# Etude de l'émission prompte des sursauts gamma: Expérience HETE-2

Abstract : Gamma ray bursts are very short cosmological phenomena (lasting a few milliseconds to several hundred seconds), extremely intense and powerful: the emitted energy is received essentially as photons of a few hundreds of keV. Since their discovery, at the end of the sixties by American satellites of the VELA series, the emission mechanism of gamma ray bursts remains poorly understood by astrophysicists. For 30 years, these phenomena have been sources of surprises and interrogations but at the same time of technological, physical, and theoretical advances. The BATSE experiment has detected during nine years (1991--2000) an average of one burst per day and collected successfully many data on gamma ray bursts. It has showed in particular that gamma ray bursts were distributed isotropically but nonhomogeneously on the sky, which strongly favoured a cosmological origin of gamma ray bursts. The later dectection, thanks to the BeppoSAX satellite (1996--2002), of optical, X and radio counterparts, called afterglow'' whose redshifts lie between z=0.1 and z=4, definitely confirmed the cosmological origin of gamma ray bursts. With almost 3000 bursts, BATSE allowed to determine temporal and spectral characteristics of the gamma emission of gamma ray bursts, called the prompt emission. Concerning light curves, they are diverses and each burst is unique. Spectra, on the other hand, are non thermal and can be fit by an empirical model, called the Band model, which is defined by two power laws smoothly connected. The HETE-2 satellite, launched in October 2000, is dedicated to the observation of gamma ray bursts in a broad energy range from 2 keV to 400 keV. The main instrument, the gamma detector FREGATE (6 -- 400 keV), was built by the CESR in Toulouse. I present in this thesis the spectral study of 64 gamma ray bursts detected by HETE-2 between October 2000 and April 2004. In particular, HETE-2 has confirmed the existence of soft gamma ray bursts, previously discovered by BeppoSAX. Two classes of bursts were defined: X-Ray Flashes and X-Ray Rich bursts which form with  classical'' gamma ray bursts a single phenomenon whose typical energy range extend from few keV to few MeV (including hard gamma ray bursts detected by BATSE). Analysing jointly data of both FREGATE and the WXM (X-ray instrument of HETE-2 which energy range is 2--25 keV) allowed me to study spectra in a particularly broad energy range, covering the low energy portion of the spectrum (2 -- 400 keV). Such an energy range allowed me to study in details spectra of soft gamma ray burst, which represent almost three quarters of the total sample of bursts detected by HETE-2. I have shown that these kind of bursts have the same temporal and spectral characteristics as BASTE bursts. Then I have studied a simplified theoretical model of the internal shocks, which explains the prompt emission of gamma ray bursts. This model, nowadays accepted by the majority of the community, involves a relativistic wind into which shocks are formed, the internal shocks which emit high energy photons, by emission synchotron mainly, forming the prompt emission. This study, using numerical simulations, allowed me to gerenate a significant number of gamma ray bursts. Using the constrains derived from the spectral characteristics of X-Ray-Flashes and X-Ray-Rich bursts detected by HETE-2, I determined the main intrinsic characteristics of the model which allow the production of X ray flashes.
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https://tel.archives-ouvertes.fr/tel-00006477
Contributor : Celine Barraud <>
Submitted on : Friday, July 16, 2004 - 12:09:21 PM
Last modification on : Thursday, October 15, 2020 - 4:07:17 AM
Long-term archiving on: : Friday, April 2, 2010 - 8:18:17 PM

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• HAL Id : tel-00006477, version 1

### Citation

Céline Barraud. Etude de l'émission prompte des sursauts gamma: Expérience HETE-2. Astrophysique [astro-ph]. Université Paris-Diderot - Paris VII, 2004. Français. ⟨tel-00006477⟩

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