Gamma-ray Line Emission Research Articles

Strong Upper Limits on Sterile Neutrino Warm Dark Matter

Sterile neutrinos are attractive dark matter candidates. Their parameter space of mass and mixing angle has not yet been fully tested despite intensive efforts that exploit their gravitational clustering properties and radiative decays. We use the limits on gamma-ray line emission from the Galactic center region obtained with the SPI spectrometer on the INTEGRAL satellite to set new constraints, which improve on the earlier bounds on mixing by more than 2 orders of magnitude, and thus strongly restrict a wide and interesting range of models.

Circumscribing late dark matter decays model-independently

A number of theories, spanning a wide range of mass scales, predict dark matter candidates that have lifetimes much longer than the age of the Universe, yet may produce a significant flux of gamma rays in their decays today. We constrain such late-decaying dark matter scenarios model-independently by utilizing gamma-ray line emission limits from the Galactic Center region obtained with the SPI spectrometer on INTEGRAL, and the determination of the isotropic diffuse photon background by SPI, COMPTEL, and EGRET observations. We show that no more than $\ensuremath{\sim}5%$ of the unexplained MeV background can be produced by late dark matter decays either in the Galactic halo or cosmological sources.

Observing cosmic nuclei in gamma rays

Nucleosynthesis events in cosmic objects create new nuclei, admixtures of radioactive isotopes being part of the matter ejected into interstellar space by these events. Gamma rays are emitted in radioactive decays, and can be measured with space-based gamma-ray telescopes. Four mission years of INTEGRAL have led to discoveries of new sources and to detailed astronomical refinements of already-known gamma-ray line emission. As part of the main science objectives of INTEGRAL's spectrometer SPI, diffuse emission from annihilation of positrons has presented a new puzzle, as the Galactic distribution of their presumed sources does not agree with the gamma-ray image. Recent massive-star nucleosynthesis is traced throughout the Galaxy with radioactivites seen in 26Al and now also 60Fe gamma rays; precision line spectroscopy now reveals Doppler shifts for the 26Al line, separately for different parts of the Galaxy. This provides new insights into the dynamics of hot interstellar gas. The processes generating new atomic nuclei in stars and supernovae are reflected in the abundances of the ejected radioactivities. For individual supernovae, measurements of 44Ti with its 85 year decay time probes the symmetry of the supernova interior. Since such gamma-ray measurements probe the generation of new nuclei in cosmic sites through a radiation process which is independent of environmental parameters such as temperature or ionization, it will remain worthwhile to further develop instrumentation in this window to the nuclear-physics universe.

LIGHT SUPERCONDUCTING STRINGS IN THE GALAXY

Observations of the Milky Way by the SPI/INTEGRAL satellite have confirmed the presence of a strong 511 keV gamma ray line emission from the bulge, which requires an intense source of positrons in the galactic center. These observations are hard to account for by conventional astrophysical scenarios, whereas other proposals, such as light DM, face stringent constraints from the diffuse gamma ray background. Here we suggest that light superconducting strings could be the source of the observed 511 keV emission. The associated particle physics, at the ~ 1 TeV scale, is within the reach of planned accelerator experiments, while the distinguishing spatial distribution, proportional to the galactic magnetic field, could be mapped by SPI or by future, more sensitive satellite missions.

SPI observations of the diffuse60Fe emission in the Galaxy

Aims. Gamma-ray line emission from radioactive decay of 60 Fe provides constraints on nucleosynthesis in massive stars and supernovae.Methods. spectrometer SPI on board INTEGRAL has accumulated nearly three years of data on gamma-ray emission from the Galactic plane. We have analyzed these data with suitable instrumental-background models and sky distributions to produce high-resolution spectra of Galactic emission.Results. We detect the γ -ray lines from 60 Fe decay at 1173 and 1333 keV, obtaining an improvement over our earlier measurement of both lines with now 4.9σ significance for the combination of the two lines. The average flux per line is (4.4 ± 0.9) 10-5 ph cm-2 s-1 rad-1 for the inner Galaxy region. Deriving the Galactic 26 Al gamma-ray line flux with using the same set of observations and analysis method, we determine the flux ratio of 60 Fe/26 Al gamma-rays as 0.148 ± 0.06.Conclusions. The current theoretical predictions are consistent with our result. We discuss the implications of these results for the widely-held hypothesis that 60 Fe is synthesized in core-collapse supernovae, and also for the closely-related question of the precise origin of 26 Al in massive stars.

