MeV Gamma-ray Line Research Articles

Thallium-208: A Beacon of InSitu Neutron Capture Nucleosynthesis.

We demonstrate that the well-known 2.6MeV gamma-ray emission line from thallium-208 could serve as a real-time indicator of astrophysical heavy element production, with both rapid (r) and intermediate (i) neutron capture processes capable of its synthesis. We consider the r process in a Galactic neutron star merger and show Tl-208 to be detectable from ∼12 hours to ∼ten days, and again ∼1-20 years postevent. Detection of Tl-208 represents the only identified prospect for a direct signal of lead production (implying gold synthesis), arguing for the importance of future MeV telescope missions which aim to detect Galactic events but may also be able to reach some nearby galaxies in the Local Group.

Improved inertial confinement fusion gamma reaction history 12C gamma-ray signal by direct subtraction.

The Gamma Reaction History (GRH) diagnostic located at the National Ignition Facility (NIF) measures time resolved gamma rays released from inertial confinement fusion experiments by converting the emitted gamma rays into Cherenkov light. Imploded capsules have a bright 4.4 MeV gamma ray from fusion neutrons inelastically scattering with carbon atoms in the remaining ablator. The strength of the 4.4 MeV gamma ray line is proportional to the capsule's carbon ablator areal density and can be used to understand the dynamics and energy budget of a carbon-based ablator capsule implosion. Historically, the GRH's four gas cells use the energy thresholding from the Cherenkov process to forward fit an estimation of the experiment's complete gamma ray spectrum by modeling the surrounding environment in order to estimate the 4.4 MeV neutron induced carbon gamma ray signal. However, the high number of variables, local minima, and uncertainties in detector sensitivities and relative timing had prevented the routine use of the forward fit to generate carbon areal density measurements. A new, more straightforward process of direct subtraction of deconvolved signals was developed to simplify the extraction of the carbon areal density. Beryllium capsules are used as a calibration to measure the capsule environment with no carbon signal. The proposed method is then used to appropriately subtract and isolate the carbon signal on shots with carbon ablators. The subtraction algorithm achieves good results across all major capsule campaigns, achieving similar results to the forward fit. This method is now routinely used to measure carbon areal density for NIF shots.

Scintillators and Cherenkov detectors for the registration of 10.8 MeV gamma rays

The identification of nitrogen by neutron activation has been utilized in both explosive detection and in-vivo metabolic analysis. The 10.8 MeV gamma ray line emitted by thermal neutron capture provides a unique signature, however, due to its high energy its registration is non-trivial. Conventional approaches have used large dense inorganic scintillators which inevitably entail considerable expense. We examine the capabilities of arrays of smaller scintillation detectors and the use of glass Cherenkov detectors as an alternative.

Diagnostics of the region of nuclear interactions of neutrons in the solar flare of January 20, 2005 by gamma-ray lines

Based on an analysis of the time history of the count rate in the 2.223 MeV neutron-capture line from the extreme solar event of January 20, 2005, we investigate the density of the surrounding medium and the 3He content in the area of the 2.223 MeV gamma-line production. The analysis uses data on count rates in the 2.223 MeV gamma-ray line, as well as in lines ranging from 4 to 7 MeV, which were acquired with the AVS-F apparatus from the SONG-D detector onboard the CORONAS-F satellite. It is shown that simulation of the time history of the 2.223 MeV gamma-ray line from the flare event would require an assumption of the elevated abundance of 3He. It is found that in the area of the 2.223 MeV gamma-line production the ratio n(3He)/n(1H) is (1.4 ± 0.15) × 10−4, although, based on the data from different flares, on average it is approximately 2 × 10−5. The elevated abundance of 3He may be considered as an additional indirect argument for the possible presence of reaction products of neutron radiative absorption by the 3He isotope within a small increase in gamma emission in the 15–21 MeV range over the background level (which was detected previously from the CORONAS-F/AVS-F data).

Diffuse MeV gamma rays and galactic 511 keV line from decaying WIMP dark matter

The origin of both the diffuse high-latitude MeV gamma-ray emission and the 511 keV line flux from the Galactic bulge are uncertain. Previous studies have invoked dark matter physics to independently explain these observations, though as yet none has been able to explain both of these emissions within the well-motivated framework of weakly-interacting massive particles (WIMPs). Here we use an unstable WIMP dark matter model to show that it is in fact possible to simultaneously reconcile both of these observations, and in the process show a remarkable coincidence: decaying dark matter with MeV mass splittings can explain both observations if positrons and photons are produced with similar branching fractions. We illustrate this idea with an unstable branon, which is a standard WIMP dark matter candidate appearing in brane-world models with large extra dimensions. We show that because branons decay via three-body final states, they are additionally unconstrained by searches for Galactic MeV gamma-ray lines. As a result, such unstable long-lifetime dark matter particles provide novel and distinct signatures that can be tested by future observations of MeV gamma rays.

Positron Annihilations at the Galactic Center: Generating More Questions Than Answers

The bulge of our Galaxy is illuminated by the 0.511 MeV gamma-ray line flux from annihilations 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 26 Al) 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.

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.

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.

