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Highest energy phenomena in the universe

Description: One of the most enduring and frustrating mysteries in astrophysics is the nature of the cosmic gamma-ray burst (GRB). GRB` s were first reported in the 1970` s, and since then have been observed from over a dozen different satellites. Two recently launched satellites have revolutionized this field. One, the most sensitive, is known as the Compton Gamma-Ray Observatory (CGRO). This orbiting observatory carries an experiment known as the Burst and Transient Source Experiment (BATSE), which was intended to help elucidate the nature and origin of GRB` s. The other is the Italian-Dutch satellite Beppo-SAX, which is substantially less sensitive than BATSE, but can provide greatly superior positional accuracy for the GRBs it detects. The purpose of this LDRD project was to maximize the combined return of these satellites and two ground-based programs (MACHO [Massive Compact Halo Objects] and LOTIS [Livermore Optical Transient Imaging System]), which LLNL largely controls. We have been successful in designing and implementing real-time response systems, and have successfully responded to GRBs with the MACHO Telescope System.
Date: May 13, 1998
Creator: Alcock, C. R., LLNL
Partner: UNT Libraries Government Documents Department

Super LOTIS a high sensitive optical counterpart search experiment

Description: We are constructing a 0.6 meter telescope system to search for early time gamma-ray burst (GRB) optical counterparts. Super-LOTIS (Super-Livermore Optical Transient Imaging System) is an automated telescope system that has a 0.8 x 0.8{degree} field-of-view, is sensitive to M{sub v} {approximately} 19 and responds to a burst trigger within 5 min. This telescope will record images of the gamma-ray burst coordinates that is given by the GCN (GRB Coordinate Network). A measurement of GRB light curves at early times will greatly enhance our understanding of GRB physics.
Date: November 17, 1997
Creator: Park, H.S., Ables, E. & Band, D.L
Partner: UNT Libraries Government Documents Department

Limits on expanding relativistic shells from Gamma-Ray Burst temporal structure

Description: The author calculates the expected envelope of emission for relativistic shells under the assumption of local spherical symmetry. Gamma-Ray Burst envelopes rarely conform to the expected shape, which has a fast rise and a smooth, slower decay. Furthermore, the duration of the decay phase is related to the time the shell expands before converting its energy to gamma rays. From this, one can estimate the energy required for the shell to sweep up the ISM. The energy greatly exceeds 10{sup 53} erg unless the bulk Lorentz factor is less than 75. This puts extreme limits on the {open_quotes}external{close_quotes} shock models. However, the alternative, {open_quotes}internal{close_quotes} shocks from a central engine, has one extremely large problem: the entire long complex time history lasting hundreds of seconds must be postulated at the central site.
Date: December 31, 1996
Creator: Fenimore, E. E.
Partner: UNT Libraries Government Documents Department

First year results from LOTIS

Description: LOTIS (Livermore Optical Transient Imaging System) is a gamma-ray burst optical couterpart search experiment located near Lawrence Livermore National Laboratory in California. The system is linked to the GCN (GRB Coordinates Network) real-time coordinate distribution network and can respond to a burst trigger in 6-15 seconds. LOTIS has a total field-of-view of 17.4 degrees x 17.4 degrees with a completeness sensitivity of mv {approximately} 11 for a 10 second integration time. Since operations began in October 1996, LOTIS has responded to over 30 GCN/BATSE GRB triggers. Seven of these triggers are considered good events subject to the criteria of clear weather conditions, {lt}60 S RESPONSE TIME, AND {gt}50% coverage of the final BATSE 3(sigma) error circle. We discuss results from the first year of LOTIS operations with an emphasis on the observations and analysis of GRB 971006 (BATSE trigger 6414).
Date: November 17, 1997
Creator: Williams, G.G.; Parks, H.S. & Ables, E.
Partner: UNT Libraries Government Documents Department

