C. Barat

Constraints on neutron star models of gamma-burst sources from the Einstein observatory

Six Einstein observations of five gamma-ray burst sources are presented and discussed. With one possible exception, no point source was detected in any of the observations. The data are interpreted in the framework of neutron star models for gamma bursters. Upper limits are derived for the surface temperatures of the neutron stars assumed to be responsible for the bursts. It is shown that the lack of soft X-ray emission may impose stringent constraints on accretion rates onto neutron stars.

Time history, energy spectrum, and localization of an unusual gamma-ray burst

A ..gamma..-ray burst lasting approx.48 ms was observed on 1979 June 13 by four instruments in the interplanetary network. The event is unusual not only by virtue of its extremely short duration, but also by the presence of rise and fall times at the 2 ms level in the time history, and because the energy spectrum is hard, extending to 2 MeV. The 0.7 arcmin/sup 2/ error box contains no optical counterpart on the POSS plate (limiting magnitude approx.21). The spectral characteristics measured by experiments in the interplanetary network are substantially different from those previously reported for this burst.

Locations and time histories of five 1979 gamma-ray bursts

We have studied the locations and time histories of five ..gamma..-ray bursts that occurred between 1979 March 7 and March 31. The error box for GB 790325 has a typical dimension of approx.15. The other localizations, while not precise enough for thorough optical examination, contribute to distribution studies and allow radio and X-ray observations, catalog searches, and other archival work. A search through selected catalogs did reveal one object, the star FY Aql (cataloged as a Mira-type variable but probably a dwarf nova) inside one of the ..gamma..-ray burst boxes. Given the parameters of this particular search, the probability of at least one chance association is 0.03. Both recent and archival optical examinations of some of the error boxes were carried out, and no indications of any actual physical associations were seen. One event, GB 790331, had an interesting spectral behavior, in that the leading edges of the two main peaks within the burst had harder spectra than the remainder of the event.

Optical search for gamma-ray bursts

Abstract The preliminary results from optical search for light pulses associated with gamma ray bursts by means of the Czechoslovak Fireball Network plate collection at the Ondřejov observatory are given. Optical monitoring represents more than 7700 hours, but no real optical counterpart was found. Problems associated with the optical search for gamma ray bursts are discussed.

Spectral variations in gamma‐ray bursts

We report the initial results of an analysis of 19 gamma‐ray burst energy spectra recorded by the Venera‐13 and 14 SIGNE experiments. The complete data‐base includes some 150 events, with 5 channel energy spectra taken in the 50–700 keV range with 0.5 s time resolution, for 64 s. The observed spectra of each event analyzed were fitted with a power law, bremsstrahlung, and thermal synchrotron model, for each time interval containing a statistically significant number of counts. Although specific examples may be found which exhibit hard‐to‐soft evolution, or a correlation between the effective temperature kT of a fit and the time history, cases of ‘‘random’’ spectral evolution are also present, and there appears to be no single way to characterize spectral evolution. There is also no clear preference for a particular model fit in the data analyzed so far.

Possible short annihilation flashes in the 1978 November 4 gamma-ray burst

Les experiences Franco-Sovietiques Signe a bord de Venera 11 et Venera 12 ont rendu possible lanalyse spectrale des sursauts Rγ avec une resolution temporelle de 250 ms. Plusieurs caracteristiques spectrales courtes (<250 ms) et intenses (jusqua 28% de lenergie du continuum) apparaissent dans le spectre de levenement du 4 novembre 1978 autour de 400 keV. On les attribue a lannihilation des paires e − /e +

Techniques for fine gamma-ray burst spectroscopy

Abstract A better understanding of the origin of gamma-ray bursts requires a significant improvement in present detector sensitivity, particularly for fine line spectroscopy in the 5–200 keV energy range. This paper presents a critical analysis of some detectors which may be used to obtain high energy resolution measurements of photon spectra from cosmic gamma-ray burst sources.

Evidence for e−/e+ annihilation mechanism as an initial radiation process in gamma-ray bursts

Abstract An unusual spectrum has been obtained over 126 ms at the onset of the intense 1978 November 19 gamma-ray burst recorded by the Franco-Soviet Signe experiments. Evidence is presented for two distinct components above and below 200 keV : a soft emission with a possible low-energy cutoff and a peak around 400 keV with an accompanying high-energy tail. As this peak contains > 95 percent of the total fluence at this time, we suggest the role of the e − /e + annihilation as an initial radiation process in gamma-ray bursts and we propose a possible interpretation of the high-energy tail.

New results on the time histories of gamma-ray bursts

Abstract A study of the morphology of 14 short (⪝ 1 s) gamma ray bursts observed by the Franco Soviet SIGNE detectors onboard the VENERA spacecraft between 1978 and 1982 is presented. We find two major groups of short bursts characterised not only by their different durations (∼ 1 s and ∼ 100 ms respectively), but also by different e-folding rise and decay times. A study of the time history of the impulsive portion of the 1979 March 5 event at 2 ms resolution shows evidence for a previously undiscovered ⋍ 23ms quasi periodicity. These results are discussed in the context of neutron star models for gamma ray bursters.

EXOSAT observations of two gamma-ray burst sources

Abstract EXOSAT observations of the 19 Nov 1978 and 25 Mar 1979b gamma bursters are presented, for the 0.02–2.5 keV energy range. No source was detected in either case, leading to temperature upper limits for the neutron stars assumed to be present of 10 5 –10 6 °K. Polar cap accretion rates are constrained to values of 10 −12 –10 −18 M ⊙ /y km 2 . The EXOSAT non-detection of the source discovered by the Einstein satellite in the 19 Nov 1978 error box may be explained if the burster is more distant than about 1.5 kpc.