Positron Annihilations at the Galactic Center: Stringent Constraint on Positron Injection Energies

The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. The emission is strongly concentrated at the Galactic Center (GC), in contrast to gamma-ray maps tracing nucleosynthesis (e.g., the 1.809 MeV line from decaying 26Al) or cosmic ray processes (e.g., the 1-30 MeV continuum), which reveal a bright disk with a much less prominent central region. If positrons are generated at relativistic energies, higher-energy gamma rays will also be produced from inflight annihilation of positrons on ambient electrons. The comparison of the gamma-ray spectrum from inflight annihilation to the observed diffuse Galactic gamma-ray data constrains the injection energies of Galactic positrons to be less than 3 MeV.

Time delay between γ-ray lines and hard X-ray emissions during the 23 July 2002 solar flare interpreted by a trap plus precipitation model

Context. The 23 July 2002 event was the first -ray flare observed by the Ramaty High Energy Solar Spectrosc opic Imager (RHESSI). Analysis of the time profiles of the hard X-ray and gamma-ray radiation s of this flare shows a time delay between hard X-ray at 150 keV a nd gamma ray line emissions. Aims. We aim to interpret this delay in terms of transport of the particles accelerated during the flare. Methods. In this paper, we focus on the interpretation of this delay in the context of a trap plus precipitation model for energetic particles. Results. The time profiles of hard-X-ray and prompt gamma-ray line flux es can be reproduced given that electrons and ions are injected and partially trapped in different coronal loop systems with slightly different characteristics such as density and length, and that t he energetic electron-to-ion ratio varies from peak to peak during the fla re. Conclusions. The results obtained from this analysis are discussed with respect to the constraints provided by the X-ray and gamma-ray images previously obtained, as well as with previously published analysis of the same event

Puzzles and potential for gamma-ray line observations of solar flare ion acceleration

The best tool for understanding ion acceleration in solar flares is gamma-ray line emission from nuclear de-excitation, positron annihilation, and neutron capture. These techniques have not yet come close to reaching their potential due to limited counting statistics in the lines. Instruments with focusing optics and large effective areas promise real breakthroughs in understanding high-energy solar processes. I discuss what can be learned from the various lines and the instrumental requirements for future focusing observations.

Stringent Constraint on Galactic Positron Production

The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. If these positrons are injected at even mildly relativistic energies, higher-energy gamma rays will also be produced. We calculate the gamma-ray spectrum due to inflight annihilation and compare it with the observed diffuse Galactic gamma-ray data. Even with a simplified but conservative treatment, we find that the positron injection energies must be less than or similar to 3 MeV, which strongly constrains models for Galactic positron production.

Delayed X‐ and Gamma‐Ray Line Emission from Solar Flare Radioactivity

We have studied the radioactive line emission expected from solar active regions after large flares, following the production of long-lived radioisotopes by nuclear interactions of flare-accelerated ions. This delayed X- and gamma-ray line emission can provide unique information on the accelerated particle composition and energy spectrum, as well as on mixing processes in the solar atmosphere. Total cross sections for the formation of the main radioisotopes by proton, 3-He and alpha-particle reactions are evaluated from available data combined with nuclear reaction theory. Thick-target radioisotope yields are provided in tabular form, which can be used to predict fluxes of all of the major delayed lines at any time after a gamma-ray flare. The brightest delayed line for days after the flare is found to be the 511 keV positron-electron annihilation line resulting from the decay of several beta+ radioisotopes. After ~2 days however, the flux of the 511 keV line can become lower than that of the 846.8 keV line from the decay of 56-Co. Our study has revealed other delayed gamma-ray lines that appear to be promising for detection, e.g. at 1434 keV from the radioactivity of both the isomer and the ground state of 52-Mn, 1332 and 1792 keV from 60-Cu, and 931.1 keV from 55-Co. The strongest delayed X-ray line is found to be the Co Kalpha at 6.92 keV, which is produced from both the decay of the isomer of 58-Co by the conversion of a K-shell electron and the decay of 57-Ni by orbital electron capture. Prospects for observation of these lines with RHESSI or future space instruments are discussed.

Damping of Magnetohydrodynamic Turbulence in Solar Flares

We describe the cascade of plasma waves or turbulence injected, presumably by reconnection, at scales comparable to the size of a solar flare loop, L ~ 109 cm, to scales comparable to elementary particle gyroradii and evaluate their damping at small scales by various mechanisms. We show that the classical viscous damping valid on scales larger than the collision mean free path (~108 cm) is unimportant for magnetically dominated or low-β plasmas and the primary damping mechanism is the collisionless damping by the background particles. We show that the damping rate is proportional to the total random momentum density of the particles. For solar flare conditions this means that in most flares, except the very large ones in which essentially all background electrons are accelerated into a nonthermal distribution, the damping is dominated by thermal background electrons. In general, damping by protons is negligible compared to that of electrons except for rare proton-dominated flares with strong nuclear gamma-ray line emission and for quasi-perpendicular propagating waves. We also determine the critical scale below which the damping becomes important and the spectrum of the turbulence steepens. We show that this scale has a strong dependence on the propagation angle of the waves with respect to the background magnetic field, resulting in a highly anisotropic spectral distribution, with quasi-parallel and quasi-perpendicular waves cascading undamped to small scales.