Binary neutron star GRB model

Description: In this paper we present the preliminary results of a model for the production of gamma-ray bursts (GRBs) through the compressional heating of binary neutron stars near their last stable orbit prior to merger. Recent numerical studies of the general relativistic (GR) hydrodynamics in three spatial dimensions of close neutron star binaries (NSBs) have uncovered evidence for the compression and heating of the individual neutron stars (NSs) prior to merger. This effect will have significant effect on the production of gravitational waves, neutrinos and, ultimately, energetic photons. The study of the production of these photons in close NSBs and, in particular, its correspondence to observed GRBs is the subject of this paper. The gamma-rays arise as follows. Compressional heating causes the neutron stars to emit neutrino pairs which, in turn, annihilate to produce a hot electron-positron pair plasma. This pair- photon plasma expands rapidly until it becomes optically thin, at which point the photons are released. We show that this process can indeed satisfy three basic requirements of a model for cosmological gamma-ray bursts: 1) sufficient gamma-ray energy release (> 10{sup 51} ergs) to produce observed fluxes, 2) a time-scale of the primary burst duration consistent with that of a ``classical`` GRB ({approximately} 10 seconds), 3) peak of photon number spectrum matches that of ``classical`` GRB ({approximately} 300 keV).
Date: November 11, 1997
Creator: Wilson, J.R.; Salmonson, J.D. & Mathews, G.J.
Partner: UNT Libraries Government Documents Department

DMSP satellite detections of gamma-ray bursts

Description: Gamma-ray burst detectors are aboard six U. S. Air Force defense Meteorological Satellite Program (DMSP) spacecraft, two of which are currently in use. Their 800-km altitude orbits give a field of view to 117 degrees from the zenith. A great many bursts have been detected, usually in coincidence with detections by GRO or other satellites such as PVO or ULYSSES. The directions of the sources can be determined with considerable accuracy from such correlated observations, even when GRO/BATSE with its directional capabilities is not involved. Thus these DMSP data, especially in conjunction with other observations, should be helpful in trying to understand the true nature of gamma-ray bursts. 8 refs., 5 figs.
Date: December 31, 1995
Creator: Terrell, J.; Lee, P. & Klebesadel, R.W.
Partner: UNT Libraries Government Documents Department

Gamma-ray bursts from fast, Galactic neutron stars

Description: What makes a Galacic model of gamma-ray bursts (GBs) feasible is the observation of a new population of objects, fast neutron stars, that are isotropic with respect to the Galaxy following a finite period, {approximately}30My, after their formation. Our Galactic model for the isotropic component of (GBs) is based upon these high-velocity neutron stars (NSs) that have accretion disks. The fast NSs are formed in tidally locked binaries, where tidal locking occurs due to the meridional circulation caused by the conservation of angular momentum of the tidal lobes. These same lobes perturb the subsequent collapse to a supernova and forming a slowly rotating NS. Following the collapse to a NS and explosion, subsequent accretion occurs on the rear side of the initially perturbed NS, resulting in a run-away acceleration of the neutron star by neutrino emission from the hot accreted matter. The recoil momentum of the relativistic neutrino emission from the localized, down flowing matter far exceeds the momentum drag of the accreted matter. The recoil of the NS may be oriented towards the companion, but misses because of the initial orbital motion. The near miss captures matter from the companion and forms a disk around the NS. Accretion onto the neutron star from this initially gaseous disk due to the ``alpha`` viscosity results initially in the soft gamma-ray repeater phase, {approximately}10{sup 4} yr. After the neutron star has moved {approximately}30 kpc from its birthplace, solid bodies form in the disk, and accrete to planetoid size bodies after {approximately}3 {times} 10{sup 7} years. Some of these planetoid bodies, with a mass of {approximately}10{sup 21to22} g, are perturbed into being captured by the magnetic field of the NS to create GBs. The high velocity and millions of years delay in forming planetoids, results in isotropy.
Date: July 1, 1995
Creator: Colgate, S.A. & Leonard, P.J.T.
Partner: UNT Libraries Government Documents Department

GRBs from the First Stars

Description: We present an estimate of the Gamma Ray Bursts which should be expected from metal-free, elusive first generation of stars known as PopulationIII (PopIII). We derive the GRB rate from these stars from the Stellar Formation Rate obtained in several Reionization scenarios available in the literature. In all of the analyzed models we find that GRBs from PopIII are subdominant with respect to the ''standard'' (PopII) ones up to z {approx} 10.
Date: April 16, 2007
Creator: Iocco, Fabio & /Naples U. /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department

