GRB 050408: A Bright Gamma‐Ray Burst Probing an Atypical Galactic Environment

The bright GRB 050408 was localized by HETE-II near local midnight, enablingan impressive ground-based followup effort as well as space-based followup fromSwift. The Swift data from the X-Ray Telescope (XRT) and our own opticalphotometry and spectrum of the afterglow provide the cornerstone for ouranalysis. Under the traditional assumption that the visible waveband was abovethe peak synchrotron frequency and below the cooling frequency, the opticalphotometry from 0.03 to 5.03 days show an afterglow decay corresponding to anelectron energy index of p_lc = 2.05 +/- 0.04, without a jet break as suggestedby others. A break is seen in the X-ray data at early times (at ~12600 secafter the GRB). The spectral slope of the optical spectrum is consistent withp_lc assuming a host-galaxy extinction of A_V = 1.18 mag. The optical-NIRbroadband spectrum is also consistent with p = 2.05, but prefers A_V = 0.57mag. The X-ray afterglow shows a break at 1.26 x 10^4 sec, which may be theresult of a refreshed shock. This burst stands out in that the optical andX-ray data suggest a large H I column density of N_HI ~ 10^22 cm^-2; it is verylikely a damped Lyman alpha system and so the faintness of the host galaxy (M_V> -18 mag) is noteworthy. Moreover, we detect extraordinarily strong Ti IIabsorption lines with a column density through the GRB host that exceeds thelargest values observed for the Milky Way by an order of magnitude.Furthermore, the Ti II equivalent width is in the top 1% of Mg IIabsorption-selected QSOs. This suggests that the large-scale environment of GRB050408 has significantly lower Ti depletion than the Milky Way and a largevelocity width (delta v > 200 km/s).Comment: ApJ submitte