Mass loss from a magnetically driven wind emitted by a disk orbiting a stellar mass black hole

The source of cosmic gamma-ray bursts (hereafter GRBs) is usually believed to be a stellar mass black hole accreting material from a thick disk. The mechanism for the production of a relativistic wind by such a system is still unknown. We investigate here one of the proposal where the disk energy is extracted by a magnetic field amplified to very large values B \sim 10^15 G. Using some very simple assumptions we compute the mass loss rate along magnetic field lines and then estimate the Lorentz factor \Gamma at infinity. We find that \Gamma can reach high values only if severe constraints on the field geometry and the conditions of energy injection are satisfied. We discuss the results in the context of different scenarios for GRBs.Comment: 5 pages, 1 figure, to appear in the proceedings of the 5th Huntsville Gamma-Ray Burst Symposiu

The runaway instability of thick discs around black holes. I. The constant angular momentum case

We present results from a numerical study of the runaway instability of thick discs around black holes. This instability is an important issue for most models of cosmic gamma-ray bursts, where the central engine responsible for the initial energy release is such a system consisting of a thick disc surrounding a black hole. We have carried out a comprehensive number of time-dependent simulations aimed at exploring the appearance of the instability. Our study has been performed using a fully relativistic hydrodynamics code. The general relativistic hydrodynamic equations are formulated as a hyperbolic flux-conservative system and solved using a suitable Godunov-type scheme. We build a series of constant angular momentum discs around a Schwarzschild black hole. Furthermore, the self-gravity of the disc is neglected and the evolution of the central black hole is assumed to be that of a sequence of exact Schwarzschild black holes of varying mass. The black hole mass increase is thus determined by the mass accretion rate across the event horizon. In agreement with previous studies based on stationary models, we find that by allowing the mass of the black hole to grow the disc becomes unstable. Our hydrodynamical simulations show that for all disc-to-hole mass ratios considered (between 1 and 0.05), the runaway instability appears very fast on a dynamical timescale of a few orbital periods, typically a few 10 ms and never exceeding 1 s for our particular choice of the mass of the black hole ($2.5 \mathrm{M_\odot}$) and a large range of mass fluxes (\dot{m} \ga 10^{-3} \mathrm{M_{\odot}/s}). The implications of our results in the context of gamma-ray bursts are briefly discussed.Comment: 20 pages, 16 figures, to appear in MNRA

The physics of pulses in gamma-ray bursts: emission processes, temporal profiles and time lags

We present a simple, semi-analytical model to explain GRB temporal and spectral properties in the context of the internal shock model. Each individual pulse in the temporal profiles is produced by the deceleration of fast moving material by a comparatively slower layer within a relativistic wind. The spectral evolution of synthetic pulses is first obtained with standard equipartition assumptions to estimate the post-shock magnetic field and electron Lorentz factor. We get Ep propto t^-delta with delta=7/2 which is much steeper than the observed slopes delta(obs) <= 1.5. We therefore consider the possibility that the equipartition parameters depend on the shock strength and post-shock density. We then get a much better agreement with the observations and our synthetic pulses satisfy both the hardness-intensity and hardness-fluence correlations. We also compute time lags between profiles in different energy channels and we find that they decrease with increasing hardness. We finally compare our predicted time lag - luminosity relation to the Norris et al. (2000) result obtained from 6 bursts with known redshift.Comment: 6 pages, 7 figures, to be published in MNRA

The expected thermal precursors of gamma-ray bursts in the internal shock model

The prompt emission of gamma-ray bursts probably comes from a highly relativistic wind which converts part of its kinetic energy into radiation via the formation of shocks within the wind itself. Such "internal shocks" can occur if the wind is generated with a highly non uniform distribution of the Lorentz factor. We estimate the expected photospheric emission of such a wind when it becomes transparent. We compare this thermal emission (temporal profile + spectrum) to the non-thermal emission produced by the internal shocks. In most cases, we predict a rather bright thermal emission that should already have been detected. This favors acceleration mechanisms for the wind where the initial energy input is under magnetic rather than thermal form. Such scenarios can produce thermal X-ray precursors comparable to those observed by GINGA and WATCH/GRANAT.Comment: 11 pages, 7 figures, to be published in MNRA