Intensity distributions of gamma-ray bursts

Description: Observations of individual bursts chosen by the vagaries of telescope availability demonstrated that bursts are not standard candles and that their apparent energy can be as great as 10{sup 54} erg. However, determining the distribution of their apparent energy (and of other burst properties) requires the statistical analysis of a well-defined burst sample; the sample definition includes the threshold for including a burst in the sample. Thus optical groups need to the criteria behind the decision to search for a spectroscopic redshift. Currently the burst samples are insufficient to choose between lognormal and power law functional forms of the distribution, and the parameter values for these functional forms differ between burst samples. Similarly, the actual intensity distribution may be broader than observed, with a low energy tail extending below the detection threshold.
Date: January 1, 2001
Creator: Band, D. L. (David L.)
Partner: UNT Libraries Government Documents Department

Simulation of Prompt Emission from GRBs with a Photospheric Component and its Detectability By GLAST

Description: The prompt emission from gamma-ray bursts (GRBs) still requires a physical explanation. Studies of time-resolved GRB spectra, observed in the keV-MeV range, show that a hybrid model consisting of two components, a photospheric and a non-thermal component, in many cases fits bright, single-pulsed bursts as well as, and in some instances even better than, the Band function. With an energy coverage from 8 keV up to 300 GeV, GLAST will give us an unprecedented opportunity to further investigate the nature of the prompt emission. In particular, it will give us the possibility to determine whether a photospheric component is the determining feature of the spectrum or not. Here we present a short study of the ability of GLAST to detect such a photospheric component in the sub-MeV range for typical bursts, using simulation tools developed within the GLAST science collaboration.
Date: November 29, 2011
Creator: Battelino, Milan; Ryde, Felix; Observ., /Stockholm; Omodei, Nicola; /INFN, Pisa; Longo, Francesco et al.
Partner: UNT Libraries Government Documents Department

The Structure and Dynamics of GRB Jets

Description: There are several lines of evidence which suggest that the relativistic outflows in gamma-ray bursts (GRBs) are collimated into narrow jets. The jet structure has important implications for the true energy release and the event rate of GRBs, and can constrain the mechanism responsible for the acceleration and collimation of the jet. Nevertheless, the jet structure and its dynamics as it sweeps up the external medium and decelerates, are not well understood. In this review I discuss our current understanding of GRB jets, stressing their structure and dynamics.
Date: October 25, 2006
Creator: Granot, Jonathan
Partner: UNT Libraries Government Documents Department

Gamma ray burst model

Description: We present a model for gamma ray bursts based on the compression of neutron stars in close binary systems. Our general relativistic hydrodynamiccomputer simulations of close neutron star binaries have found that as the orbit shrinks the density of the neutron stars rises. This compressional effect has been estimated to produce thermal energies in the neutron stars of the order of magnitude 10{sup 52}to 10{sup 53} ergs on a timescale of a few seconds.This is a possible source of energy for gamma-ray bursts. The hot neutron stars will emit neutrino pairs which will partially recombine to form an electron positron pair plasma. The pair plasma will recombine after expansion to produce photons which closely mimic the characteristics of gamma-raybursts.
Date: February 17, 1998
Creator: Wilson, J. R.; Salmonson, J. D. & Mathews, G. J.
Partner: UNT Libraries Government Documents Department

The expected rates of X-ray flashes from GRBs in ROSAT and preliminary search results

Description: The recent long (up to days) and relatively bright soft X-ray afterglows from GRBs detected by BeppoSAX raise the possibility of detecting similar events in the ROSAT database. The authors perform detailed calculations on the expected number of ROSAT events based on the extended halo scenario. They use the realistic ROSAT capabilities and the actual exposure time on bright nearby galaxies. They find that the expected number of x-ray events is quite low, {approximately} 1, and depends on the peak x-ray luminosity. Thus a nondetection of such events from nearby galaxies (within 12 Mpc) can not rule out most of the viable halo models. They also present the preliminary search results from the ROSAT PSPC database.
Date: December 1, 1998
Creator: Li, H.; Sun, X. & Fenimore, E.E.
Partner: UNT Libraries Government Documents Department

