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Colloquium speaker

Andrei Beloborodov

Title: Emission Mechanism of Gamma-Ray Bursts

Astronomy Spring Colloquia March 13, 2012 Tuesday, 4:30pm-5:30pm, Peyton Hall auditorium:

 Gamma-ray bursts are emitted by super-powerful ultra-relativistic jets

from compact engines, most likely just born black holes or magnetars. A

long debated problem is how the jet emits the observed gamma rays. A

simplest model resembles the big bang: the opaque hot plasma expands to

transparency and releases its thermal photons. The expected spectrum from

a radiation-dominated jet is Planckian with a peak near 1 MeV. The

observed burst spectra do peak around 1 MeV, however they have

non-Plankian shapes, with extended high-energy tails. I will discuss

physical processes generating non-Plankian radiation, which can tap

magnetic or kinetic energy of the jet. An old phenomenological model

invokes synchrotron emission from internal shocks; it has serious problems

-- it appears to contradict both shock physics and observations. I will

describe a different emission mechanism: internal motions in the

neutron-proton jet generate electron heat via nuclear and Coulomb

collisions, and the electrons radiate the received energy. This mechanism

invokes no phenomenological parameters, and the produced radiation can be

calculated from first principles. Remarkably, the obtained theoretical

spectra agree with observations.

I will also discuss the central engine and the mechanism driving the jet.

The jet may be a strongly magnetized outflow driven by rotation of the

central object. Alternatively, the jet may be driven by thermal pressure

due to neutrino heating around the central object. Both mechanisms can

supply the observed luminosities and require extreme rotation rates.