Late X-ray flares from the interaction of a reverse shock with a stratified ejecta in GRB afterglows: simulations on a moving mesh

Late activity of the central engine is often invoked in order to explain the flares observed in the early X-ray afterglow of gamma-ray bursts, either in the form of an active neutron star remnant or (fall-back) accretion onto a black hole. However, these scenarios are not always plausible, in particular when flares are delayed to very late times after the burst. Recently, a new scenario was proposed that suggests X-ray flares can be the result of the passing of a long-lived reverse shock through a stratified ejecta, with the advantage that it does not require late-time engine activity. In this work, we numerically demonstrate this scenario to be physically plausible, by performing onedimensional simulations of ejecta dynamics and emission using our novel moving-mesh relativistic hydrodynamics code. Improved efficiency and precision over previous work enables the exploration of a broader range of setups. We can introduce a more physically realistic description of the circumburst medium mass density. We can also locally trace the cooling of electrons when computing the broadband emission from these setups. We show that the synchrotron cooling timescale can dominate the flare decay time if the stratification in the ejecta is constrained to a localised angular region inside the jet, with size corresponding to the relativistic causal connection angle, and that it corresponds to values reported in observations. We demonstrate that this scenario can produce a large range of observed flare times, suggesting a connection between flares and initial ejection dynamics rather than with late-time remnant activity.Comment: 17 pages, 22 figures, accepted by MNRAS (18 May 2020

Gravitational Waves from the First Stars

We consider the stochastic background of gravitational waves produced by an early generation of Population III stars coupled with a normal mode of star formation at lower redshift. The computation is performed in the framework of hierarchical structure formation and is based on cosmic star formation histories constrained to reproduce the observed star formation rate at redshift z \la 6, the observed chemical abundances in damped Lyman alpha absorbers and in the intergalactic medium, and to allow for an early reionization of the Universe at $z\sim 10-20$ as indicated by the first year results released by WMAP. We find that the normal mode of star formation produces a gravitational wave background which peaks at 300-500 Hz and is within LIGO III sensitivity. The Population III component peaks at lower frequencies (30-100 Hz depending on the model), and could be detected by LIGO III as well as the planned BBO and DECIGO interferometers.Comment: 16 pages, 8 figure

Cosmic Star Formation, Reionization, and Constraints on Global Chemical Evolution

Motivated by the WMAP results indicating an early epoch of reionization, we consider alternative cosmic star formation models which are capable of reionizing the early intergalactic medium. We develop models which include an early burst of massive stars (with several possible mass ranges) combined with standard star formation. We compute the stellar ionizing flux of photons and we track the nucleosynthetic yields for several elements: D, He4, C, N, O, Si, S, Fe, Zn. We compute the subsequent chemical evolution as a function of redshift, both in the intergalactic medium and in the interstellar medium of forming galaxies, starting with the primordial objects which are responsible for the reionization. We apply constraints from the observed abundances in the Lyman alpha forest and in Damped Lyman alpha clouds in conjunction with the ability of the models to produce the required degree of reionization. We also consider possible constraints associated with the observations of the two extremely metal-poor stars HE 0107-5240 and CS22949-037. We confirm that an early top-heavy stellar component is required, as a standard star formation model is unable to reionize the early Universe and reproduce the abundances of the very metal-poor halo stars. A bimodal (or top-heavy) IMF (40 - 100 M_\odot) is our preferred scenario compared to the extreme mass range (\ga 100 M_\odot) often assumed to be responsible for the early stages of reionization. A mode of even more extreme stellar masses in the range (\ge 270 M_\odot) has also been considered. All massive stars in this mode collapse entirely into black holes, and as a consequence, chemical evolution and reionization are de-correlated. [Abstract abbreviated.]Comment: 45 pages, 18 eps figures, as accepted in Ap