New studies of nuclear decay {gamma}-rays from novae

Description: The cause of the nova outburst is a thermonuclear runaway (TNR) in hydrogen rich material transferred by a companion onto a white dwarf. Studies of this phenomenon have shown that the TNR produces large concentrations of the short lived positron unstable isotopes of the CNO nuclei which are transported to the surface by convection so that early in the outburst we expect significant numbers of radioactive decays to occur at the surface. The resulting {gamma}-ray emission may be detectable from nearby novae early in their outbursts. The TNR is also expected to produce substantial amounts of {sup 7}Be and {sup 22}Na. Their decays also yield potentially detectable levels of {gamma}-ray emission for relatively nearby novae. We are also interested in the role played by novae in the production of the {approximately}2M{circle_dot} of {sup 26}Al found in the galaxy. In order to improve our predictions of this phenomenon, we have performed a new set of calculations of TNR`s on ONeMg and CO white dwarfs with an updated nuclear reaction network and opacities.
Date: November 1, 1997
Creator: Starrfield, S.; Truran, J.W. & Wiescher, M.C.
Partner: UNT Libraries Government Documents Department

Observational and theoretical studies of the nova outburst

Description: A nova outburst is one consequence of the accretion of hydrogen rich material onto a white dwarf in a close binary system. The strong electron degeneracy of a massive white dwarf drives the temperatures in the nuclear burning region to values exceeding 108K under all circumstances. As a result, a major fraction of the CNO nuclei in the envelope are transformed into e{sup +}-decay nuclei, which constrains the nuclear energy generation and yields non-solar CNO isotopic abundance ratios. In addition, the observations demonstrate that white dwarf core material is dredged up into the accreted layers and these nuclei are the catalysts for producing peak rates of energy generation that can exceed 10{sup 16} erg gm{sup -1}s{sup -1}. Observations show that there are two compositional classes of novae, one that occurs on a carbon-oxygen white dwarf and the other that occurs on an oxygen-neon-magnesium white dwarf.
Date: April 1, 1998
Creator: Starrfield, S.; Vanlandingham, K. & Schwarz, G.
Partner: UNT Libraries Government Documents Department

Gamma-Ray Bursts: Relativistic shells or central engines?

Description: In many models of Gamma-Ray Bursts (GRBs) relativistic shells are responsible for the overall envelope of emission. The authors use kinematics and symmetry to calculate the time history and spectral evolution expected from a relativistic shell including effects from intrinsic variations in the shell`s intensity and spectra. They find that the decay phase of an envelope is produced by photons delayed by the shell`s curvature. These delayed photons are produced by regions that are off-axis such that the spectra evolve according to a universal function ({proportional_to} T{sup {minus}1}) regardless of intrinsic variations in the rest frame of the shell. They compare these predictions to the overall envelope of emission of GRBs. The observed spectra evolve faster ({approximately} T{sup {minus}3}). Intrinsic variations cannot make the spectra evolve that fast, which adds strength to the shell symmetry problem: models, in particular, the external shock model, that involve relativistic shells must either confine the material to narrow pencil beams, be very inefficient, or break the local spherical symmetry so that the shell acts like a parallel slab. In the case of the internal shock models involving winds (i.e., central engines), it will probably be easier to break the local spherical symmetry, but the none must postulate nearly continuous energy generation at 10{sup 51} erg s{sup {minus}1} lasting up to hundreds of seconds at the central site.
Date: August 1, 1997
Creator: Fenimore, E.E. & Summer, M.C.
Partner: UNT Libraries Government Documents Department

X-ray spectral properties of {gamma}-ray bursts

Description: The authors summarize the spectral characteristics of a sample of 22 bright gamma-ray bursts detected with the gamma-ray burst sensors aboard the satellite Ginga. This instrument employed a proportional and scintillation counter to provide sensitivity to photons in the 2--400 keV range, providing a unique opportunity to characterize the largely unexplored X-ray properties of gamma-ray bursts. The photon spectra of the Ginga bursts are well described by a low energy slope, a bend energy, and a high energy slope. In the energy range where they can be compared, this result is consistent with burst spectral analyses obtained from the BATSE experiment aboard the Compton Observatory. However, below 20 keV they find evidence for a positive spectral number index in approximately 40% of their burst sample, with some evidence for a strong rolloff at lower energies in a few events. They find that the distribution of spectral bend energies extends below 10 keV. The observed ratio of energy emitted in the X-rays relative to the gamma-rays can be much larger than a few percent and, in fact, is sometimes larger than unity. The average for their sample is 24%.
Date: September 1, 1997
Creator: Strohmayer, T.E.; Fenimore, E.E.; Murakami, Toshio & Yoshida, Atsumasa
Partner: UNT Libraries Government Documents Department


