Evidence for Circumburst Extinction of Gamma-Ray Bursts with Dark Optical Afterglows and Evidence for a Molecular Cloud Origin of Gamma-Ray Bursts

First, we show that the gamma-ray bursts with dark optical afterglows (DOAs) cannot be explained by a failure to image deeply enough quickly enough, and argue that circumburst extinction is the most likely solution. If so, many DOAs will be ``revived'' with rapid follow up and NIR searches in the HETE-2 and Swift eras. Next, we consider the effects of dust sublimation and fragmentation, and show that DOAs occur in clouds of size R > 10L_{49}^{1/2} pc and mass M > 3x10^5L_{49} M_{sun}, where L is the luminosity of the optical flash. Stability considerations show that such clouds cannot be diffuse, but must be molecular. Consequently, we compute the expected column density distribution of bursts that occur in Galactic-like molecular clouds, and show that the column density measurements from X-ray spectra of afterglows, DOAs and otherwise, satisfy this expectation in the source frame.Comment: Invited Review. To appear in Procs. of Gamma-Ray Burst and Afterglow Astronomy 2001: A Workshop Celebrating the First Year of the HETE Mission, 8 pages, 8 figures, LaTe

The Detectability of Gamma-Ray Bursts and Their Afterglows at Very High Redshifts

There is increasingly strong evidence that gamma-ray bursts (GRBs) are associated with star-forming galaxies, and occur near or in the star-forming regions of these galaxies. These associations provide indirect evidence that at least the long GRBs detected by BeppoSAX are a result of the collapse of massive stars. The recent evidence that the light curves and the spectra of the afterglows of GRB 970228 and GRB 980326 appear to contain a supernova component, in addition to a relativistic shock wave component, provide more direct clues that this is the case. Here we establish that GRBs and their afterglows are both detectable out to very high redshifts (z > 5).Comment: To appear in Proc. of the 10th Annual October Astrophysics Conference in Maryland: Cosmic Explosions, 4 pages, LaTe

Gamma-Ray Bursts as a Probe of Cosmology

We show that, if the long GRBs are produced by the collapse of massive stars, GRBs and their afterglows may provide a powerful probe of cosmology and the early universe.Comment: 6 pages, 5 PostScript figures. To appear in the proceedings of the October 2000 Rome Workshop on Gamma-Ray Bursts in the Afterglow Er

Gamma-Ray Burst Dust Echoes Revisited: Expectations at Early Times

Gamma-ray burst (GRB) dust echoes were first proposed as an alternative explanation for the supernova-like (SN-like) components to the afterglows of GRB 980326 and GRB 970228. However, the spectroscopic identification of Type Ic SN 2003dh associated with GRB 030329, as well as the identification of SN-like components to the afterglows of other GRBs, appears to have confirmed the GRB/SN paradigm. However, the likely progenitors of Type Ic SNe are Wolf-Rayet WC stars, and late-type WC stars have been observed to be surrounded by dust, at a distance of 10^14 -- 10^15 cm from the star. Consequently, we revisit the possibility of GRB dust echoes, not on a timescale of weeks after the burst but on a timescale of minutes to hours. We find that if the optical flash is sufficiently bright and the jet sufficiently wide, GRB afterglows may be accompanied by chromatic variations on this timescale. From these signatures, such model parameters as the inner radius of the dust distribution, the initial opening angle of the jet, etc., may be deduced. With rapid and regular localizations of GRBs by HETE-2, Integral, and now Swift, and new and improved robotic telescope systems, these early-time GRB dust echoes may soon be detected. We describe one such robotic telescope system, called PROMPT, that the University of North Carolina at Chapel Hill is building at the Cerro Tololo Inter-American Observatory in greater detail.Comment: Accepted to The Astrophysical Journal, 15 pages, 5 figures, LaTe

Construction of the Variability -> Luminosity Estimator

We present a possible Cepheid-like luminosity estimator for the long-duration gamma-ray bursts based on the variability of their light curves.Comment: To appear in Procs. of Gamma-Ray Bursts in the Afterglow Era: 2nd Workshop, 3 pages, 2 figures, LaTe

Constraints on the Redshift and Luminosity Distributions of Gamma Ray Bursts in an Einstein-de Sitter Universe

Two models of the gamma ray burst population, one with a standard candle luminosity and one with a power law luminosity distribution, are chi^2-fitted to the union of two data sets: the differential number versus peak flux distribution of BATSE's long duration bursts, and the time dilation and energy shifting versus peak flux information of pulse duration time dilation factors, interpulse duration time dilation factors, and peak energy shifting factors. The differential peak flux distribution is corrected for threshold effects at low peak fluxes and at short burst durations, and the pulse duration time dilation factors are also corrected for energy stretching and similar effects. Within an Einstein-de Sitter cosmology, we place strong bounds on the evolution of the bursts, and these bounds are incompatible with a homogeneous population, assuming a power law spectrum and no luminosity evolution. Additionally, under the implied conditions of moderate evolution, the 90% width of the observed luminosity distribution is shown to be < 10^2, which is less constrained than others have demonstrated it to be assuming no evolution. Finally, redshift considerations indicate that if the redshifts of BATSE's faintest bursts are to be compatible with that which is currently known for galaxies, a standard candle luminosity is unacceptable, and in the case of the power law luminosity distribution, a mean luminosity < 10^57 ph s^-1 is favored.Comment: Accepted to the Astrophysical Journal, 18 pages, LaTe