Engineering topics

Fishman, G.

Publications and source records attributed to Fishman, G..

15 records

NASA NTRS2012

GRB110721A: An Extreme Peak Energy and Signatures of the Photosphere

GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its two instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The burst consisted of one major emission episode which lasted for approximately 24.5 s (in the GBM) and had a peak flux of (5.7 +/- 0.2) 10(exp -5) erg s(exp -1) cm(exp -2). The time-resolved emission spectrum is best modeled with a combination of a Band function and a blackbody spectrum. The peak energy of the Band component was initially 15 +/- 2 MeV, which is the highest value ever detected in a GRB. This measurement was made possible by combining GBM/BGO data with LAT Low Energy events to achieve continuous 10-100 MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89 +/- 0.10. The temperature of the blackbody component also decreased, starting from approximately 80 keV, and the decay showed a significant break after approximately 2s. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.

Axelsson, M.↗

NASA NTRS2004

Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Shocks

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (m) code, we have investigated particle acceleration associated with a relativistic jet front propagating through an ambient plasma with and without initial magnetic fields. We find only small differences in the results between no ambient and weak ambient parallel magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates particles perpendicular and parallel to the jet propagation direction. New simulations with an ambient perpendicular magnetic field show the strong interaction between the relativistic jet and the magnetic fields. The magnetic fields are piled up by the jet and the jet electrons are bent, which creates currents and displacement currents. At the nonlinear stage, the magnetic fields are reversed by the current and the reconnection may take place. Due to these dynamics the jet and ambient electron are strongly accelerated in both parallel and perpendicular directions.

Nishikawa, Ken-IchiI.↗

NASA NTRS1997

Are Abell Clusters Correlated with Gamma-Ray Bursts?

A recent study has presented marginal statistical evidence that gamma-ray burst (GRB) sources are correlated with Abell clusters, based on analyses of bursts in the BATSE 3B catalog. Using precise localization information from the Third Interplanetary Network, we have reanalyzed this possible correlation. We find that most of the Abell clusters that are in the relatively large 3B error circles are not in the much smaller IPN/BATSE error regions. We believe that this argues strongly against an Abell cluster-GRB correlation.

Hurley, K.↗

NASA NTRS1997

BASIS: A new GRB mission concept

The burst acrsecond imaging and spectroscopy (BASIS) mission concept is reviewed. The scientific objectives of the mission are to accurately locate bursts, determine their distance scale and measure the physical characteristics of the emission region. Arcsecond burst positions would be obtained for approximately 100 bursts/yr in the 10 to 200 keV emission region, allowing deep, unconfused counterpart surveys at other wavelengths. The key enabling technology development is the production of CdZnTe room temperature semiconductor detectors with a spatial resolution of the order of 100 microns. Fine spectroscopy would be obtained between 0.2 and 200 keV. The 0.2 keV threshold would allow measurements of the absorption in our Galaxy and possible host galaxies.

Teegarden, B. J.↗

NASA NTRS1997

GRB 970228

We made radio observations (5 GHz, 3".8 x 17" beamwidth) with the Westerbork array of the SAX/WFC error box of GRB 970228 (IAUC 6572) on Feb. 28 (19.9 hr after the onset of GRB 970228) for 1.2 hr, and on Mar. 1 and 2 for 12 hr each. The error box contains no radio sources above 0.7 mJy (4 sigma).

Galama, T. J.↗

NASA NTRS1997

Evidence for Highly Inhomogeneous mm-Wave Sources During the Impulsive Flare of May 9, 1991

In this paper multiwavelength observations of an impulsive flare of May 9, 1991 are presented. This event was observed with the 48 GHz multibeam focal array used at the Itapetinga radio telescope, the microwave patrol telescopes at Bem and the BATSE high time resolution hard X-ray spectrometer on board CGRO. While spatially unresolved low sensitivity observations show two major impulsive peaks, the mm-wave observations with the ability of spatially high resolved tracking of the emission centroids suggest a primarily bipolar source configuration. For the first time two mm-wave sources with a spacing below the HPBW could be separated with the multibeam technique. The general features of the observations are explained as emission of partially trapped electrons. Furthermore we present evidence for highly inhomogeneous substructures within one of the two mm-wave sources for which the positional scatter of the emission center, within 2s, is less than 2".

Hermann, R.↗

NASA NTRS1996

All-Sky Search for Transient Sources near 0.5 MeV with the Burst and Transient Source Experiment (BATSE)

We present a search for bright, transient emission lines near 0.5 MeV in nearly two years of data from the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory, Such features have been reported from black hole candidates Nova Muscae and 1E 1740.7-2942 and from the Crab, lasting for about 1 day. Our survey covers the whole sky and is sensitive to events with durations from 0.5-3.0 day. No transients are observed, and the systematic errors are low enough that the upper limits are significantly below the fluxes of the two most significant events previously reported.

Smith, D. M.↗

NASA NTRS1995

The first search for a gamma-ray burst quiescent counterpart in the extreme ultraviolet with EUVE

The opening of the extreme ultraviolet window by the Extreme Ultraviolet Explorer (EUVE) satellite has provided the unique opportunity to perform the first search for a quiescent gamma-ray burst counterpart at these wavelengths. Such emission might be expected if some bursts are related to nearby hot neutron stars or neutron stars with accretion disks, among other objects. We report here on a 40 ks observation on the 1992 March 25 gamma-ray burst error box, determined by triangulation with the Third Interplanetary Network. No quiescent 40-190 A Extreme Ultraviolet (EUV) source was identified using the Deep Survey instrument, and a 3 sigma upper limit of 2.9 x 10(exp -14) erg/sq cm/s was obtained. Similarly, upper limits to the 140-380 and 280-760 A fluxes were obtained with the medium- and long-wavelength spectrometers; they are 1.1 x 10(exp -12) and 5.0 x 10(exp -13) erg/sq cm/s, respectively. We discuss the constraints which these limits impose on thermally radiating quiescent counterparts.

