Where are Swift Gamma-ray bursts beyond the 'synchrotron deathline'?

In spite of the fact that the phenomenon of the γ-ray bursts (GRB) was discovered more than 40 years ago, its origin and basic physical mechanisms of the observed 0.1-100 second long pulses of hard X-ray /soft γ-ray emission remain obscure. A significant progress in understanding of the GRB phenomenon was achieved over the last decade with the discovery of X-ray, optical and radio afterglows of long (duration > 2 s) (van Paradijs et al. 1997;Frail et al. 1997) and short (duration 2 s) GRBs (Gehrels et al. 2005; Barthelmy et al. 2005;Fox et al. 2005;Hjorth et al. 2005). Localization of long GRBs in star formation regions, where more than 90% of the supernovae occur, indicates that long GRBs may be produced by the death of massive stars in supernova explosions. This is confirmed by the direct observation of appearance of supernovae at the GRB positions (Galama et al. 1998;Stanek et al. 2003).

The mechanism of production of GRBs is constrained not only by the identification of their multi-wavelength counterparts, but also by the intrinsic properties of the γ-ray emission. Observations of fast, millisecond-scale, variability during the prompt emission phase (Bhat et al. 1992;Walker et al. 2000) point to the presence of a compact “central engine” powering the GRB, which is naturally associated with a stellar mass black hole or a neutron star formed in result of the gravitational collapse of a massive star at the on-set of a supernova explosion. γ-rays can escape from a relatively compact emission region only if the emitting medium moves with a large bulk Lorentz factor γ b > 100 (Fenimore et al. 1993;Woods & Loeb 1995).

Although it is clear that the observed emission is produced by relativistically moving plasma ejected by a newly formed black hole or neutron star, the physical mechanism of the γ-ray emission is not well constrained by the available data. One possibility is that this emission is synchrotron emission from electrons accelerated on “internal” shocks formed in collisions of plasma blobs moving with different velocities (Zhang & Mészáros 2004;Piran 2005). Alternatively, prompt γ-ray emission can be produced via inverse Compton scattering of lower energy synchrotron photons originating from the relativistic plasma itself (the so-called synchrotron-self-compton [SSC] model) (Panaitescu & Mészáros 2000;Kumar & McMahon 2008;Piran et al. 2008) or of the photons from external ambient radiation fields (external Compton [EC] model) (Shemi 1994;Shaviv & Dar 1995;Lazatti et al. 2000;Dar & De Rújula 2004;Piran et al. 2008).

Recent detections of strong prompt optical emission from several GRBs seem to indicate the presence of a separate lower energy prompt emission component of the GRB emission (Akerlof et al. 1999;Verstrand et al. 2005Verstrand et al. , 2006;;Racusin et al. 2008). A natural interpretation of this observation would be that optical and soft γray emission are produced, respectively, via synchrotron and inverse Compton mechanisms by one and the same population of relativistic electrons, thus favoring the inverse Compton (SSC or EC) scenario for the 0.01-10 MeV band emission. It is, however, possible that optical and γ-ray components of prompt emission are produced by two separate electron populations and/or in different emission regions (Zou et al. 2008). A strong test of the inverse Compton model of the prompt γ-ray emission can be given in the nearest future via (non)detection of the predicted secondorder inverse Compton scattering component in the 1-100 GeV energy band by Fermi/GLAST and/or ground-based γ-ray telescopes (Racusin et al. 2008;Savchenko et al. 2009).

The synchrotron and inverse Compton models of the prompt γ-ray emission could be distinguished via a study of the spectral characteristics of the prompt GRB emission. If the prompt emission is optically thin synchrotron emission from the shockaccelerated electrons in a relativistic “fireball”, the spectrum of γ-ray emission could not have photon index harder than the one of the synchrotron emission from a monoenergetic electron distribution, Γ synch,lim = 2/3 (Katz 1994;Tavani 1995). At the same time, the spectrum of inverse Compton emission can be as hard as Γ IC,lim = 0 (Baring & Braby 2004). Observation of episodes of hardening of GRB spectra beyond Γ synch,lim would provide a strong argument against the synchrotron model of the prompt emission (see, however, (Epstein 1973;Lloyd & Petrosian 2000;Lloyd-Ronning & Petrosian 2002;Medvedev 2000; Baring & Braby 2004) for modifications of the synchrotron model which are able to accommodate photon indices harder than 2/3).

The time-resolved spectral characteristics of the prompt GRB emission were studied in details by Preece et al. (1998aPreece et al. ( ,b, 2000) ) and Kaneko et al. (2006) using a set of bright GRBs detected by the BATSE instrument on board of CGRO. The study of BATSE GRBs shows that approximately 30% of all the spectra violate the “synchrotron deathline” Γ = 2/3 (Pree

…(Full text truncated)…

This content is AI-processed based on ArXiv data.