Showing papers by "E. E. Fenimore published in 1996"

Expanding Relativistic Shells and Gamma-Ray Burst Temporal Structure

Abstract: Many models of gamma-ray bursts (GRBs) involve a shell expanding at extreme relativistic speeds. The shell of material expands in a photon-quiet phase for a period t0 and then becomes gamma-ray active, perhaps due to inhomogeneities in the interstellar medium or the generation of shocks. Based on kinematics, we relate the envelope of the emission of the event to the characteristics of the photon-quiet and photon-active phases. We initially assume local spherical symmetry wherein, on average, the same conditions prevail over the shell's surface within angles the order of Γ–1, where Γ is the Lorentz factor for the bulk motion. The contribution of the curvature to the temporal structure is comparable to the contribution from the overall expansion. As a result, GRB time histories from a shell should have an envelope similar to "FRED" (fast rise, exponential decay) events in which the rise time is related to the duration of the photon-active phase and the fall time is related to the duration of the photon-quiet phase. This result depends only on local spherical symmetry and, since most GRBs do not have such envelopes, we introduce the "shell symmetry" problem: the observed time history envelopes of most GRBs do not agree with that expected for a relativistic expanding shell.Although FREDs have the signature of a relativistic shell, they may not be due to a single shell, as required by some cosmological models. Some FREDs have precursors in which the peaks are separated by more than the expansion time required to explain FRED shape. Such a burst is most likely explained by a central engine; that is, the separation of the multiple peaks occurs because the central site produced multiple releases of energy on timescales comparable to the duration of the event. Alternatively, there still could be local spherical symmetry of the bulk material, but with a low "filling factor"; that is, only a few percent of the viewable surface (which is already very small, 4πΓ–2) ever becomes gamma-ray active.Long complex bursts present a myriad of problems for the models. The duration of the event at the detector is ~t0/(2Γ2). The long duration cannot be due to large t0, since it requires too much energy to sweep up the interstellar medium. Nor can it be due to small Σ if the time variation is due to ambient objects, since the density of such objects is unreasonable (~1018Γ–4 pc–3 for typical parameters). Long events must explain why they almost always violate local spherical symmetry or why they have low filling factors.Both precursor and long complex events are likely to be "central engines" that produce multiple releases of energy over ~100 s. One promising alternative scenario is one in which the shell becomes thicker than the radius of the curvature within Γ–1. Then it acts as a parallel slab, eliminating the problems associated with local spherical symmetry.

Expanding relativistic shells and gamma-ray burst temporal structure

Abstract: Many models of gamma-ray bursts (GRBs) involve a shell expanding at extreme relativistic speeds The shell of material expands in a photon-quiet phase for a period t0 and then becomes gamma-ray active, perhaps due to inhomogeneities in the interstellar medium or the generation of shocks Based on kinematics, we relate the envelope of the emission of the event to the characteristics of the photon-quiet and photon-active phases We initially assume local spherical symmetry wherein, on average, the same conditions prevail over the shell's surface within angles the order of Γ–1, where Γ is the Lorentz factor for the bulk motion The contribution of the curvature to the temporal structure is comparable to the contribution from the overall expansion As a result, GRB time histories from a shell should have an envelope similar to "FRED" (fast rise, exponential decay) events in which the rise time is related to the duration of the photon-active phase and the fall time is related to the duration of the photon-quiet phase This result depends only on local spherical symmetry and, since most GRBs do not have such envelopes, we introduce the "shell symmetry" problem: the observed time history envelopes of most GRBs do not agree with that expected for a relativistic expanding shellAlthough FREDs have the signature of a relativistic shell, they may not be due to a single shell, as required by some cosmological models Some FREDs have precursors in which the peaks are separated by more than the expansion time required to explain FRED shape Such a burst is most likely explained by a central engine; that is, the separation of the multiple peaks occurs because the central site produced multiple releases of energy on timescales comparable to the duration of the event Alternatively, there still could be local spherical symmetry of the bulk material, but with a low "filling factor"; that is, only a few percent of the viewable surface (which is already very small, 4πΓ–2) ever becomes gamma-ray activeLong complex bursts present a myriad of problems for the models The duration of the event at the detector is ~t0/(2Γ2) The long duration cannot be due to large t0, since it requires too much energy to sweep up the interstellar medium Nor can it be due to small Σ if the time variation is due to ambient objects, since the density of such objects is unreasonable (~1018Γ–4 pc–3 for typical parameters) Long events must explain why they almost always violate local spherical symmetry or why they have low filling factorsBoth precursor and long complex events are likely to be "central engines" that produce multiple releases of energy over ~100 s One promising alternative scenario is one in which the shell becomes thicker than the radius of the curvature within Γ–1 Then it acts as a parallel slab, eliminating the problems associated with local spherical symmetry

