Simulations and Analytic Models of Relativistic Magnetized Jets
Sasha Tchekhovskoy (Harvard) - Oct 5, 2009 at 12:10 pm
Long-duration gamma-ray bursts (GRBs) occur when the core of a
progenitor star collapses, forms a magnetized rotating compact object
and powers a pair of magnetized relativistic jets. These jets emerge
from the star with Lorentz factors \gamma of a few hundred and opening
angles \theta <~ 0.1 radians. Achromatic breaks in GRB afterglow light
curves indicate that \gamma\theta >> 1. However, magnetohydrodynamic
(MHD) simulations of collimated jets generally give \gamma\theta <~ 1,
indicating a conflict between these models and observations. In this
talk we present a new class of MHD jet simulations in which the
simulated jet is confined by a GRB progenitor star, modeled as a rigid
confining wall that extends out to a certain radius. Beyond this radius,
the jet becomes deconfined. We find that the onset of deconfinement
outside the star causes a burst of acceleration with negligible change
in the opening angle. In our fiducial model with a stellar radius equal
to 10^4.5 times that of the central compact object, the jet achieves an
asymptotic \gamma ~ 500 far outside the star and an asymptotic \theta ~
0.04 rad, giving \gamma\theta ~ 20. These values are consistent with
observations of typical long-duration GRBs and explain the occurrence of
jet breaks within the context of MHD jets.
The seminar will be held in 544 Campbell Hall.
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