Hurley, K.↗

NASA NTRS1995

The LXeCAT instrument for gamma-ray astrophysics

The Liquid Xenon Coded Aperture Telescope (LXeCAT) and its capability to image astrophysical gamma-ray sources in the MeV region is described. The gamma-ray detector is a Liquid Xenon Time Projection Chamber (LXeTPC) triggered by the primary scintillation light. Effective background rejection is a direct consequence of the intrinsic three-dimensional imaging capability of the LXeTPC. Initial results with a 10 liter prototype confirm an energy resolution of 6% FWHM, a position resolution of 1 mm RMS and a light triggering efficiency higher than 90% for 1 MeV gamma-rays.

Aprile, E.↗

NASA NTRS1994

Network synthesis localization of two soft gamma repeaters

We introduce the method of 'network synthesis,' which allows the detection of very weak gamma-ray transient signals in the data of the Ulysses gamma-ray burst (GRB) experiment from repeating sources. It consists of defining a grid of alpha, delta values, and for each BATSE detection of a burst from a soft gamma repeater, predicting the arrival time of the burst at Ulysses and co-adding the Ulysses data rephased so that the burst signals are aligned in time and produce a detectable pulse. We demonstrate that this method identifies the position of the soft repeater SGR 1806-20, and apply it to the repeater B1900+14. We show that the counterpart to this burst source is probably in or in the vicinity of the Galactic supernova remnant G42.8+0.6.

Hurley, Kevin↗

NASA NTRS1994

The optical and X-ray content of the 1992 May 1 gamma-ray burst error box

A gamma-ray burst which occurred on 1992 May 1 was observed by three spacecraft in the third interplanetary network, and rapidly localized to a small error box. The coordinates were promptly circulated to a wide astronomical community, and radio, optical, and X-ray counterpart searches were carried out. A weak X-ray source was found in the error box, and two radio sources are discovered outside the error box, but in alignment with the X-ray source. The X-ray source position contains approximately 25 optical objects down to 23d magnitude. We discuss the prospects for identifying the burster counterpart.

Hurley, K.↗

NASA NTRS1994

Do gamma-ray burst sources repeat?

The demonstration of repeated gamma-ray bursts from an individual source would severely constrain burst source models. Recent reports of evidence for repetition in the first BATSE burst catalog have generated renewed interest in this issue. Here, we analyze the angular distribution of 585 bursts of the second BATSE catalog (Meegan et al. 1994). We search for evidence of burst recurrence using the nearest and farthest neighbor statistic ad the two-point angular correlation function. We find the data to be consistent with the hypothesis that burst sources do not repeat; however, a repeater fraction of up to about 20% of the bursts cannot be excluded.

Meegan, C. A.↗

NASA NTRS1992

Ulysses/BATSE observations of cosmic gamma ray bursts

The gamma ray burst detector aboard the ESA-NASA Ulysses spacecraft, in operation since Nov. 1990, has detected numerous gamma bursts in conjunction with the BATSE experiment aboard the Compton Observatory. Initial results are presented on burst locations for three events (21 April, 2 May, and 3 May, 1991) obtained by arrival time analysis, and they are compared with the BATSE locations. The arrival time analysis annuli have typical widths of 5'. The preliminary analysis indicates that both experiments are likely to have unresolved systematic errors, but that further work will improve the location accuracy substantially.

Hurley, K.↗

NASA NTRS1992

Spectral evolution of gamma-ray bursts

BATSE's Spectral Detectors provide a series of high resolution spectra over the duration of a gamma-ray burst; fits to these spectra show the evolution of the continuum as the burst progresses. The burst continuum can usually be fit by the spectral form AE sup alpha exp(-E/kT) from around 25 keV to more than 3 MeV, with varying trends in the value and evolution of the spectral parameters. As a result of limited statistics for E greater than 1 - 2 MeV in the individual spectra, a high energy power law is not required. Only long duration strong bursts can be studied by fitting a series of spectra, and therefore our conclusions concern only this class of burst. The bursts we analyzed tend to be characterized by a hard-to-soft trend both for individual intensity spikes and for the burst as a whole: the hardness leads the count rate in spectra which resolve the temporal variations, while the hardness of successive spikes decreases. We also summarize the performance of the Spectral Detectors and the development of analysis tools to date.

Band, D.↗

NASA NTRS1992

A quantitative measure of the structure of gamma-ray burst time profiles

A cursory examination of cosmic gamma-ray burst time profiles indicates an inhomogeneous distribution of structure. In the first approximation, there seem to be two types of profiles; smooth ones with little structure and highly variable ones with lots of structure. To put this observation to the test, we have examined the statistical nature of the profile derivative to choose which parameter might best be called the burst 'spikiness'. We have found that a good estimator is given by a count of the number of 'spikes' (defined by a specific numerical recipe) and not by the rms deviations from either a pre-burst background or any type of moving average background. The application of this parameter to 30 burst time histories shows it to be consistent over a wide range of profile types. The analysis also reveals a preferred average time between spikes of approximately 1.5 seconds.

Lestrade, John P.↗