Dynamos, Super-pulsars and Gamma-ray bursts

The remnant of a neutron star binary coalescence is expected to be temporarily stabilised against gravitational collapse by its differential rotation. We explore the possibility of dynamo activity in this remnant and assess the potential for powering a short-duration gamma-ray burst (GRB). We analyse our three-dimensional hydrodynamic simulations of neutron star mergers with respect to the flow pattern inside the remnant. If the central, newly formed super-massive neutron star remains stable for a good fraction of a second an efficient low-Rossby number $\alpha-\Omega$-dynamo will amplify the initial seed magnetic fields exponentially. We expect that values close to equipartition field strength will be reached within several tens of milliseconds. Such a super-pulsar could power a GRB via a relativistic wind, with an associated spin-down time scale close to the typical duration of a short GRB. Similar mechanisms are expected to be operational in the surrounding torus formed from neutron star debris.Comment: 5 pages, 1 figure, Proceedings of the Gamma-ray Burst Symposium 2003, Santa Fe; Reference adde

Photospheric signatures imprinted on the gamma-ray burst spectra

A solution is presented for the spectrum of high-energy gamma-ray burst photons confined to a quasi-thermal baryonic photosphere. The solution is valid in the steady-state limit assuming the region under consideration is optically thick to the continuously injected photons. It is shown that for a high luminosity photosphere, the non-thermal electrons resulting from gamma-ray Compton cooling lose their energy by upscattering the soft thermalised radiation. The resulting spectral modifications offer the possibility of diagnosing not only the burst comoving luminosity but also the baryon load of the ejecta. This model leads to a simple physical interpretation of X-ray rich bursts and anomalous low-energy slopes.Comment: 7 pages, 3 figures; to appear in MNRAS pink page

Time Scales in Long GRBs

We analyze a sample of bright long bursts and find that the pulses duration have a lognormal distribution while the intervals between pulses have an excess of long intervals (relative to lognormal distribution). This excess can be explained by the existence of quiescent times, long periods with no signal above the background. The lognormal distribution of the intervals (excluding the quiescent times) is similar to the distribution of the pulses width. This result suggests that the quiescent times are made by a different mechanism than the rest of the intervals. It also suggests that the intervals (excluding the quiescent times) and the pulse width are connected to the same parameters of the source. We find that there is a correlation between a pulse width and the duration of the interval preceding it. There is a weaker, but still a significant, correlation between a pulse width and the interval following it. The significance of the correlation drops substantially when the intervals considered are not adjacent to the pulse.Comment: 5 pages, 8 figures, accepted for publication by MNRA

The Formation of Rapidly Rotating Black Holes in High Mass X-ray Binaries

High mass X-ray binaries (HMXRBs) like Cygnus X-1, host some of the most rapidly spinning black holes (BHs) known to date, reaching spin parameters $a \gtrsim 0.84$. However, there are several effects that can severely limit the maximum BH spin parameter that could be obtained from direct collapse, such as tidal synchronization, magnetic core-envelope coupling and mass loss. Here we propose an alternative scenario where the BH is produced by a {\it failed} supernova (SN) explosion that is unable to unbind the stellar progenitor. A large amount of fallback material ensues, whose interaction with the secondary naturally increases its overall angular momentum content, and therefore, the spin of the BH when accreted. Through SPH hydrodynamic simulations, we studied the unsuccessful explosion of a $8M_{\odot }$ pre-SN star in a close binary with a $12M_{\odot}$ companion with an orbital period of $\approx1.2$ days, finding that it is possible to obtain a BH with a high spin parameter $a\gtrsim0.8$ even when the expected spin parameter from direct collapse is $a \lesssim 0.3$. This scenario also naturally explains the atmospheric metal pollution observed in HMXRB stellar companions.Comment: 6 pages, 4 figure