Photospheres, Comptonization and X-ray Lines in Gamma Ray Bursts

Steep X-ray spectral slopes, X-ray excesses and preferred spectral energy breaks in the 01-0.3 MeV range are discussed as the possible consequences of the photospheric component of the GRB relativistic outflow, and of pair breakdown in internal shocks leading to comptonization on semirelativistic electrons or MHD waves. We also discuss the X-ray and UV spectra of GRB afterglows occuring in a dense environment characteristic of massive stellar progenitors, including their ability to produce detectable Fe or other metallic line features. I PHOTOSPHERES, SHOCKS AND PAIRS A significant fraction of bursts appear to have low energy spectral slopes steeper than 1/3 in energy [19,5]. This has motivated consideration of a thermal or nonthermal [11,12] comptonization mechanism, while leaving the astrophysical model largely unspecified. There is also evidence that the apparent clustering of the break energy of GRB spectra in the 50-500 keV range may not be due to observational selection [19,4,6]. Models using Compton attenuation [3] require reprocessing by an external medium whose column density adjusts itself to a few g cm. More recently a preferred break has been attributed to a blackbody peak at the comoving pair recombination temperature in the fireball photosphere [8]. In order for such photospheres to occur at the pair recombination temperature in the accelerating regime requires an extremely low baryon load. For very large baryon loads, a different explanation has been invoked [21] involving scattering of photospheric photons off MHD waves in the photosphere, which upscatters the adiabatically cooled photons up to the observed break energy. Motivated by the above observations, these ideas have been synthesized [14] to produce a generic scenario in which the presence of a photospheric component as well as shocks subject to pair breakdown can produce steep low energy spectra and preferred breaks (see Figure 1). In some of our previξ = 1 ηl = 321 ξ = 1 ηm = 143