New ideas about GRB emission

New results presented at last week’s 2012 Fermi/Swift GRB conference in Munich, Germany, has shed new light on the emission mechanism of gamma-ray bursts (GRBs).  GRBs are brief but very intense bursts of gamma radiation. Gamma-ray telescopes like the Fermi and Swift space telescopes are detecting about one GRB per day coming from all directions in the sky, with each flash lasting from a few milliseconds to a few hundred seconds.  There are two flavours of GRBs defined by the length of their burst: long bursts (longer than two seconds), which are associated with supernova explosions, and short bursts (shorter than two second) are thought to originate from the merger of two compact objects such as neutron stars. Both types are GRBs are likely signal the birth of a black hole.

For the past 20 years it has been assumed that γ-rays emitted from GRBs result from synchrotron radiation, whereby electrons emit high-energy light as they are accelerated along magnetic field lines embedded in the shockwaves that rippling out from the explosion. However, using the high resolution of  Fermi it seems that the γ-rays are actually generated by thermal emission  at the surface of the fireball.  By analysing the spectral energy distribution of dozens of GBRs, Sylvain Guiriec of Goddard Spaceflight Center suggests that the small bump in the power-law rise of the energy distribution points to thermal emission.

Preliminary model of the spectral energy distribution of GRB 120323A, with a bump thought to arise from thermal emission, suggesting that synchrotron emissions alone cannot explain the burst. (Credit: Sylvain Guiriec, GSFC.)

For more details, see

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