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All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Space-Borne Observations of Intense Gamma-Ray Flashes (TGFs) Above Thunderstorms

Intense millisecond flashes of MeV photons are being observed with space-borne detectors. These terrestrial gamma-ray flashes (TGFs) were discovered with the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory (CGRO) in the early 1990s. They are now being observed with several other instruments, including the Gamma-ray Burst Monitor (GBM) detectors on the Fermi Gamma-ray Space Telescope. Although Fermi-GBM was designed and optimized for the observation of cosmic gamma-ray bursts (GRBs), it has unprecedented capabilities for TGF observations. The TGFs usually have extremely hard continuous spectra, typical of highly-Comptonized bremsstrahlung radiation. These spectral are harder than those of GRBs, with photons extending to over 40 MeV. The most likely origin of these high-energy photons is bremsstrahlung radiation produced by a relativistic runaway avalanche electron beam. Such a beam is expected to be produced in an extended, intense electric field in or above thunderstorm regions. The altitude of origin and beaming characteristics of the radiation are quite uncertain. These TGFs may produce an appreciable radiation dose to passengers and crew in nearby aircraft. They have generated considerable observational and theoretical interest in recent years. Instruments are being designed specifically for TGF observations from new spacecraft as well as from airborne platforms.

Fishman, Gerald J.↗
NASA NTRS2011

Suzaku Detection of Diffuse Hard X-Ray Emission Outside Vela X

Vela X is a large, 3 deg x 2 deg, radio-emitting pulsar wind nebula (PWN) powered by the Vela pulsar in the Vela supernova remnant. Using four Suzaku/XIS observations pointed just outside Vela X, we find hard X-ray emission extending throughout the fields of view. The hard X-ray spectra are well represented by a power-law. The photon index is measured to be constant at Gamma approximates 2.4, similar to that of the southern outer part of Vela X. The power-law flux decreases with increasing distance from the pulsar. These properties lead us to propose that the hard X-ray emission is associated with the Vela PWN. The larger X-ray extension found in this work strongly suggests that distinct populations relativistic electrons form the X-ray PWN and Vela X, as was recently inferred from multiwavelength spectral modeling of Vela X.

Katsuda, Satoru↗
NASA NTRS2018

Accretion Disc MHD Winds and Blazar Classification

The Fermi Gamma-Ray Space Telescope observations of blazars show a strong correlation between the spectral index of their -ray spectra and their synchrotron peak frequency synpk ;additionally, the rate of Compton dominance of these sources also seems to be a function of synpk .In this work, we adopt the assumption that the non-thermal emission of blazars is primarily due to radiation by a population of Fermi-accelerated electrons in a relativistic outflow (jet) along the symmetry axis of the blazar's accretion disc. Furthermore, we assume that the Compton component is related to an external photon field of photons, which are scattered from particles of the magnetohydrodynamic wind emanating from the accretion disc. Our results reproduce well the aforementioned basic observational trends of blazar classification by varying just oneparameter, namely the mass accretion rate on to the central black hole.

Boula, Stella↗
DOE OSTI2016

THE {sup 7}Be ii RESONANCE LINES IN TWO CLASSICAL NOVAE V5668 SGR AND V2944 OPH

We report spectroscopic observations of the resonance lines of singly ionized {sup 7}Be in the blueshifted absorption line systems found in the post-outburst spectra of two classical novae—V5668 Sgr (Nova Sagittarii 2015 No. 2) and V2944 Oph (Nova Ophiuchi 2015). The unstable isotope {sup 7}Be should have been created during the thermonuclear runaway (TNR) of these novae and decayed to form {sup 7}Li within a short period (a half-life of 53.22 days). These confirmations of {sup 7}Be are the second and the third ones following the first case found in V339 Del by Tajitsu et al. The blueshifted absorption line systems in both novae are clearly divided into two velocity components, both of which contain {sup 7}Be. This means that the absorbing gases in both velocity components consist of products of TNR. We estimated the amounts of {sup 7}Be produced during the outbursts of both novae and concluded that significant {sup 7}Li should have been created. These findings strongly suggest that the explosive production of {sup 7}Li via the reaction {sup 3}He(α,γ){sup 7}Be and its subsequent decay to {sup 7}Li occurs frequently among classical novae and contributes to the process of Galactic Li enrichment.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Resonance Self-shielding Impact on Neutron Spectrum Determination for Missouri S and T Reactor