The 1979 March 5 Gamma-Ray Transient: Was It a Classic Gamma-Ray Burst?

Abstract: The 1979 March 5 gamma-ray transient has long been thought to be fundamentally different from the classic gamma-ray bursts (GRBs). It had recurrences, pulsations, and a soft spectral component unlike classic GRBs. With the exception of the soft component reported from the KONUS experiment, the unusual characteristics of the March 5 transient were detectable primarily because it was extremely bright. Computer limitations, satellite transmission effects or pulse pileup, and dead-time effects have prevented, until now, the analysis of spectra from the {ital International} {ital Cometary} {ital Explorer} ({ital ICE}) and the {ital Pioneer} {ital Venus} {ital Orbiter} ({ital PVO}). The {ital ICE}-{ital PVO} spectrum of the main peak differs markedly from the published KONUS spectrum. Rather than being dominated by a soft component similar to that observed in the soft gamma repeaters (SGRs), the {ital ICE}-{ital PVO} spectrum appears to be consistent with a classic GRB spectrum, especially above 100 keV. Above 100 keV, the spectrum is consistent with thermal bremsstrahlung with a temperature of {approximately}200 keV, somewhat soft but within the range of classic GRBs. We believe that, given the {ital ICE}-{ital PVO} spectral observations, the March 5 transient would have been classified as a classic GRB when itmore » was discovered. Although a formal analysis has not been done, the pulsations and recurrences might still be unique features that distinguish the March 5 transient from GRBs. The {ital ICE} spectrum provides evidence for a low-energy cutoff at 100 keV. If high-velocity neutron stars are born as misaligned rotators with their velocities aligned with their spin axes and if their emissions are beamed, then when they are young their spatial distribution will be similar to the SGRs. If torques can align the field with the spin axis, then when they are old their spatial distribution will be isotropic like classic GRBs. (Abstract Truncated)« less

The autocorrelation of gamma-ray burst time profiles

Abstract: The autocorrelation of time profiles of emission from gamma-ray bursts has previously been proved to be a valuable diagnostic for the study of the timing behavior of these bursts. In particular, comparative studies benefited from this diagnostic. However, in these previous studies, the particular shape of the autocorrelation function has not been addressed in great detail. In the present paper we try to explain the autocorrelation shape and behavior. We propose an empirical model that aids in the evaluation of uncertainties in autocorrelation analyses of gamma-ray burst time profiles. Our most important conclusions are that analyses based on the autocorrelation function are not dominated by mathematical properties not connected with the gamma-ray burst phenomenon, and that the uncertainty in the relative time stretching between different photon energy ranges found through such an analysis previously from {ital CGRO} BATSE data is less than or equal to 10{percent}. {copyright} {ital 1996 The American Astronomical Society.}

