furiously whirling around each other, completing an orbit in a little more than a day."[10]

Plasma objects[edit]

A hot plasma ion 'steals' charge from a cold neutral atom to become an Energetic Neutral Atom (ENA).[11] Credit Mike Gruntman.
The ENA leaves the charge exchange in a straight line with the velocity of the original plasma ion.[11] Credit: Mike Gruntman.

"In 1951, prior to the Space Age, the existence of energetic neutral hydrogen atoms (as high as 70 keV in energy) in space plasma was discovered."[12]

"ENA imaging permits study of the ways in which our entire plasma environment -- including the magnetopause, ring current, plasmasphere, auroral zones, plasma sheet, and the ionosphere -- reacts to the changing conditions of the solar wind (Williams, 1990)."[12]


Main sources: Chemicals/Hydrogens and Hydrogens

"Proton–hydrogen charge-exchange collisions [such as those shown at right] are often the most important process in space plasma because hydrogen is the most abundant constituent of both plasmas and background gases and hydrogen charge-exchange occurs at very high velocities involving little exchange of momentum."[13]

The "efforts to study the 21 cm hydrogen line from the northern and southern hemispheres in 1954 and 1959 [...] were combined and provided the first full-galaxy radio map of neutral hydrogen in the Milky Way".[14]

A hydrogen atom is about 0.11 nm in diameter.

The "relative motion in a hydrogen atom in crossed electric and magnetic fields leads to peculiar quasi-ionized states with an electron localized very far from a proton."[15]

The Solar Wind Anisotropies (SWAN) aboard SOHO "is the only remote sensing instrument on SOHO that does not look at the Sun. It watches the rest of the sky, measuring hydrogen that is ‘blowing’ into the Solar System from interstellar space. By studying the interaction between the solar wind and this hydrogen gas, SWAN determines how the solar wind is distributed. As such, it can be qualified as SOHO’s solar wind ’mapper’."[16]


Main sources: Stars/Sun and Sun (star)

As stars are defined as luminous balls of plasma, the Sun may not qualify as its photosphere has a plasma concentration of approximately 10-4. The rest is composed of neutral atoms at about 5800 K.


Main sources: Wanderers/Mercury and Mercury

"Measurements by instruments on MESSENGER during the spacecraft's three Mercury flybys have led to discoveries of previously undetected neutral (Mg) and ionized (Ca+) species in Mercury's neutral and ionized exosphere and mapped these and previously known constituents (Na, Ca) on the anti-sunward side of the planet and over the poles. [...] Some ions and neutrals can be released directly from mineral surfaces by electron-stimulated desorption (ESD). Because cross sections of neutrals can be higher than photon-stimulated desorption (PSD) cross sections and because active electron precipitation on both the day and night side of Mercury can produce ESD of ions, at least part of the ionized exosphere is produced directly from surface materials by ESD."[17]


Main source: Earth

"Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid-state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange."[18]

"The IMAGE mission ... High Energy Neutral Atom imager (HENA) ... images [ENAs] at energies between 10 and 60 keV/nucleon [to] reveal the distribution and the evolution of energetic [ions, including protons] as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre‐storm levels."[19]


Main source: Moon
This is a computer generated model of the Lunar Atmosphere and Dust Environment Explorer (LADEE). Credit: NASA.

"The Lunar Atmosphere and Dust Environment Explorer (LADEE, at left) launched 07 September 2013 at 03:27 UT (06 September 11:27 EDT) on a Minotaur-V from Wallops Flight Facility. LADEE is designed to characterize the tenuous lunar atmosphere and dust environment from orbit. The scientific objectives of the mission are:(1) determine the global density, composition, and time variability of the fragile lunar atmosphere; and, (2) determine the size, charge, and spatial distribution of electrostatically transported dust grains and assess their likely effects on lunar exploration and lunar-based astronomy. Further objectives are to determine if the Apollo astronaut sightings of diffuse emission at 10s of km above the surface were Na glow or dust and document the dust impactor environment (size-frequency) to help guide design engineering for outpost and future robotic missions."[20]