Neutron Activation Analysis is a useful technique for characterizing the neutron flux at nuclear facilities. It involves irradiating samples to produce unstable isotopes, which enables qualitative and quantitative analyses of unknown samples. In the process, samples are bombarded with neutrons; new elements then become radioactive isotopes due to absorption of neutrons. These emit different amounts of gamma energy during the decay process, and gamma rays are counted using HPGe. Each reaction product has a known half-life and cross-section, and by measuring the amount of gamma radiation energy emitted from a radioactive isotope, it is possible to identify the isotope and activity. However, the neutron spectrum is not precise or representative of the actual neutron environment because of the self-shielding effect. This occurs with large samples that have large cross-sections or thicknesses in the resonance range of the neutron flux spectrum. Several methods have been used to mitigate this problem, such as thinning or diluting alloys samples, calculating the self-shielding factor for each material, and generating a corrected library for the unfolding procedure code. The Missouri University of Science and Technology Reactor (MSTR) is a pool-type research reactor that operates up to 200 kW. It uses light-water for both moderation and natural convective heat removal. The reactor core consists of 15 fuel elements positioned in a 9 x 6 aluminum grid plate, with four elements used for reactor power control. The fuel element is 19.75% low enriched uranium in 18 curved fuel plates. The reactor has seen various configurations starting with configuration designation 101W and subsequent reconfiguration increases the numeric designation by 1. The current configuration, 120W, was implemented in 2010. The new configuration provides more in-core locations for activation experiments and uses more fuel elements than previous ones. One of the irradiation locations in the MSTR core is the source holder. It is desired to improve the documentation of neutronic characteristics of the MSTR which are cardinal to experiments supported by the facility. One of these characteristics is the neutron spectrum at various locations in the reactor. Successful efforts yielded the neutron spectrum at a known location through the unfolding of the activity of irradiated foils. The purpose of this current study is to investigate the impact of resonance self-shielding of some of the activated foils on the determined spectrum.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2015

Neutrino and cosmic-ray emission and cumulative background from radiatively inefficient accretion flows in low-luminosity active galactic nuclei

We study high-energy neutrino and cosmic-ray (CR) emission from the cores of low-luminosity active galactic nuclei (LLAGN). In LLAGN, the thermalization of particles is expected to be incomplete in radiatively inefficient accretion flows (RIAF), allowing the existence of non-thermal particles. In this work, assuming stochastic particle acceleration due to turbulence in RIAFs, we solve the Fokker–Planck equation and calculate spectra of escaping neutrinos and CRs. The RIAF in LLAGN can emit CR protons with ≳10 PeV energies and TeV–PeV neutrinos generated via pp and/or pγ reactions. We find that, if ∼1% of the accretion luminosity is carried away by non-thermal ions, the diffuse neutrino intensity from the cores of LLAGN may be as high as E{sub ν}{sup 2}Φ{sub ν}∼3×10{sup −8} GeV cm{sup −2} s{sup −1} sr{sup −1}, which can be compatible with the observed IceCube data. This result does not contradict either of the diffuse gamma-ray background observed by Fermi or observed diffuse CR flux. Our model suggests that, although very-high-energy gamma-rays may not escape, radio-quiet active galactic nuclei with RIAFs can emit GeV gamma-rays, which could be used for testing the model. We also calculate the neutron luminosity from RIAFs of LLAGN, and discuss a strong constraint on the model of jet mass loading mediated by neutrons from the diffuse neutrino observation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Examining The Fermi-LAT Third Source Catalog in search of dark matter subhalos

Dark matter annihilations taking place in nearby subhalos could appear as gamma-ray sources without detectable counterparts at other wavelengths. In this study, we consider the collection of unassociated gamma-ray sources reported by the Fermi Collaboration in an effort to identify the most promising dark matter subhalo candidates. While we identify 24 bright, high-latitude, non-variable sources with spectra that are consistent with being generated by the annihilations of ∼ 20–70 GeV dark matter particles (assuming annihilations to b b-bar ), it is not possible at this time to distinguish these sources from radio-faint gamma-ray pulsars. Deeper multi-wavelength observations will be essential to clarify the nature of these sources. It is notable that we do not find any such sources that are well fit by dark matter particles heavier than ∼100 GeV. We also study the angular distribution of the gamma-rays from this set of subhalo candidates, and find that the source 3FGL J2212.5+0703 prefers a spatially extended profile (of width ∼ 0.15°) over that of a point source, with a significance of 4.2σ (3.6σ after trials factor). Although not yet definitive, this bright and high-latitude gamma-ray source is well fit as a nearby subhalo of m{sub χ} ≅ 20–50 GeV dark matter particles (annihilating to b b-bar ) and merits further multi-wavelength investigation. Based on the subhalo distribution predicted by numerical simulations, we derive constraints on the dark matter annihilation cross section that are competitive to those resulting from gamma-ray observations of dwarf spheroidal galaxies, the Galactic Center, and the extragalactic gamma-ray background.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Monte Carlo simulations of photoneutron activation in a medical LINAC

