The bright GRB 050408 was localized by HETE-II near local midnight, enabling
an impressive ground-based followup effort as well as space-based followup from
Swift. The Swift data from the X-Ray Telescope (XRT) and our own optical
photometry and spectrum of the afterglow provide the cornerstone for our
analysis. Under the traditional assumption that the visible waveband was above
the peak synchrotron frequency and below the cooling frequency, the optical
photometry from 0.03 to 5.03 days show an afterglow decay corresponding to an
electron energy index of p_lc = 2.05 +/- 0.04, without a jet break as suggested
by others. A break is seen in the X-ray data at early times (at ~12600 sec
after the GRB). The spectral slope of the optical spectrum is consistent with
p_lc assuming a host-galaxy extinction of A_V = 1.18 mag. The optical-NIR
broadband spectrum is also consistent with p = 2.05, but prefers A_V = 0.57
mag. The X-ray afterglow shows a break at 1.26 x 10^4 sec, which may be the
result of a refreshed shock. This burst stands out in that the optical and
X-ray data suggest a large H I column density of N_HI ~ 10^22 cm^-2; it is very
likely 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 II
absorption lines with a column density through the GRB host that exceeds the
largest values observed for the Milky Way by an order of magnitude.
Furthermore, the Ti II equivalent width is in the top 1% of Mg II
absorption-selected QSOs. This suggests that the large-scale environment of GRB
050408 has significantly lower Ti depletion than the Milky Way and a large
velocity width (delta v > 200 km/s).Comment: ApJ submitte
