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

Quiescent times in gamma-ray bursts: I. An observed correlation between the durations of subsequent emission episodes

Although more than 2000 astronomical gamma-ray bursts (GRBs) have been detected, the precise progenitor responsible for these events is unknown. The temporal phenomenology observed in GRBs can significantly constrain the different models. Here we analyse the time histories of a sample of bright, long GRBs, searching for the ones exhibiting relatively long (more than 5 per cent of the total burst duration) quiescent times, defined as the intervals between adjacent episodes of emission during which the gamma-rays count rate drops to the background level. We find a quantitative relation between the duration of an emission episode and the quiescent time elapsed since the previous episode. We suggest here that the mechanism responsible for the extraction and the dissipation of energy has to take place in a meta-stable configuration, such that the longer the accumulation period, the higher is the stored energy available for the next emission episode.Comment: 5 pages, 3 figures, with final revision

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

Constraining Collapsar r-Process Models through Stellar Abundances

We use observations of heavy elements in very metal-poor stars ([Fe/H] < -2.5) in order to place constraints on the viability of collapsar models as a significant source of the r-process. We combine bipolar explosion nucleosynthesis calculations with recent disk calculations to make predictions of the observational imprints these explosions would leave on very metal-poor stars. We find that a source of low (~ 0.1-0.5 $M_\odot$) Fe mass which also yields a relatively high (> 0.08 $M_\odot$) r-process mass would, after subsequently mixing and forming new stars, result in [r/Fe] abundances up to three orders of magnitude higher than those seen in stars. In order to match inferred abundances, 10-10$^3 M_\odot$ of Fe would need be efficiently incorporated into the r-process ejecta. We show that Fe enhancement and hence [r/Fe] dilution from other nearby supernovae is not able to explain the observations unless significant inflow of pristine gas occurs before the ejecta are able to form new stars. Finally, we show that the inferred [Eu/Fe] abundances require levels of gas mixing which are in conflict with other properties of r-process enhanced metal-poor stars. Our results suggest that early r-process production is likely to be spatially uncorrelated with Fe production, a condition which can be satisfied by neutron star mergers due to their large kick velocities and purely r-process yields.Comment: 6 pages, 2 figures, accepted for publication in ApJ