Description: The High Energy Transient Explorer uses a triggering algorithm for gamma-ray bursts that can achieve near the statistical limit by fitting to several background regions to remove trends. Dozens of trigger criteria run simultaneously covering time scales from 80 msec to 10.5 sec or longer. Each criteria is controlled by about 25 constants which gives the flexibility to search wide parameter spaces. On orbit, we have been able to operate at 6{sigma}, a factor of two more sensitive than previous experiments.
Date: May 1, 2001
Partner: UNT Libraries Government Documents Department

A continuous watch of the northern sky above 40 TeV with the CYGNUS array

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The aim of the project has been to continuously monitor the northern sky for transient high-energy gamma-ray emission from astrophysical sources. Potential objects of such emission include gamma-ray bursts and flares from active galaxies. At the start of this project, the CYGNUS extensive air shower array was used for the monitoring; CYGNUS has an energy threshold of {approximately}40 TeV. In August, 1996, the CYGNUS data-acquisition computer suffered a fatal hardware problem so data-taking with the array ended. The Milagrito detector, which is much more sensitive than CYGNUS, started taking data in February 1997 and has continued the sky monitoring. The authors are presently honing reconstruction algorithms for Milagrito. When this is complete, the data taken since February will be analyzed for transient emission.
Date: November 1, 1998
Creator: Haines, T.J.; Miller, R. & Sinnis, C.
Partner: UNT Libraries Government Documents Department

The galactic model of GRBs

Description: The galactic model of gamma ray bursts (GRBs) is based upon the observed production of soft gamma ray repeaters (SGRs) in the galaxy and the consequences of a reasonable model to explain them. In this view GRBs are the long term result of the burn-out conditions of the SGRs in this and in other galaxies. A delay of {approximately} 30 million years before GRBs are being actively produced can be understood as the time required for the ejected matter during the SGR phase to cool, condense, and form planetesimals that are eventually captured by the central neutron star. The amount of disk matter and the interaction between each GRB and the disk determine the rate of burst production and turn-off time of GRBs. The x-ray afterglow as well as optical emission is derived from x-ray fluorescence and ionization of previously ablated matter.
Date: September 1, 1997
Creator: Colgate, S.A. & Li, H.
Partner: UNT Libraries Government Documents Department

Laboratory laser acceleration and high energy astrophysics: {gamma}-ray bursts and cosmic rays

Description: Recent experimental progress in laser acceleration of charged particles (electrons) and its associated processes has shown that intense electromagnetic pulses can promptly accelerate charged particles to high energies and that their energy spectrum is quite hard. On the other hand some of the high energy astrophysical phenomena such as extremely high energy cosmic rays and energetic components of {gamma}-ray bursts cry for new physical mechanisms for promptly accelerating particles to high energies. The authors suggest that the basic physics involved in laser acceleration experiments sheds light on some of the underlying mechanisms and their energy spectral characteristics of the promptly accelerated particles in these high energy astrophysical phenomena.
Date: August 20, 1998
Creator: Tajima, T. & Takahashi, Y.
Partner: UNT Libraries Government Documents Department

Gamma-ray burst spectra and time histories from 2 to 400 keV

Description: The Gamma-Ray burst detector on Ginga consisted of a proportional counter to observe the x-rays and a scintillation counter to observe the gamma-rays. Both instrument recorded the time histories in phase with each other and with 0.03125 s temporal resolution. The author compares the average of 21 gamma-ray bursts to determine the delay, in any, between the peak of the x-rays and the peak of the gamma-rays. The delay is less than or about equal to 30 msec. Thus, models must content with two average features of the temporal behavior. First, as a function of energy, the time structure scales as {approximately}E{sup {minus}0.45}. Second, the x-rays are not appreciable delayed relative to the gamma-rays. Some cooling models might have difficulties explaining these features.
Date: July 1, 1998
Creator: Fenimore, E.E.
Partner: UNT Libraries Government Documents Department