"The orbiter will carry a Neutral Mass Spectrometer (NMS), an Ultraviolet/Visible Spectrometer (UVS), and a Lunar Dust Experiment (LDEX)."[20]

"The NMS is a quadrupole mass spectrometer designed ot detect species up to 150 amu and will look for CH4, S, O, Si, Kr, Xe, Fe, Al, Ti, Mg, OH, and H2O. The UVS will detect Al, Ca, Fe, K, Li, Na, Si, T, Ba, Mg, H2O, and O and will monitor the dust composition. The LDEX is an impact ionization dust detector designed to measure particles down to 0.3 microns at the spacecraft altitude. The LLCD is a test of a high data-rate optical (laser) communications system."[20]


Main sources: Wanderers/Mars and Mars

"The major atmospheric gases on Earth, Venus, and Mars were probably CO2, H2O, and N2. [The ions from the upper parts of an atmosphere] are often suprathermal, and their interactions can produce suprathermal neutral atoms as well [The] ionopause [...] separates the bound ionosphere from an outer region in which the solar wind is diverted and flows around and past the planet. This region still contains some neutral gas, and if such atoms are ionized by solar photons or electron impact, they are swept up in the flow."[21]

"There are strong reasons to believe that Mars once had much more atmospheric CO2 and H2O than it now has ... (Impacts, which may have eroded even larger amounts, operated at an earlier period.) ... The visible polar caps are thought to contain relatively small quantities. [...] More recently it has been proposed (35) that Mars may have had several episodes of high atmospheric pressure, warm conditions, and substantial precipitation of rain and snow, with a north polar ocean and southern glaciers."[21]

Comet Halley[edit]

“During the Halley Monitoring Program at La Silla from Feb.17 to Apr.17,1986 ... In the light of the neutral CN-radical a continuous formation and expansion of [cyan] gas-shells could be observed.”[22] “The gas-expansion velocity decreases with increasing heliocentric distance from 1 km/s in early March to 0.8 km/s in April.”[22]


"The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself."[23]

Interstellar medium[edit]

This image is an all-sky map of neutral atoms streaming in from the interstellar boundary. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

"In 2009, NASA's Interstellar Boundary Explorer (IBEX) mission science team constructed the first-ever all-sky map [at right] of the interactions occurring at the edge of the solar system, where the sun's influence diminishes and interacts with the interstellar medium. A 2013 paper provides a new explanation for a giant ribbon of energetic neutral atoms – shown here in light green and blue -- streaming in from that boundary."[24]

"[T]he boundary at the edge of our heliosphere where material streaming out from the sun interacts with the galactic material ... emits no light and no conventional telescope can see it. However, particles from inside the solar system bounce off this boundary and neutral atoms from that collision stream inward. Those particles can be observed by instruments on NASA’s Interstellar Boundary Explorer (IBEX). Since those atoms act as fingerprints for the boundary from which they came, IBEX can map that boundary in a way never before done. In 2009, IBEX saw something in that map that no one could explain: a vast ribbon dancing across this boundary that produced many more energetic neutral atoms than the surrounding areas."[24]

""What we are learning with IBEX is that the interaction between the sun's magnetic fields and the galactic magnetic field is much more complicated than we previously thought," says Eric Christian, the mission scientist for IBEX at NASA's Goddard Space Flight Center in Greenbelt, Md. "By modifying an earlier model, this paper provides the best explanation so far for the ribbon IBEX is seeing.""[24]

H I regions[edit]

"An H I region is an interstellar cloud composed of neutral atomic hydrogen (H I), in addition to the local abundance of helium and other elements."[25]

SIMBAD contains some 6,010 entries of the astronomical object type 'HI' (H I region).

"These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. ... Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies."[25].

"The degree of ionization in an H I region is very small at around 10−4 (i.e. one particle in 10,000). The temperature of an H I region is about 100 K,[26] and it is usually considered as isothermal, except near an expanding H II region.[27]"[25].