Possible association of a quiescent x-ray source with a gamma-ray burster

Abstract: We report on the first repeatedly detected statistically significant coincidence (chance probability {approx_equal}10{sup {minus}2}{endash}10{sup {minus}3}) between an X-ray source and a gamma-ray burst error box. We present three {ital ROSAT} observations of the field of the gamma-ray burst of 1992 May 1. The first, a 2000 s target of opportunity observation, was carried out 18 days after the burst. A weak X-ray source was identified, but with too few photons to determine its spectral characteristics. The second, a 30 ks PSPC observation, resulted in the detection of 118 net photons over the 0.07{endash}2.4 keV energy range. We find that the spectrum is consistent with thermal bremsstrahlung from a 7{times}10{sup 6} K plasma with about 10{sup 22} cm{sup {minus}2} HI column density. The unabsorbed flux is {approximately}9.4{times}10{sup {minus}13} ergscm{sup {minus}2}s{sup {minus}1} (corresponding absorbed flux 4.8{times}10{sup {minus}14} ergscm{sup {minus}2}s{sup {minus}1}). Analysis of the photon arrival times indicates that the source may be variable. Using the HI column density from the spectral fit, we set a lower limit to the source distance of at least several kpc; an extragalactic source cannot be ruled out. If the gamma-ray burst is indeed related to the X-ray source, its total energy output would have been atmore » least 2{times}10{sup 37} ergs. The third observation, 6200 s with the HRI, defines a source error circle of 6{double_prime} radius. We discuss optical observations of this region, and consider various possibilities for the nature of the X-ray source. {copyright} {ital 1996 The American Astronomical Society.}« less

BASIS mission concept for gamma-ray burst imaging and spectroscopy

Abstract: We are studying a gamma-ray burst mission concept called burst arcsecond imaging and spectroscopy (BASIS) as part of NASA's new mission concepts for astrophysics program. The scientific objectives are to accurately locate bursts, determine their distance scale, and measure the physical characteristics of the emission region. Arcsecond burst positions (angular resolution approximately 30 arcsec, source positions approximately 3 arcsec) will be obtained for approximately 100 bursts per year using the 10 - 100 keV emission. This will allow the first deep, unconfused counterpart searches at other wavelengths. The key technological breakthrough that makes such measurements possible is the development of CdZnTe room-temperature semiconductor detectors with fine (approximately 100 micron) spatial resolution. Fine spectroscopy will be obtained between 0.2 and 150 keV. The 0.2 keV threshold will allow the first measurements of absorption in our galaxy and possible host galaxies, constraining the distance scale and host environment.

Expanding Relativistic Shells and Gamma-Ray Burst Temporal Structure

Abstract: Many models of cosmological gamma-ray bursts involve the sudden release of $\sim 10^{51}$ erg which produce shells which expand at relativistic speeds (Lorentz $\Gamma$ factors of $10^{2-3}$). We investigate the kinematic limits on the source size due to the observed time structure in three types of bursts: short spikes, FREDs (Fast Rise, Exponentail decay), and long complex bursts. The emitting shell keeps up with the photons it produces reducing apparent durations by $\Gamma^2$ so that source sizes can be very large ($2c\Gamma^2 T_{dur}). However, the thickness of the emitting region is not effected by the shell motion so it must always be small, $c\Delta T$ where $\Delta T$ is the subpeak time scale. We argue that one can only view the bulk motion head-on so it is inappropriate to treat GRBs as viewing the sides of a jet. Although photons come from a region within an angle $\Gamma^{-1}$, we show that the curvature of the shell within that angle creates delays comparable to those associated with the duration of the event. As a result, most bursts should be like FREDs with sharp rises related to how long the shell emits and power law decays related to how long the shell expanded before becoming gamma-ray active. Few bursts have the long decay phases required for large shells resulting in unacceptable high densities for ISM objects to cause the observed subpeaks. To be consistent with the observations, perhaps very thick shells (which act as parallel slabs) are required to avoid the effects of the curvature, or the duration is dictated by a central engine.

Detectability of Gamma-Ray Burst Excess toward M31

Abstract: We calculate the expected excess of gamma-ray bursts (GRBs) from the direction of M31 assuming that GRBs come from an extended halo around both M31 and our own Galaxy. Specifically, we consider the delayed turn-on and halo-beaming models for GRBs. We express the ratio of GRB rate from M31 to our Galaxy as a function of instrument sensitivities, the observing solid angles toward M31, and the burst luminosity functions. This ratio tends to be much larger than 1 for a small observing angle (<10°) when the instrument can sample close to the distance of M31. Assuming that a hypothetical gamma-ray instrument is 10 times more sensitive than BATSE, we find in a 1 yr observation, for both imaging or nonimaging instruments, that the delayed turn-on model might be tested but no statistically significant signal is expected for the halo-beaming model. Thus, it is extremely difficult to rule out the halo-beaming model by observing M31.