Energy Spectra of Prompt Gamma Rays from Al and Fe Thick Targets Irradiated by Helium and Proton Beams: Concerning Planetary Gamma-Ray Spectroscopy

Bombardment experiments with thick targets made of each of Al and Fe were performed using 230 MeV/nucleon helium and 210 MeV proton beams. The energies and intensities of prompt gamma rays emitted by inelastic nuclear scattering of energetic neutrons were measured by a high-purity Ge detector. This work presents the first experimental results concerning the emission of prompt gamma rays from thick targets anticipated by helium component in galactic cosmic rays, since no foregoing work related to this one has been done. It is concluded that helium has the capability of prompt gamma ray line emission through the inelastic nuclear reaction of neutrons with target material by a factor of 3.5 on average in comparison with proton. As regards the lunar and planetary nuclear spectroscopy, a consideration is given on the role of helium particles to the total production of gamma rays emitted from a planetary surface.

The Role of Nuclei-Nuclei Interactions in the Production of Gamma-ray Lines in Solar Flares

Dramatic extensions of experimental possibilities (spacecraft RHESSI, CORONAS-F and others) in solar gamma-ray astronomy call for urgent, detailed theoretical consideration of a set of physical problems of solar activity and solar-terrestrial relationships that earlier may have only been outlined. Here we undertake a theoretical analysis of issues related to the production of gamma-radiation in the processes of interactions of energetic (accelerated) heavy and middle nuclei with the nuclei of the solar atmosphere (the so-called i-j interactions). We also make an estimate of the contribution of these interactions to the formation of nuclear and isotopic abundances of the solar atmosphere in the range of light and rare elements. The analysis is carried out for solar flares in the wide range of their intensities. We compare our theoretical estimates with RHESSI observations for the flare of 2002 July 23. It was shown that the 24Mg gamma-ray emission in this event was produced by the newly generated Mg nuclei. With a high probability, the gamma-ray line emission of 28Si nuclei from this flare was generated by the same processes.

Astrophysical constraints from gamma-ray spectroscopy

Gamma-ray lines from cosmic sources provide unique isotopic information, since they originate from energy level transitions in the atomic nucleus. Gamma-ray telescopes explored this astronomical window in the past three decades, detecting radioactive isotopes that have been ejected in interstellar space by cosmic nucleosynthesis events and nuclei that have been excited through collisions with energetic particles. Astronomical gamma-ray telescopes feature standard detectors of nuclear physics, but have to be surrounded by effective shields against local instrumental background, and need special detector and/or mask arrangements to collect imaging information. Due to exceptionally-low signal/noise ratios, progress in the field has been slow compared with other wavelengths. Despite the difficulties, this young field of astronomy is well established now, in particular due to advances made by the Compton Gamma-Ray Observatory in the 90ies. The most important achievements so far concern: short-lived radioactivities that have been detected in a couple of supernovae ( 56Co and 57Co in SN1987A, 44Ti in Cas A), the diffuse glow of long-lived 26Al that has been mapped along the entire plane of the Galaxy, several excited nuclei that have been detected in solar flares, and, last but not least, positron annihilation that has been observed in the inner Galaxy since the 70ies. High-resolution spectroscopy is now being performed: since 2002, ESAs INTEGRAL and NASAs RHESSI, two space-based gamma-ray telescopes with Ge detectors, are in operation. Recent results include: imaging and line shape measurements of e − – e + annihilation emission from the Galactic bulge, which can hardly be accounted for by conventional sources of positrons; 26Al emission and line width measurement from the inner Galaxy and from the Cygnus region, which can constrain the properties of the interstellar medium; and a diffuse 60Fe gamma-ray line emission which appears rather weak, in view of current theoretical predictions. Recent Galactic core-collapse supernovae are studied through 44Ti radioactivity, but, apart from Cas A, no other source has been found; this is a rather surprising result, assuming a canonical Galactic supernova rate of ∼ 1 / 50 years. The characteristic signature of 22Na-line emission from a nearby O Ne Mg novae is expected to be measured during INTEGRALs lifetime.

Optical and Near-IR Observations of SN 1998bw

Thus, the observation of gamma-ray line emission from a young supernova seems very promising in the near future. The observation, or even a null observation at a low threshold, will give significance in the fields of nuclear astrophysics and supernova theory. The scientific importance of a positive measurement would be analogous with and comparable to the importance of successful detection of neutrinos from the Sun.Clayton, Colgate, and Fishman [2].

The Galactic 26 Al Problem and the Close Binary Type Ib/c Supernova Solution?