The first step of the present study was to simulate the γ-radioactivity produced by photoneutrons in the treatment head of a decommissioned medical LINAC. These results, obtained with MCNPX and CINDER'90, are then compared with γ-ray spectrometric measurements of parts of the treatment head. Our ultimate objective is to simulate the associated production by photonuclear reactions of pure β-emitters, which are more difficult to access experimentally than the γ-ray emitters. (authors)

43 PARTICLE ACCELERATORS↗
NASA NTRS1977

Evidence for mass loss in the mid-ultraviolet spectra of Be stars

The spectral region about the Mg II doublet lines near 2800 A of Be and shell stars has been observed. The star Gamma Cas shows narrow components to each resonance doublet line shifted 215 km/s toward shorter wavelengths. The resonance and subordinate lines of Zeta Tau are shifted toward shorter wavelengths by 75 km/s, and the subordinate lines are greatly strengthened. The star Kappa Dra, which is much later in spectral type, shows a comparatively normal doublet spectrum.

Morgan, T. H.↗
DOE OSTI2017

Benchmarking of activation rate in special core in LR-0 reactor

The experiments focused on the determination of activation reaction rates of selected IRDFF materials activated in a well-defined special core assembled in the LR-0 reactor are presented in the paper. The used core is designed and verified for IRDFF library testing by reactivity characterization, fission rates distribution and also neutron spectra measurement at the center of the core. {sup 23}Na(n,2n), {sup 75}As(n,2n), {sup 89}Y(n,2n) and {sup 90}Zr(n,2n) reaction cross sections were measured in this special core. The presented neutron cross sections were derived from experimentally determined reaction rates of irradiated samples of NaF, As{sub 2}O{sub 3}, Y{sub 2}O{sub 3}, and ZrO{sub 2}. These reaction rates are derived from gamma activities of the activation products of studied activation reactions. Net Peak Areas (NPA) of photons accompanying decay were measured using a semiconductor High Purity Germanium (HPGe) detector. Despite the fact that mentioned reactions have a threshold above 10 MeV, where the neutron spectrum in the special LR-0 core is nearly undistinguishable from {sup 235}U prompt fission neutron spectrum, the measured integral cross sections averaged in LR-0 spectrum were corrected for spectral shift. The obtained values of {sup 235}U spectral average cross section are: (3.84 ± 0.19) μb for {sup 23}Na(n,2n); (0.320 ± 0.015) mb for {sup 75}As(n,2n); (0.171 ± 0.006) mb for {sup 89}Y(n,2n); (0.107 ± 0.005) mb for {sup 90}Zr(n,2n). (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
NASA NTRS1999

Development of CDZNTE Detectors for Low-Energy Gamma-Ray Astronomy

Under this grant the UC Berkeley PI, K. Hurley, joined a Goddard-led effort to develop large area, multi-pixel Cadmium-Zinc-Telluride (CdZnTe, or CZT) detectors for gamma-ray astronomy. His task was to advise the project of new developments in the area of cosmic gamma-ray bursts, in order to focus the detector development effort on the construction of an instrument which could be deployed on a spacecraft to localize and measure the energy spectra of bursts with good angular and energy resolution, respectively. UC Berkeley had no hardware role in this proposal. The result of this effort was the production, at Goddard, of five CZT prototype modules. A proposal was written for SWIFT, a MIDEX mission to study cosmic gamma-ray bursts. One experiment aboard SWIFT is the Burst Arcminute Telescope (BAT), which consists of a 5200 sq cm hard X-ray detector and a coded mask. The detector comprises 256 CZT modules, each containing 128 4 x 4 x 2 mm CZT detectors. Each detector is read out using an ASIC. The angular resolution achieved with this mask/array combination is 22 arcminutes, and a strong gamma-ray burst can be localized to an accuracy of 4 arcminutes in under 10 seconds. The energy resolution is typically 5 keV FWHM at 60 keV, and the energy range is 10 - 150 keV. The BAT views 2 steradians, and its sensitivity is such that the instrument can detect 350 gamma-ray burst/year, localizing 320 of them to better than 4 arcminute accuracy. The BAT concept therefore met the science goals for gamma-ray bursts. The UCB effort in the SWIFT proposal included the scientific objectives for gamma-ray bursts, and the assembly of a team of optical and radio observers who would use the BAT data to perform rapid multi-wavelength searches for the counterparts to bursts. This proposal was submitted to NASA and peer-reviewed. In January 1999 it was one of five such proposals selected for a Phase A study. This study was completed in June, and SWIFT was formally presented to NASA in September. It appeared that a final selection of two MIDEX missions for flight was imminent. However, the final selection was deferred at the last minute due to the uncertainty in NASA's OSS budget.

