One favored progenitor model for short duration gamma-ray bursts ( GRBs ) is the coalescence of two neutron stars ( NS - NS ) .
One possible outcome of such a merger would be a rapidly spinning , strongly magnetized neutron star ( known as a millisecond magnetar ) .
These magnetars may be “ supra-massive , ” implying that would collapse to black holes after losing centrifugal support due to magnetic dipole spin down .
By systematically analyzing the Burst Alert Telescope ( BAT ) -XRT light curves of all short GRBs detected by Swift , we test how consistent the data are with this central engine model of short GRBs .
We find that the so-called “ extended emission ” feature observed with BAT in some short GRBs is fundamentally the same component as the “ internal X-ray plateau ” as observed in many short GRBs , which is defined as a plateau in the light curve followed by a very rapid decay .
Based on how likely a short GRB is to host a magnetar , we characterize the entire Swift short GRB sample into three categories : the “ internal plateau ” sample , the “ external plateau ” sample , and the “ no plateau ” sample .
Based on the dipole spin down model , we derive the physical parameters of the putative magnetars and check whether these parameters are consistent with expectations from the magnetar central engine model .
The derived magnetar surface magnetic field B _ { p } and the initial spin period P _ { 0 } fall into a reasonable range .
No GRBs in the internal plateau sample have a total energy exceeding the maximum energy budget of a millisecond magnetar .
Assuming that the beginning of the rapid fall phase at the end of the internal plateau is the collapse time of a supra-massive magnetar to a black hole , and applying the measured mass distribution of NS - NS systems in our Galaxy , we constrain the neutron star equation of state ( EOS ) .
The data suggest that the NS EOS is close to the GM1 model , which has a maximum non-rotating NS mass of M _ { TOV } \sim 2.37 M _ { \odot } .