Showing papers by "E. E. Fenimore published in 1994"

The x-ray spectrum of the soft gamma repeater 1806-20

Abstract: Soft Gamma Repeaters (SGRs) are a class of rare, high-energy galactic transients that have episodes of short (~0.1 sec), soft (~30 keV), intense (~100 Crab), gamma-ray bursts. We report an analysis of the x-ray emission from 95 SGR1806-20 events observed by the International Cometary Explorer. The spectral shape remains remarkably constant for bursts that differ in intensity by a range of 50. Below 15 keV the number spectrum falls off rapidly such that we can estimate the total intensity of the events. Assuming that SGR1806-20 is associated with the supernova remnant G10.0-0.3 (Kulkarni and Frail, Murakami \\etal), the brightest events had a total luminosity of ~1.8 x 10^42 erg sec^-1, a factor of 2 x 10^4 above the Eddington limit. A third of the emission was above 30 keV. There are at least three processes that are consistent with the spectral rollover below 15 keV. (1)The rollover is consistent with some forms of self absorption. Typical thermal temperatures are ~20 keV and require an emitting surface with a radius between 10 and 50 km. The lack of spectral variability implies that only the size of the emitting surface varies between events. If the process is thermal synchrotron the required magnetic field might be too small to confine the plasma against the super Eddington flux. (2)The low energy rollover could be due to photoelectric absorption by ~10^24 Hydrogen atoms cm^-2 of neutral material with a cosmic abundance assuming a continuum similar to TB with T= ~22 keV. (3) Emission in the two lowest harmonics from a 1.3 x 10^12 Gauss field would appear as Doppler broadened lines and fall off rapidly below 15 keV.

The X-ray Spectrum of Soft Gamma Repeater 1806-20

Abstract: Soft Gamma Repeaters (SGRs) are a class of rare, high-energy galactic transients that have episodes of short (~0.1 sec), soft (~30 keV), intense (~100 Crab), gamma-ray bursts. We report an analysis of the x-ray emission from 95 SGR1806-20 events observed by the International Cometary Explorer. The spectral shape remains remarkably constant for bursts that differ in intensity by a range of 50. Below 15 keV the number spectrum falls off rapidly such that we can estimate the total intensity of the events. Assuming that SGR1806-20 is associated with the supernova remnant G10.0-0.3 (Kulkarni and Frail, Murakami \etal), the brightest events had a total luminosity of ~1.8 x 10^42 erg sec^-1, a factor of 2 x 10^4 above the Eddington limit. A third of the emission was above 30 keV. There are at least three processes that are consistent with the spectral rollover below 15 keV. (1)The rollover is consistent with some forms of self absorption. Typical thermal temperatures are ~20 keV and require an emitting surface with a radius between 10 and 50 km. The lack of spectral variability implies that only the size of the emitting surface varies between events. If the process is thermal synchrotron the required magnetic field might be too small to confine the plasma against the super Eddington flux. (2)The low energy rollover could be due to photoelectric absorption by ~10^24 Hydrogen atoms cm^-2 of neutral material with a cosmic abundance assuming a continuum similar to TB with T= ~22 keV. (3) Emission in the two lowest harmonics from a 1.3 x 10^12 Gauss field would appear as Doppler broadened lines and fall off rapidly below 15 keV.

Constraints on the Gamma-ray Burst Luminosity Function from PVO and BATSE

Abstract: We examine the width of the gamma-ray burst luminosity function through the distribution of GRB peak fluxes as detected by the Pioneer Venus Orbiter (PVO) and the Burst and Transient Source Experiment (BATSE). The strength of the analysis is greatly enhanced by using a merged catalog of peak fluxes from both instruments with good cross-calibration of their sensitivities. The range of peak fluxes is increased by approximately a factor of 20 relative to the BATSE catalog. Thus, more sensitive investigations of the $\log N-\log P$ distribution are possible. We place constraints on the width of the luminosity function of gamma-ray bursts brighter than the BATSE completeness limit by comparing the intensity distribution in the merged catalog with those produced by a variety of spatial density and luminosity functions. For the models examined, $90\%$ of the {\em detectable\/} bursts have peak luminosities within a range of 10, indicating that the peak luminosities of gamma-ray bursts span a markedly less wide range of values than many other of their measurable properties. We also discuss for which slopes of a power-law luminosity function the observed width is at the upper end of the constrained range. This is important in determining the power-law slopes for which luminosity-duration correlations could be important.

MOXE x-ray all-sky monitor for the Spectrum X-Gamma

Abstract: The MOnitoring X-ray Experiment (MOXE) is an X-ray all-sky monitor to be launched on the Russian Spectrum-X-Gamma satellite. It will monitor several hundred X-ray sources on a daily basis, and will be the first instrument to monitor most of the X-ray sky most of the time. MOXE will alert users of more sensitive instruments on Russia's giant high energy astrophysics observatory and of other instruments to transient activity. MOXE consists of an array of 6 X-ray pinhole cameras, sensitive from 2 to 25 keV, which views 4(pi) steradians (except for a 20 degree(s) X 80 degree(s) patch which includes the Sun). The pinhole apertures of 0.625 X 2.556 cm2 imply an angular resolution of 2 degree(s).4 X 9 degree(s).7 (FWHM on-axis). The flight instrument will mass approximately 118 kg and draw 38 Watts. For a non-focussing all-sky instrument that is limited by sky background, the limiting sensitivity is a function only of detector area. MOXE will, for a 24 hrs exposure, have a sensitivity of approximately 2 mCrab. MOXE distinguishes itself with respect to other all-sky monitors in its high duty cycle, thus having unprecedented sensitivity to transient phenomena with time scales between minutes and hours.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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

Abstract: 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.