Gehrels, N.↗
NASA NTRS2004

Physics of Gamma Ray Burst Sources

During this grant period, the physics of gamma-ray bursts was investigated. A number of new results have emerged. The importance of pair formation in high compactness burst spectra may help explain x-ray flashes; a universal jet shape is a likely explanation for the distribution of jet break times; gravitational waves may be copiously produced both in short bursts from compact mergers and in long bursts arising from collapsars; x-ray iron lines are likely to be due to interaction with the stellar atmosphere of the progenitor; prompt optical flashes from reverse shocks will give diagnostics on the Lorentz factor and the environment; GeV and TeV emission from bursts may be expected in the external shock; etc. The group working with the PI included postdocs Dr. Bing Zhang (now assistant professor at University of Nevada); Dr. Shiho Kobayashi; graduate student Lijun Gou; collaborators Drs. Tim Kallman and Martin Rees. Meszaros shared with Rees and Dr. Bohan Paczynsky the AAS Rossi Prize in 2000 for their work on the theory of gamma ray bursts. The refereed publications and conference proceedings resulting from this research are summarized below. The PI gave a number of invited talks at major conferences, also listed.

Meszaros, Peter↗
DOE OSTI2017

NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare

We report a Nuclear Spectroscopic Telescope Array ( NuSTAR ) observation of a solar microflare, SOL2015-09-01T04. Although it was too faint to be observed by the GOES X-ray Sensor, we estimate the event to be an A0.1 class flare in brightness. This microflare, with only ∼5 counts s{sup −1} detector{sup −1} observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager ( RHESSI ), is fainter than any hard X-ray (HXR) flare in the existing literature. The microflare occurred during a solar pointing by the highly sensitive NuSTAR astrophysical observatory, which used its direct focusing optics to produce detailed HXR microflare spectra and images. The microflare exhibits HXR properties commonly observed in larger flares, including a fast rise and more gradual decay, earlier peak time with higher energy, spatial dimensions similar to the RHESSI microflares, and a high-energy excess beyond an isothermal spectral component during the impulsive phase. The microflare is small in emission measure, temperature, and energy, though not in physical size; observations are consistent with an origin via the interaction of at least two magnetic loops. We estimate the increase in thermal energy at the time of the microflare to be 2.4 × 10{sup 27} erg. The observation suggests that flares do indeed scale down to extremely small energies and retain what we customarily think of as “flare-like” properties.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Gamma-ray burst spectrum with decaying magnetic field

In the internal shock model for gamma-ray bursts (GRBs), the synchrotron spectrum from the fast cooling electrons in a homogeneous downstream magnetic field (MF) is too soft to produce the low-energy slope of GRB spectra. However, the MF may decay downstream with distance from the shock front. Here we show that the synchrotron spectrum becomes harder if electrons undergo synchrotron and inverse-Compton cooling in a decaying MF. To reconcile this with the typical GRB spectrum with low-energy slope νF {sub ν}∝ν, the postshock MF decay time must be comparable to the cooling time of the bulk electrons (corresponding to a MF decaying length typically of ∼10{sup 5} skin depths); that the inverse-Compton cooling should dominate synchrotron cooling after the MF decay time; and/or that the MF decays with comoving time roughly as B∝t {sup –1.5}. An internal shock synchrotron model with a decaying MF can account for the majority of GRBs with low-energy slopes not harder than ν{sup 4/3}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SUPERLUMINOUS SUPERNOVAE POWERED BY MAGNETARS: LATE-TIME LIGHT CURVES AND HARD EMISSION LEAKAGE

