MONSTER IN THE DARK: THE ULTRALUMINOUS GRB 080607 AND ITS DUSTY ENVIRONMENT

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