2 Spectral turnover interpretations

We have systematically examined all of the conventional absorption mechanisms that may lead to a spectral break at a few hundred keVs. External photoelectric absorption by a cold static ISM or CBM (circumburster medium) requires a huge column density $(\gt 10^{26}~{\rm cm}^{-2})$ for solar abundance unless the CBM is very Fe-rich (Boettcher et al. 1999). In that case the required Fe column density would be $\gt 10^{21}~{\rm cm}^{-2}$. But even in this case the low-energy spectral shape is inconsistent with those observed by Beppo/SAX and Ginga. However this idea merits further study in view of the recent claims of Fe-fluorescence line emission during the afterglows (Piro et al. 1999).

Internal synchrotron self-absorption up to keV energies if the bulk Lorentz factor $\Gamma=$ hundreds requires very strong fields $(B\gt 10^7~{\rm G})$ (Rybicki & Lightman 1979). Internal bremsstrahlung (Rybicki & Lightman 1979) or plasma (Razin-Tsytovich) absorption requires very high density $(\gt 10^{26}~{\rm cm}^{-3})$(Melrose 1980). None of these options are viable based on current relativistic fireball scenarios (Piran et al. 1993). Internal double Compton or photo-electric absorption remains to be investigated but is unlikely to produce spectral shapes that match those observed. Hence the remaining option is saturated Comptonization (Rybicki & Lightman 1979), a mechanism we have concentrated on for the last two years.