Recently, research performed by two groups has revealed that the magnetar spin-down energy injection model with full energy trapping can explain the early-time light curves of SN 2010gx, SN 2013dg, LSQ12dlf, SSS120810, and CSS121015 but fails to fit the late-time light curves of these superluminous supernovae (SLSNe). These results imply that the original magnetar-powered model is challenged in explaining these SLSNe. Our paper aims to simultaneously explain both the early- and late-time data/upper limits by considering the leakage of hard emissions. We incorporate quantitatively the leakage effect into the original magnetar-powered model and derive a new semianalytical equation. Comparing the light curves reproduced by our revised magnetar-powered model with the observed data and/or upper limits of these five SLSNe, we found that the late-time light curves reproduced by our semianalytical equation are in good agreement with the late-time observed data and/or upper limits of SN 2010gx, CSS121015, SN 2013dg, and LSQ12dlf and the late-time excess of SSS120810, indicating that the magnetar-powered model might be responsible for these SLSNe and that the gamma-ray and X-ray leakages are unavoidable when the hard photons were down-Comptonized to softer photons. To determine the details of the leakage effect and unveil the nature of SLSNe, more high-quality bolometric light curves and spectra of SLSNe are required.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Long-term monitoring of the black hole candidate Swift J1753.5–0127 with INTEGRAL/SPI

The black hole candidate (BHC) Swift J1753.5–0127 went into outburst in 2005 June. Rather than fade into quiescence as most BHC transients do, it has remained in a low hard spectral state for most of the ∼9 years after its outburst. Its persistent emission during the hard state is reminiscent of the black hole Cyg X-1 and the BHCs 1E 1740.7–2942 and GRS 1758–258. Thus far, hard X-ray/soft gamma-ray results have mainly focused on the 2005 flare, with a few additional observations during 2007. Here, we present results from INTEGRAL/SPI observations from 2005–2010 spanning the 22–650 keV energy range. Spectral analysis shows a weak high-energy excess (∼2.9σ) above a cutoff power-law model that is well fit by a power law, suggesting an additional spectral component. Observations of Cyg X-1, 1E 1740.7–2942, and GRS 1758–258 have shown similar spectra requiring an additional high-energy component. The SPI results are compared to previously reported results for Swift J1753.5–0127 as well as observations of other sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Detailed investigation of the gamma-ray emission in the vicinity of SNR W28 with Fermi-LAT

We present a detailed investigation of the γ-ray emission in the vicinity of the supernova remnant (SNR) W28 (G6.4–0.1) observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. We detected significant γ-ray emission spatially coincident with TeV sources HESS J1800–240A, B, and C, located outside the radio boundary of the SNR. Their spectra in the 2-100 GeV band are consistent with the extrapolation of the power-law spectra of the TeV sources. We also identified a new source of GeV emission, dubbed Source W, which lies outside the boundary of TeV sources and coincides with radio emission from the western part of W28. All of the GeV γ-ray sources overlap with molecular clouds in the velocity range from 0 to 20 km s{sup –1}. Under the assumption that the γ-ray emission toward HESS J1800–240A, B, and C comes from π{sup 0} decay due to the interaction between the molecular clouds and cosmic rays (CRs) escaping from W28, they can be naturally explained by a single model in which the CR diffusion coefficient is smaller than the theoretical expectation in the interstellar space. The total energy of the CRs escaping from W28 is constrained through the same modeling to be larger than ∼2 × 10{sup 49} erg. The emission from Source W can also be explained with the same CR escape scenario.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

The 1993 multiwavelength campaign on 3C 279: The radio to gamma-ray energy distribution inlow state

Simultaneous observations of 3C 279 at radio, millimeter, near-infrared, optical, ultraviolet (with IUE) and X-ray (with ROSAT) wavelengths were obtained in 1992 December-1993 January, during a three week pointing at the source by the Compton Gamma Ray Observatory. The blazar was in a quiescent or 'low' state during this period. Comparing the multiwavelength energy distribution to that from 1991 June, when 3C 279 was in its brightest recorded gamma-ray state, we find the following: (1) 3C 279 faded dramatically at all frequencies above 10(exp 14) Hz, while the flux variations at low frequencies (radio to millimeter wavelengths) were minor. (2) The near-infrared-optical-ultraviolet spectral shape was softer (steeper) in the quiescent state, and the X-ray spectra also appear softer, although the spectral indix measured by ROSAT refer to a lower energy band than that measured earlier with Ginga. (3) The ratio of the gamma-ray luminosity to that across all other frequencies decreased from a value of approximately equal to 10 in the flaring state to a value approximately equal to 1 in the quiescent state. These findings imply that the production of gamma-rays is closely related to the optical-ultraviolet continuum, in agreement with models where gamma-rays are produced through inverse Compton (IC) scattering by relativistic electrons emitting the synchrotron continuum. The observed nonlinear relation between the synchrotron and IC requires both a change in the electron spectrum and an associated change in the seed photons.

Maraschi, L.↗