HYDRODYNAMIC PROPERTIES of GAMMA-RAY BURST OUTFLOWS DEDUCED from the THERMAL COMPONENT

• We study the properties of a significant thermal emission component that was identified in 47 gamma-ray bursts observed by different instruments. Within the framework of the "fireball" model, we deduce the values of the Lorentz factor Gamma, and the acceleration radius, r(0), for these bursts. We find that all the values of Gamma in our sample are in the range 10(2) <= Gamma <= 10(3), with = 370. We find a very weak dependence of Gamma on the acceleration radius r0, Gamma proportional to r(0)(alpha) with alpha - -0.10 +/- 0.09 at the sigma - 2.1 confidence level. The values of r(0) span a wide range, 10(6.5) <= r(0) <= 10(9.5) cm, with a mean value of < r(0)> similar to 10(8) cm. This is higher than the gravitational radius of a 10 M-circle dot black hole by a factor of approximate to 30. We argue that this result provides indirect evidence for jet propagation inside a massive star, and suggests the existence of recollimation shocks that take place close to this radius.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

gamma rays

stars

hydrodynamics

methods: data analysis

radiation mechanisms

thermal

radiative transfer

Publikations- och innehållstyp

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