Germanium gamma-ray spectrometer PGS for the MARS-96 mission

Abstract: The Precision Gamma-ray Spectrometer (PGS) on the Russian MARS-96 spacecraft is designed to measure 0.1--8 MeV gamma rays in order to determine the elemental composition of the Martian surface, to study solar flares, and to determine energy spectra and times of arrival of gamma-ray bursts. The PGS instrument contains two high-purity, n-type germanium crystals, each similar to the one used on the Mars Observer mission. Each crystal is contained in a titanium can with Helicoflex cryogenic metal seals. An annealing capability allows repair of radiation damage. The detectors are cooled via nitrogen heat pipes attached to a passive radiator mounted on the back side of a solar panel. The radiators are designed to keep the Ge detectors below 100 K during the interplanetary flight. The electronics include first-stage electronics mounted on each crystal can and 4096-channel pulse height analyzers. Two parallel channels of electronics are provided and can be cross-switched by telecommands. In November 1995 integration of the flight detectors with flight electronics and testing of the complete system cooled by the passive radiator were successfully completed. The energy resolution degrades to about 3 keV in the flight configuration. Warming the radiators indicated that for the worst case when the radiator views Mars at the equator the maximum temperature of the detectors will be limited by the diode action of the heat pipes to 118 K. Extensive calibrations with radioactive sources are in progress. The authors conclude that they have an improved design for planetary and gamma-ray burst studies and the PGS instrument is ready for launch in November 1996.

Burst locations with an arc second telescope (BLAST)

Abstract: Burst locations with an arc second telescope (BLAST) is a new mission concept being studied for NASA's medium explorer (MIDEX) mission opportunities. The principal scientific objectives of the BLAST mission are (1) to localize gamma- ray burst (GRB) positions to arcsec accuracy; (2) to search for enhancements in the rate of GRBs toward M31; and (3) to conduct the most sensitive sky survey to date of x-ray sources in the 7 - 200 keV regime. These objectives are achieved using a large array of position-sensitive scintillation detectors with a total area of 17,000 cm2. This array is combined with a large field of view telescope (greater than 1 steradian) comprising two separate imaging systems. A coded aperture telescope provides arcminute source localization. For low energy x-rays (less than 50 keV), the aperture is also defined by phase modulation grids with provide complementary arcsecond information. The grid system consists of two aperture planes with 'checker board' patterns of slightly different pitch. The beating between the two grid pitches casts a broad interference pattern on the detector plane. Determining the phase of this interference pattern in both coordinates gives the location of a point source source in the sky, with aliased positions at approximately 1 arcmin spacing. The arcmin ambiguity is resolved by the coded aperture image. BLAST has a sensitivity to bursts of 0.03 photons cm-2 s-1, almost ten times more sensitive than BATSE. We expect to position 20 bursts per year to better than 2 arcsec accuracy and 35 bursts per year to better than 5 arcsec. BLAST will provide an all sky survey in hard x-rays with a sensitivity of 0.2 milliCrab at low energies.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Performance of Wide-field X-ray Monitor on board HETE (High-Energy Transient Experiment)

Abstract: The wide-field x-ray monitor (WXM) is one of the three scientific instruents onboard high energy transient experiment (HETE) satellite, which was launched in 1996. The primary objective of HETE is to carry out the first multi- wavelength study of gamma-ray bursts with UV, x-ray, and gamma-ray instruments mounted on a single, compact spacecraft. WXM has been designed to undertake comprehensive x-ray spectra observations and quickly determine small error boxes of GRB locations within a large field of view of about 1.5 steradian. It is based on the principle of coded aperture imaging. It has four identical one-dimensional position sensitive proportional counters (PSPCs), one pair in each of two orthogonal directions. Each PSPC is filled with 1.4 atm Xe (97%) and CO2 (3%), equipped with three resistive carbon anodes of 10 micrometer diameter, and sensitive to x-rays between 2 and 25 keV. It provides position resolution of about 1.0 mm (FWHM), and energy resolution of about 17% (FWHM) at 8 keV.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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

Abstract: 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.