Short Gamma-ray Bursts: the mass of the accretion disk and the initial
  radius of the outflow
  In this work we estimate the accretion-disk mass in the specific scenario of
binary-neutron-star-merger with current observational data. Assuming that the
outflows of short Gamma-ray Bursts (GRBs) are driven via neutrino-antineutrino
annihilation we estimate the disk mass of about half of short bursts in the
sample to be $\sim 0.01-0.1 M_\odot$, in agreement with that obtained in the
numerical simulations. Massive disks ($\sim {\rm several} 0.1 M_\odot$) found
in some other short GRBs may point to the more efficient magnetic process of
energy extraction or the neutron star and black hole binary progenitor. Our
results suggest that some short bursts may be really due to the coalescence of
double neutron stars and are promising gravitational wave radiation sources.
For future short GRBs with simultaneous gravitational-wave detections, the disk
mass may be reliably inferred and the validity of our approach will be tested.
We also propose a method to constrain the initial radius of a baryonic outflow
where it is launched ($R_0$) without the need of identifying an ideal thermal
spectrum. We then apply it to GRB 090510 and get that $R_0 \lesssim 6.5\times
10^{6}(\Gamma_{\rm ph}/2000)^{-4}$ cm, suggesting that the central engine is a
black hole with a mass $< 22 M_\odot(\Gamma_{\rm ph}/2000)^{-4}$, where
$\Gamma_{\rm ph}$ is the bulk Lorentz factor of the outflow at the photospheric
radius.