The origin of the long-lived (1.07 Myr mean life) radioactive 26Al, which has been observed in the Galactic interstellar medium from its 1.809 MeV decay gamma-ray line emission, has been a persistent problem for over 20 years. Wolf-Rayet (W-R) winds were thought to be the most promising source, but their calculated 26Al yields are not consistent with recent analyses of the 1.809 MeV emission from the nearest W-R star and nearby OB associations. The expected 26Al yield from the W-R star exceeds, by as much as a factor of 3, that set by the 2 ? upper limit on the 1.809 MeV emission, while the W-R yields in the OB associations are only about of that required by the 1.809 MeV emission. We suggest that a solution to these problems may lie in 26Al from a previously ignored source: explosive nucleosynthesis in the core-collapse Type Ib/c supernovae (SNe Ib/c) of W-R stars that have lost most of their mass to close binary companions. Recent nucleosynthetic calculations of SNe Ib/c suggest that their 26Al yields depend very strongly on the final pre-SN mass of the W-R star and that those with final masses around 6-8 M? are expected to produce as much as 10-2 M? of 26Al per SN. Such binary SNe Ib/c make up only a small fraction of the current SNe Ib/c and only about 1% of all Galactic core-collapse SNe. But they appear to be such prolific sources that the bulk of the present 26Al in the Galaxy may come from just a few hundred close binary SNe Ib/c, and the intense 1.809 MeV emission from nearby OB associations may come from just one or two such SNe. More extensive SN Ib/c calculations of the 26Al yields versus pre-SN mass are clearly needed to test this possibility.

Gamma-ray line emission from galaxy clusters

We report first results of a study dedicated to a search for γ-ray line emission from rich galaxy clusters, finding traces of emission from the region of Virgo and Coma near the Northern Galactic Pole (NGP). The study is based on the 6 years of COMPTEL data collected during the extended observational programme of the Compton Gamma Ray Observatory (CGRO) monitoring of the Virgo and 3C273 region. We point out the advantages of performing this study using an energy range that contains the strongest excitation γ-ray lines of 16 O at 6.13, 6.9 and 7.1 MeV, as well as lines at 6.175 MeV ( 15 O*) and 6.322 MeV ( 15 N*) of the spallation products of the cosmic ray interactions with 16 O. We discuss a generation mechanism for γ-ray line emission in merging clusters via excitation of the abundant element(s) of the intracluster matter at the merger shock by the subrelativistic protons accelerated to energies of ∼ 10 MeV to ∼ 100 MeV/n in the merger process.

Study of global galactic distribution of classical novae by their gamma-ray line emission at 1.275 MeV

Classical novae (CNe) are believed to be the source of the ISM enrichment with the isotopes of 13C, 15N, 22Na and 26Al. In the γ-ray band the Galaxy is almost transparent, so that otherwise obscured bulge CNE may be detectable up to A ν∼10 3 by observation of the extended 1.275 MeV line emission from the accumulated 22Na. Indeed, COMPTEL telescope on board the Compton Gamma-Ray Observatory (CGRO) has detected this extended γ-ray line emission from the bulge population of classical novae. We discuss implications of this result for the theory of classical nova.

Cause of Proton Depletion in Solar Impulsive Particle Events

It is found from our reanalysis of the relevant data that the high electron-to-proton ratio (E/P) observed in large gamma-ray line (GRL) flares can be alternatively explained by proton depletion rather than electron richness as formerly suggested. The term impulsive in this paper is exclusively used as the large events associated with some energetic flare phenomena like GRL emissions and meter-wave type II emissions. Consequently, other weaker events that occur much more frequently, and in association with 3He-rich particle events, are outside the scope of this study. We propose that the proton depletion can be explained by either intrinsic E/Ps of escaping particles from the flare itself or a selection effect in the acceleration process at the lower coronal shock. In the latter case, we suggest that shock waves in the lower corona accelerate only a very few protons because there are no such seed particles as are usually supplied by non-Maxwellian suprathermal high-energy tails in the case of upper collisionless atmosphere. Such suprathermal seed particles are actually observed even in the steady solar wind ubiquitously. As for the former case of the explanation, the flare origin hypothesis for large particle events seems unlikely, because of the difference in the emission cone angles between the large and small events. For gradual events, on the other hand, shocks are thought to be driven by coronal mass ejections and can survive even in the upper atmosphere, accompanied by seed particles, resulting in acceleration of plenty of protons.

X- and Gamma-Ray Line Emission Processes

This chapter is intended to provide a general presentation of the atomic and nuclear processes responsible for X-ray line and gamma-ray line emission in various astrophysical environments. I consider line production from hot plasmas, from accelerated particle interactions, from the decay of radioactive nuclei synthesized in stars and from positron annihilation. Spectroscopic properties of these emissions are discussed in the light of the detection capabilities of modern space instruments.