Gamma-ray burst theory after Swift

Abstract

Afterglow observations in the pre-Swift era confirmed to a large extend the relativistic blast wave model for gamma-ray bursts (GRBs). Together with the observations of properties of host galaxies and the association with (type Ic) SNe, this has led to the generally accepted collapsar origin of long GRBs. However, most of the afterglow data was collected hours after the burst. The X-ray telescope and the UV/optical telescope onboard Swift are able to slew to the direction of a burst in real time and record the early broadband afterglow light curves. These observations, and in particular the X-ray observations, resulted in many surprises. While we have anticipated a smooth transition from the prompt emission to the afterglow, many observed that early light curves are drastically different. We review here how these observations are changing our understanding of GRBs.

GRBs in the Era of Rapid Response Telescopes

“…The actual energy involved is less by the reciprocal of the beaming factor ∼10 − 300. GRBs are thought to result from either the catastrophic core-collapse of massive stars, or from compact object mergers involving at least one neutron star, i.e., NS-NS or BH-NS (Piran & Fan 2007;Gehrels, Ramirez-Ruiz, & Fox 2009). GRBs are found via their "prompt" γ-ray emission which lasts ∼0.1 − 100 s. Since this bandpass is blocked by Earth's atmosphere, these discoveries must be made by space-based missions carrying γ−ray detectors with localization capability.…”

“…Rapid GRB follow-up observations are crucial for detecting early optical afterglow. The fireball model accounts for the afterglow peak as being either due to a forward shock passing through the circumstellar environment of the GRB, or a reverse shock moving backwards in the ejecta frame through the GRB jet plasma itself (Zhang & Kobayashi 2005;Piran & Fan 2007). Reverse shock emission lasts only a short time due to the short crossing time of the GRB ejecta.…”

Introduction: recent developments in the study of gamma-ray bursts

“…About half of the afterglows have X-ray flares superimposed on the broken power-law (Burrows et al 2007) also indicative of continuing activity within the central engine for extended periods after the initial outburst. Panaitescu (2007), Piran & Fan (2007) and others have discussed the added complexity to afterglow models needed to fully understand these new features.…”

Neutrinos from collapsars

“…However, standard versions of the internal shock model do not explain the origin of the magnetic fields needed to produce the synchrotron radiation observed in the fireball. In addition, the validity of the model has been recently compromised, because it presents many problems for reproducing the variety of lightcurves observed with Swift and Fermi satellites in past years (Piran & Fan 2007;Ackermann et al 2010). Then, alternative models proposed to explain the gamma-ray emission are being currently explored.…”