The peak energy of dissipative GRB photospheres

ABSTRACT The radiation released at the transparency radius of an ultrarelativistic flow can account forthe observed properties of gamma-ray bursts (GRBs) provided that sufficient energy is dissi-pated in the sub-photospheric region. Here, I investigate how the peak energy of the E · f(E)spectrum and its overall shape depend on the properties of the jet for various “dissipative pho-tospheres”. I find that continuous energy release over a wide range of distances may be thekey to explain the GRB emission. In this picture, the peak of the spectrum forms at a Thom-son optical depth of several tens. The peak depends mainly on the bulk Lorentz factor Γ ofthe flow and can, therefore, be used to determine it. The Γ can range from ∼ 10 to 1000 fromX-ray flashes to the brightest observed GRBs. The Amati relation can be understood if thebrightest bursts are the least baryon loaded ones. Implications from this interpretation of theGRB emission for the central engine are discussed.Key words: Gamma rays: bursts – radiation mechanisms: general – method s: statistical