26Al production in [formula omitted] Velorum

The closest Wolf-Rayet star, WR 11 in the binary system γ 2 Velorum, is the only star for which the spectral signature of the 26Al produced in its core is expected to be detectable with current gamma-ray instruments, through the 1.8 MeV gamma-ray line produced by the radioactive decay of the 26Al nuclei. We present here the current status of both model predictions, from calculations of massive star evolution including rotation of stellar interior, and from data on γ 2 Velorum obtained by the ESA's gamma-ray satellite INTEGRAL over the first year of its mission.

On Nonprimordial Deuterium Production by Accelerated Particles

Deuterium plays a crucial role in cosmology because the primordial D/H abundance, in the context of big bang nucleosynthesis (BBN) theory, yields a precise measure of the cosmic baryon content. Observations of D/H can limit or measure the true primordial abundance because D is thought to be destroyed by stars and thus D/H monotonically decreases after BBN. Recently, however, Mullan & Linsky have pointed out that D arises as a secondary product of neutrons in stellar flares that then capture on protons via n + p → d + γ and that this could dominate over direct D production in flares. Mullan & Linsky note that if this process is sufficiently vigorous in flaring dwarf stars, it could lead to significant non-BBN D production. We have considered the production of D in stellar flares, both directly and by n capture. We find that for reasonable flare spectra, n/d ≲ 10 and (n + d)/6Li ≲ 400, both of which indicate that the n-capture channel does not allow for Galactic D production at a level that will reverse the monotonic decline of D. We also calculate the 2.22 MeV gamma-ray line production associated with n capture and find that existing COMPTEL limits also rule out significant D production in the Galaxy today. Thus, we find that flares in particular and neutron captures in general are not an important Galactic source of D. On the other hand, we cannot exclude that flare production might contribute to recent Far Ultraviolet Spectroscopic Explorer (FUSE) observations of large variations in the local interstellar D/H abundance; we do, however, offer important constraints on this possibility. Finally, since flare stars should inevitably produce some n-capture events, a search for diffuse 2.22 MeV gamma rays by INTEGRAL can further constrain (or measure!) Galactic deuterium production via n-capture.

Warm thick target solar γ-ray source revisited

The 1.63 MeV gamma-ray line of Ne-20 is sensitive to protons of lower energies than most other nuclear de-excitation lines. Its unexpected strength has been taken as evidence for a solar flare fast ion distribution that remains steep at low energies, and thus has a large total energy content. It has also been suggested that its strength might instead reflect the enhancement of ion lifetimes that occurs when ambient temperatures exceed 10(7) K. Here we revisit this idea ( a) recognising that ions may be effectively trapped in high temperature regions and (b) taking account of the contribution to the line of all ions above threshold. The strength of the 1.63 MeV line relative to other de-excitation lines has been used to estimate the steepness (e.g. energy power-law index) of the ion distribution. We show that these estimates must be significantly revised if primary ions are contained in a region with temperature in the few 10(7) K range, lower than found elsewhere. Such a region would almost certainly be coronal, so we also briefly review other arguments for and against coronal gamma-ray sources.

21Na(p,gamma)22Mg reaction and oxygen-neon novae.

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in oxygen-neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E(c.m.)=205.7+/-0.5 keV with a resonance strength omegagamma=1.03+/-0.16(stat)+/-0.14(sys) meV.

Discovery of an Old, Nearby, and Overlooked Supernova Remnant Centered on the Southern Constellation Antlia Pneumatica

We report the discovery of a supernova remnant (SNR) with an angular diameter of 24°, centered on the southern constellation Antlia Pneumatica. The SNR is detected well in Hα and X-rays. Within the Antlia SNR's outline, a marginally significant feature is detected in the 1.8 MeV gamma-ray line of the radioisotope 26Al. At an estimated distance dA 60-340 pc, the Antlia SNR is perhaps the nearest SNR except for the Local Bubble. Consequently, any associated neutron star or black hole is expected to have a large proper motion. Of the trajectories of nearby pulsars with well-determined proper motions, only B0950+08's passes within the SNR outline. If the SNR and the pulsar B0950+08 indeed both originated from the same supernova, then their age t = 1.8(dA/100 pc) Myr.

