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"Close binary progenitors of gamma ray bursts" M.V. Barkov (Space Research Institute) Abstract: The strong
dependence of the neutrino annihilation mechanism on the mass accretion rate
makes it difficult to explain the LGRBs with duration in excess of 100
seconds as well as the precursors separated from the main gamma-ray pulse by
few hundreds of seconds. Even more difficult is to explain the Swift
observations of the shallow decay phase and X-ray flares, if they indeed
indicate activity of the central engine for as long as 10^4 seconds. These
data suggest that some other, most likely magnetic mechanisms have to be
considered. Since the efficiency of magnetic mechanisms does not depend that
much on the mass accretion rate, the magnetic models do not require the
development of accretion disk within the first few seconds of the stellar
collapse and hence do not require very rapidly rotating stellar cores at the
presupernova state. This widens the range of potential LGRB progenitors. In
this paper, we re-examine the close binary scenario allowing for the
possibility of late development of accretion disks in the collapsar model and
investigate the available range of mass accretion rates, black hole masses,
and spins. We find that the black hole mass can be much higher than 2-3 Msun,
usually assumed in the collapsar model, and normally exceeds half of the
presupernova mass. The black hole spin is rather moderate, a=0.4-0.8, but
still high enough for the Blandford-Znajek mechanism to remain efficient
provided the magnetic field is sufficiently strong. Our numerical simulations
confirm the possibility of magnetically driven stellar explosions, in
agreement with previous studies, but point towards the required magnetic flux
on the black hole horizon in excess of 10^28 G cm^2. At present, we cannot
answer with cirtainty whether such a strong magnetic field can be generated
in the stellar interior. Perhaps, the supernova explosions associated with
LGRBs are still neutrino-driven and their gamma-ray signature is the
precursors. The supernova blast cleares up escape channels for the
magnetically driven GRB jets, which may produce the main pulse. In this
scenario, the requirements on the magnetic field strength can be lowered. A
particularly interesting version of the binary progenitor involves merger of
a WR star with an ultra-compact companion, neutron star or black hole. In
this case we expect the formation of very long-lived accretion disks, that
may explain the phase of shallow decay and X-ray flares observed by Swift.
Similarly long-lived magnetic central engines are expected in the current
single star models of LGRB progenitors due to their assumed exceptionally
fast rotation. |
