2013MNRAS.432.3237G

Gamma-ray bursts with blackbody spectra are only a few and in most cases this spectral component is accompanied by a dominating non-thermal one. Only four bursts detected by Burst And Transient Source Experiment have a pure blackbody spectrum throughout their duration. We present the new case of GRB 100507 detected by the Gamma Burst Monitor on board the Fermi satellite. GRB 100507 has a blackbody spectrum for the entire duration ( ∼ 30s) of the prompt emission. The blackbody temperature varies between 25 and 40keV. The flux varies between 10-7 and 4x10-7 ergcm-2s. There is no clear evidence of a correlation between the temperature and the blackbody flux. If the thermal emission in GRB 100507 is due to the fireballs becoming transparent, we can estimate the radius RT and bulk Lorentz factor ΓT corresponding to this transition and the radius R0 where the fireballs are created. We compare these parameters with those derived for the other four bursts with a pure blackbody spectrum. In all but one burst, for fiducial assumptions on the radiative efficiency and distance of the sources, R0 ∼ 109-1010cm, i.e. much larger than the gravitational radius of a few solar mass black hole. Possible solutions of this apparent inconsistency are tentatively discussed considering the dependence of R0 on the unknown parameters. Alternatively, such a large R0 could be where the fireball, still opaque, converts most of its kinetic energy into internal energy (due to the impact with some material left over by the progenitor star) and starts to re-accelerate.