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Long-duration gamma-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of massive stars. Previous early optical observations of even the most exceptional GRBs (990123 and 030329) lacked both the temporal resolution to probe the optical flash in detail and the accuracy needed to trace the transition from the prompt emission within the outflow to external shocks caused by interaction with the progenitor environment. Here we report observations of the extraordinarily bright prompt optical and gamma-ray emission of GRB 080319B that provide diagnostics within seconds of its formation, followed by broadband observations of the afterglow decay that continued for weeks. We show that the prompt emission stems from a single physical region, implying an extremely relativistic outflow that propagates within the narrow inner core of a two-component jet.

Gamma-ray bursts can be used to test for the presence of spacetime foam—postulated in theories of quantum gravity. Quantum fluctuations would cause the photon speeds to vary, leading to ‘fuzziness’ and, consequently, Lorentz invariance violation.

We present a detailed spectral analysis of the prompt and afterglow emission of four nearby long-soft gamma-ray bursts (GRBs 980425, 030329, 031203, and 060218) that were spectroscopically found to be associated with type Ic supernovae, and compare them to the general GRB population. For each event, we investigate the spectral and luminosity evolution, and estimate the total energy budget based upon broadband observations. The observational inventory for these events has become rich enough to allow estimates of their energy content in relativistic and sub-relativistic form. The result is a global portrait of the effects of the physical processes responsible for producing long-soft GRBs. In particular, we find that the values of the energy released in mildly relativistic outflows appears to have a significantly smaller scatter than those found in highly relativistic ejecta. This is consistent with a picture in which the energy released inside the progenitor star is roughly standard, while the fraction of that energy that ends up in highly relativistic ejecta outside the star can vary dramatically between different events. 55 pages including 23 figures and 8 tables. Accepted for publication in ApJ. Replaced with the accepted version

Opacity effects in relativistic sources of high-energy gamma-rays, such as gamma-ray bursts (GRBs) or Blazars, can probe the Lorentz factor of the outflow and the distance of the emission site from the source, and thus help constrain the composition of the outflow (protons, pairs, magnetic field) and the emission mechanism. Most previous works consider the opacity in steady state. Here we study time dependent effects of the opacity to pair production ($����\to e^+e^-$) in impulsive relativistic sources. We present a simple, yet rich, semi-analytic model for the time and energy dependence of the optical depth, $��$, where an ultra-relativistic thin spherical shell emits isotropically in its own rest frame over a finite range of radii, $R_0 < R < R_0 + ��R$. This is very relevant for GRB internal shocks. We find that in an impulsive source ($��R ��_{1*}$ will arrive mainly near the onset of the spike or flare from to the short emission episode, as in impulsive sources it takes time to build-up the (target) photon field, and thus $��(��)$ initially increases with time and $��_1(T)$ decreases with time, so that photons of energy $��> ��_{1*}$ are able to escape the source mainly very early on while $��_1(T) > ��$. As the source approaches a quasi-steady state ($��R >> R_0$), the time integrated spectrum develops an exponential cutoff, while the power-law tail becomes increasingly suppressed. 67 pages, 13 figures, submitted to ApJ