Implications of 1.8 MeV Gamma‐Ray Observations for the Origin of26Al

Recent analysis of COMPTEL data has revealed an extremely close correlation between 53 GHz microwave free-free and 1.8 MeV gamma-ray line emission. While microwave free-free emission arises from the ionized interstellar medium, 1.8 MeV gamma rays are emitted during the radioactive decay of 26Al. We argue that the close correlation can only be understood if massive stars (M20 M☉) are at the origin of Galactic 26Al. Based on the measured proportionality factor, we estimate the 26Al yield of an equivalent O7 V to be (1.0±0.5)×10−4 M☉. Using an estimate for the total Galactic Lyman continuum luminosity of Q=3.5×1053 photons s-1, we derive the Galactic 26Al mass to be 3.1±0.9 M☉. The mass estimate is compared to theoretical nucleosynthesis predictions for 26Al from core-collapse supernovae and Wolf-Rayet stars. We circumvent the problem of using a weakly constrained star formation rate for this comparison by determining the star formation rate self-consistently from our models, using the Galactic Lyman continuum luminosity. The effects of mass loss and metallicity are considered, and the uncertainties of predicted 26Al production rates due to poorly known initial mass limits for the candidate sources are discussed. Assuming solar metallicity throughout the entire Galaxy, we predict a Galactic 26Al mass of 1.6±0.3 M☉, of which ~60% is produced by core-collapse supernovae, while ~40% originates from Wolf-Rayet stars. Taking the Galactic metallicity gradient into account increases the Galactic 26Al mass to 2.2±0.4 M☉, consistent with the observed value. The increase mainly arises from enhanced production by Wolf-Rayet stars in the metal-rich inner Galaxy; these contribute ~60% of the Galactic 26Al budget. We predict that the metallicity gradient should produce an inner-to-outer Galaxy intensity contrast of ~30% between 1.8 MeV and Galactic free-free emission, which should be observable by the future gamma-ray spectrometer SPI on the International Gamma-Ray Astrophysics Laboratory (INTEGRAL).

COMPTEL observations of the 1.809 MeV gamma-ray line from galactic Al-26

The COMPTEL experiment on the Compton Gamma-Ray Observatory (CGRO) has been demonstrated to be capable of imaging the Galaxy within its field of view of about 1 steradian in the 1.809 MeV gamma-ray line originating from radioactive Al-26. The combined data from the CGRO sky survey in 1991/1992 have been analyzed to provide a first map of the inner Galaxy in this gamma-ray line. The 1.809 MeV emission appears extended along the inner 70 deg of the Galactic plane, with a relatively sharp falloff outside this regime. Correlations with massive stars and supernova remnants as possible tracers of the candidate Al-26 sources are discussed.

Are solar gamma-ray-line flares different from other large flares?

We reevaluate evidence indicating that gamma-ray-line (GRL) flares are fundamentally different from other large flares without detectable GRL emission and find no compelling support for this proposition. For large flares observed by the Solar Maximum Mission (SMM) from 1980 to 1982, we obtain a reasonably good correlation between 4-8 MeV GRL fluences and greater than 50 keV hard X-ray fluences and find no evidence for a distinct population of large hard X-ray flares that lack commensurate GRL emission. Our results are consistent with the acceleration of the bulk of the approximately 100 keV electrons and approximately 10 MeV protons (i.e., the populations of these species that interact in the solar atmosphere to produce hard X-ray and GRL emission) by a common process in large flares of both long and short durations.

COMPTEL results on the 1.809 MeV gamma-ray line from the Galactic-center region

The COMPTEL experiment on the Compton Gamma-Ray Observatory is designed to image celestial gamma radiation in the energy range from 0.75–30 MeV within a field of view of 1 steradian. It can locate stronger point sources with an accuracy better than 0.5° and is capable of mapping diffuse emission as well. The Galactic-center region was observed by COMPTEL for several 2-week periods in 1991/1992. These observations show evidence for 1.8 MeV line emission along the Galactic disk (attributed to radioactive 26Al), extending over at least 40 degrees in longitude.

An overview about initial results from the Compton Observatory GRO

Gamma-ray astronomy provides a diagnostic tool to study high energy processes in the Universe. At present, NASA's Compton Observatory is performing the first complete sky survey in gamma-ray astronomy. The observatory was launched on April 5, 1991 by the Space Shuttle Atlantis into a near-Earth orbit. An overview about the first highlight results from this mission is given. Special topics include: pulsars (especially the Crab and Vela pulsars), the diffuse Galactic gamma-ray emission, the nuclei of active galaxies (especially quasars), the 511 keV and 1.8 MeV gamma-ray line emissions from the central region of the Galaxy, the puzzle of cosmic gamma-ray bursts and finally, the Sun during flare activities.

An overview about initial results from the Compton observatory GRO

Gamma-ray astronomy provides a diagnostic tool to study high energy processes in the Universe. At present, NASA's Compton Observatory is performing the first complete sky survey in gamma-ray astronomy. The observatory was launched on April 5, 1991 by the Space Shuttle Atlantis into a near-earth orbit. An overview about the first highlight results from this mission is given. Special topics of presentation are: pulsars (especially the Crab and Vela pulsars), the diffuse galactic gamma-ray emission from interstellar space, the nuclei of active galaxies (especially quasars), the 511 keV and 1.8 MeV gamma-ray line emissions from the central region of the galaxy, the puzzle of cosmic gamma-ray bursts and finally, the Sun during solar flare activities.

On the correlation between the solar gamma–ray line emission, radio bursts and proton fluxes in the interplanetary space

AbstractIt is shown that the correlation takes place between the 4–7 MeV gamma–ray line flare fluence F4–7 and the intensity of the > 10 MeV proton flux in the interplanetary space as well as between F4–7 and the peak flux density of microwave bursts. Besides, the energy spectral index of protons displays the definite dependence from parameters of the radio burst frequency spectrum. These testify that: a) there is a close physical association between the acceleration of electrons and protons in flares; b) protons, giving gamma–ray lines, and ones, registered in the interplanetary space, belong to the same population.