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".[28]

"When detection of neutral hydrogen (HI) absorption of the pulsar signal is possible, an estimate, or at least a limit on the distance may be obtained using a Galactic rotation model".[29]

"There is strong evidence for an elongated cavity in the neutral component of the [local insterstellar medium] LISM. This cavity surrounds the Sun and extends several hundred parsecs into quadrant 3 (Lucke 1978). The cavity appears as a region of low reddening extending 500 pc between ℓ = 210° and 255° and 1.5 kpc toward ℓ = 240°. Running counter to this is very heavy obscuration beyond ~100 pc in the first quadrant. Similarly, HI column densities derived from ultraviolet observations show a marked paucity in HI along LOSs directed towards ℓ = 230° (Frisch & York 1983; Paresce 1984). A similar morphology for this cavity is gleaned from NaI absorption measurements".[29]

"To further characterize the distribution of electrons in the LISM it is useful to relate their location to other interstellar features, such as bubbles, superbubbles, and clouds of neutral gas. There is strong evidence for an elongated cavity in the neutral component of the LISM. [...] There are several features of interest within this cavity. One of these is the local hot bubble (LHB): a volume encompassing the Sun distinguished by low neutral gas densities and a 106 K, soft X-ray emitting gas"[29]

The "neutral hydrogen column density [has] a level of N(HI)= 5 × 1019 cm−2"[29]

"Distance estimates now exist for a few hundreds of pulsars, resulting from three basic techniques: neutral hydrogen absorption (in combination with the Galactic rotation curve), trigonometric parallax and from associations with objects of known distance".[30]

Cold neutral mediums[edit]

H I regions of the ISM contain the cold neutral medium (CNM). The CNM constitutes 1-5 % by volume of the ISM, ranges in size from 100-300 pc, has a temperature between 50 and 100 K, with an atom density of 20-50 atoms/cm3.[31] The CNM has hydrogen in the neutral atomic state and emits the 21 cm line.

Warm neutral mediums[edit]

The warm neutral medium (WNM) is 10-20 % of the ISM, ranges in size from 300-400 pc, temperature between 6000 and 10000 K, is composed of neutral atomic hydrogen, has a density of 0.2-0.5 atoms/cm3, and emits the hydrogen 21 cm line.[31]

The "peak emissivity is enhanced by about 23% for the WIM [and only 11 % for the warm neutral medium (WNM)], although the peak frequency remains unchanged."[32]

Wolf-Rayet stars[edit]

"At the low density given by the spherically symmetric wind model (see Table 1), the dominant species in the gas are atomic ions while as the gas number density increases, the recombination of ions takes place and the gas composition is governed by neutral-phase chemistry, that is, the dominant species are neutral atoms and molecules although electrons and some ions are still present in relatively large amounts (for example, C+, O+ and He+)."[33]


Main source: Technology
This image shows the IBEX (photo cells forward) being surrounded by its protective nose cone. Credit: NASA (John F. Kennedy Space Center).

"The sensors on the IBEX spacecraft are able to detect energetic neutral atoms (ENAs) at a variety of energy levels."[34]

"The satellite's payload consists of two energetic neutral atom (ENA) imagers, IBEX-Hi and IBEX-Lo. Each of these sensors consists of a collimator that limits their fields-of-view, a conversion surface to convert neutral hydrogen and oxygen into ions, an electrostatic analyzer (ESA) to suppress ultraviolet light and to select ions of a specific energy range, and a detector to count particles and identify the type of each ion."[35]

"IBEX–Lo can detect particles with energies ranging from 10 electron–volts to 2,000 electron–volts (0.01 keV to 2 keV) in 8 separate energy bands. IBEX–Hi can detect particles with energies ranging from 300 electron–volts to 6,000 electron–volts (.3 keV to 6 keV) in 6 separate energy bands. ... Looking across the entire sky, interactions occurring at the edge of our Solar System produce ENAs at different energy levels and in different amounts, depending on the process."[34]

“The Submillimeter Wave Astronomy Satellite (SWAS) [is in] low Earth orbit ... to make targeted observations of giant molecular clouds and dark cloud cores. The focus of SWAS is five spectral lines: water (H2O), isotopic water (H218O), isotopic carbon monoxide (13CO), molecular oxygen (O2), and neutral carbon (C I).”[36]

See also[edit]


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