Showing papers by "E. E. Fenimore published in 1982"

γ-ray sources as comptonized X-ray sources

Abstract: γ-ray burst spectra have often been fit by optically thin thermal bremsstrahlung. However, at the high temperatures implied by such fits (kT∼300 keV), the free–free cross-section is so much smaller than the Compton cross-section that Compton scattering might dominate the spectral formation processes. We have investigated the possibility that emission mechanisms based on Compton scattering can also fit the data. In particular, Monte Carlo calculations have been used to compare the γ-ray burst spectral data with black-body spectra which have undergone inverse comptonization by a much hotter, overlying plasma. The best-fit parameters suggest that the underlying black body is an X-ray source (kTBB∼2.4 keV) while the overlying, comptonizing plasma has kTe∼150 keV and a column density of 4 × 1024 electrons cm−2. Such a model would also naturally explain some of the unusual γ-ray burst spectra. In particular, a low energy (∼400 keV) cutoff in the spectrum of GB781119 can be explained as a Wien peak produced by a higher column density in the comptonizing region, and two-component spectra (for example, GB790329) can be explained by a lower column density.

Observation of two gamma-ray bursts by Vela X-ray detectors

Abstract: Bursts of X-rays coincident in time with two gamma-ray burst events were observed by the 3-12 keV collimated X-ray detectors on the Vela spacecraft. Both of these observations show recurrence on a time scale of hundreds of seconds. For one of these events (GB 720514) the X-ray detection gives an improved position as well as information on the spectrum late in the outburst. The other event (GB 740723) is of special interest because the source, not previously located, is consistent in direction with the binary pulsar SMC X-1 in the Small Magellanic Cloud; this is the first moderately small error box for a gamma-ray burst to contain a strong X-ray source.

Problems in the Analysis of Gamma-Ray Burst Spectra

Abstract: Spectral measurements by the Solar Maximum Mission have been used to confirm the cyclotron lines in gamma-ray bursts reported from the Konus experiment. We present ISEE-3 data for the same burst (GB800419) during the same period of time which show no line. We discuss various problems in the analysis of scintillator spectra and point out that unfolded spectra are not necessarily unique and that the position of a data point in a deconvolved spectrum may vary depending on the assumed overall shape of the spectrum. As a result, if the analysis assumes a soft spectrum (such as optically thin thermal bremsstrahlung) an absorption feature might appear, whereas a harder spectrum (such as a Comptonized blackbody) would not require the feature. Since the continuum shape probably changes during the duration of a typical burst, the nonuniqueness of the spectral unfolding, combined with the assumption that the continuum is optically thin thermal bremsstrahlung, could give rise to spurious “absorption” features which vary on a time scale of seconds. Despite these problems, there is still some evidence for narrow spectral lines in the range 45 to 65 keV but not for the broad lines reported from the Konus experiment. Unfortunately, the range 45 to 65 keV is the most difficult spectral region to unfold.

Comments on the gamma-ray burst catalog of Mazetset al. (1981a)

Abstract: We have discovered a correlation between the Venera spacecraft locations and the gamma-ray burst positions reported in the KONUS catalog (Mazetset al., 1981a). The reason for the correlation is not clear, but it could be due to spatial selection effects and/or large localization errors for weak of soft bursts. Whatever the cause, it seems likely that this systematic bias might significantly affect catalog results pertaining to gamma burst locations, intensity distributions, and spectra. For example, it can explain why the KONUS galactic latitude distribution is peaked significantly south of the galactic equator. Apparent discrepancies between the KONUS and Los Alamos gamma burst data bases are noted.

Gamma-ray sources as Comptonized X-ray sources

Abstract: The possible contribution of Compton scattering to the formation of gamma-ray burst spectra is analyzed. Monte Carlo calculations are used to study the spectral distribution emerging from a plasma at temperatures typical of gamma-ray burst spectra (above 100 keV) with an underlying source of photons reprocessed by inverse Compton scattering surrounding a point source of blackbody radiation. Comparison of the computations with the observed spectrum of the burst GB781104 results in a best fit with a blackbody X-ray source at 2.4 keV, plasma temperature of about 150 eV and plasma column density of 4 x 10 to the 24th electrons/sq cm. The Comptonization model is also shown to be capable of accounting for the two-component spectra observed for bursts GB790329 and GB790524, and the moving low-energy spectral cut-off in the burst of November 18, 1978.