Monster in the Dark: The Ultraluminous GRB 080607 and its Dusty Environment

TL;DR

This paper presents detailed early-time optical to infrared observations of GRB 080607, revealing its dust environment, extinction properties, and intrinsic luminosity, providing insights into highly obscured extragalactic phenomena and dark gamma-ray bursts.

Contribution

It offers the most detailed extinction analysis of a high-redshift GRB sightline using the Fitzpatrick & Massa model, and characterizes the burst's intrinsic luminosity and environment.

Findings

Extinction properties are best modeled by Fitzpatrick & Massa parameterization.

GRB 080607 was among the most optically luminous GRBs observed.

High initial Lorentz factor (>600) indicated by early peak time.

Abstract

We present early-time optical through infrared photometry of the bright gamma-ray burst GRB 080607, starting only 6 s following the initial trigger in the rest frame. Complemented by our previously published spectroscopy, this high-quality photometric dataset allows us to solve for the extinction properties of the redshift 3.036 sightline, giving perhaps the most detailed information on the ultraviolet continuum absorption properties of any sightline outside our Local Group to date. The extinction properties are not adequately modeled by any ordinary extinction template (including the average Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud curves), partially because the 2175-Angstrom feature (while present) is weaker by about a factor of two than when seen under similar circumstances locally. However, the spectral energy distribution is exquisitely fitted by the more general Fitzpatrick & Massa (1990) parameterization of Local-Group extinction, putting it in the same family as some peculiar Milky Way extinction curves. After correcting for this (considerable, A_V = 3.3 +/- 0.4 mag) extinction, GRB 080607 is revealed to have been among the most optically luminous events ever observed, comparable to the naked-eye burst GRB 080319B. Its early peak time (t_rest < 6 s) indicates a high initial Lorentz factor (Gamma > 600), while the extreme luminosity may be explained in part by a large circumburst density. Only because of its early high luminosity could the afterglow of GRB 080607 be studied in such detail in spite of the large attenuation and great distance, making this burst an excellent prototype for the understanding of other highly obscured extragalactic objects, and of the class of "dark" GRBs in particular.