Long Gamma-Ray Bursts as standard candles

As soon as it was realized that long GRBs lie at cosmological distances, attempts have been made to use them as cosmological probes. Besides their use as lighthouses, a task that presents mainly the technological challenge of a rapid deep high resolution follow-up, researchers attempted to find the Holy Grail: a way to create a standard candle from GRB observables. We discuss here the attempts and the discovery of the Ghirlanda correlation, to date the best method to standardize the GRB candle. Together with discussing the promises of this method, we will underline the open issues, the required calibrations and how to understand them and keep them under control. Even though GRB cosmology is a field in its infancy, ongoing work and studies will clarify soon if and how GRBs will be able to keep up to the promises.Comment: To appear in the proceedings of the 16th Annual October Astrophysics Conference in Maryland "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N. Gehrels & J. Nouse

Temporal and Angular Properties of GRB Jets Emerging from Massive Stars

We study the long-term evolution of relativistic jets in collapsars and examine the effects of viewing angle on the subsequent gamma ray bursts. We carry out a series of high-resolution simulations of a jet propagating through a stellar envelope in 2D cylindrical coordinates using the FLASH relativistic hydrodynamics module. For the first time, simulations are carried out using an adaptive mesh that allows for a large dynamic range inside the star while still being efficient enough to follow the evolution of the jet long after it breaks out from the star. Our simulations allow us to single out three phases in the jet evolution: a precursor phase in which relativistic material turbulently shed from the head of the jet first emerges from the star, a shocked jet phase where a fully shocked jet of material is emerging, and an unshocked jet phase where the jet consists of a free-streaming, unshocked core surrounded by a thin boundary layer of shocked jet material. The appearance of these phases will be different to observers at different angles. The precursor has a wide opening angle and would be visible far off axis. The shocked phase has a relatively narrow opening angle that is constant in time. During the unshocked jet phase the opening angle increases logarithmically with time. As a consequence, some observers see prolonged dead times of emission even for constant properties of the jet injected in the stellar core. We also present an analytic model that is able to reproduce the overall properties of the jet and its evolution. We finally discuss the observational implications of our results, emphasizing the possible ways to test progenitor models through the effects of jet propagation in the star. In an appendix, we present 1D and 2D tests of the FLASH relativistic hydrodynamics module.Comment: 56 pages, 31 figures (47 postscript files). ApJ in press. Updated simulation results and revised according to referee comments. Manuscript with full resolution figures and movies at http://rocinante.colorado.edu/~morsony/GRB

Gamma-Ray Burst jet dynamics and their interaction with the progenitor star

The association of at least some long gamma-ray bursts with type Ic supernova explosions has been established beyond reasonable doubt. Theoretically, the challenge is to explain the presence of a light hyper-relativistic flow propagating through a massive stellar core without losing those properties. We discuss the role of the jet-star interaction in shaping the properties of the outflow emerging on the surface of the star. We show that the nature of the inner engine is hidden from the observer for most of the evolution, well beyond the time of the jet breakout on the stellar surface. The discussion is based on analytical considerations as well as high resolution numerical simulations. Finally, the observational consequences of the scenario are addressed in light of the present capabilities.Comment: Proceedings of the Royal Society dscussion meeting "Recent developments in the study of Gamma-ray bursts", Philosophical Trans. of the Royal Soc.

High-efficiency photospheric emission of long-duration gamma-ray burst jets: the effect of the viewing angle

We present the results of a numerical investigation of the spectra and light curves of the emission from the photospheres of long-duration gamma-ray burst jets. We confirm that the photospheric emission has high efficiency and we show that the efficiency increases slightly with the off-axis angle. We show that the peak frequency of the observed spectrum is proportional to the square root of the photosphere's luminosity, in agreement with the Amati relation. However, a quantitative comparison reveals that the thermal peak frequency is too small for the corresponding total luminosity. As a consequence, the radiation must be out of thermal equilibrium with the baryons in order to reproduce the observations. Finally, we show that the spectrum integrated over the emitting surface is virtually indistinguishable from a Planck law, and therefore an additional mechanism has to be identified to explain the non-thermal behavior of the observed spectra at both high and low frequencies.Comment: 6 pages, 8 figures, ApJ in press (few changes to figures

Short-duration gamma-ray bursts from off-axis collapsars

We present 2D high-resolution hydrodynamic simulations of the relativistic outflows of long-duration gamma-ray burst progenitors. We analyze the properties of the outflows at wide off-axis angles, produced by the expansion of the hot cocoon that surrounds the jet inside the progenitor star. We find that the cocoon emission at wide angles may have properties similar to those of the subclass of short-duration gamma-ray bursts with persistent X-ray emission. We compute the predicted duration distribution, redshift distribution, and afterglow brightness and we find that they are all in agreement with the observed properties of short GRBs with persistent emission. We suggest that a SN component, the properties of the host galaxies, and late afterglow observations can be used as a crucial test to verify this model.Comment: 5 pages, 6 color figures, accepted for publication in ApJ, main journa