GCN Circular 14264

T. Kruehler, D. Malesani, D. Xu, J. P. U. Fynbo (DARK/NBI), A. J. Levan
(U. Warwick), N. R. Tanvir (U. Leicester), V. D'Elia (ASDC, INAF) and 
D. Perley (Caltech) report on behalf of the X-shooter GTO GRB collaboration:

The field of GRB 100615A (Swift trigger 424733, D'Elia et al., GCN 10841
Not found
 
 
) 
was observed with the Hubble Space Telescope and WFC3
(filters F606W and F160W) under proposal ID 11840 (PI: A. J. Levan).
Observations were carried out on 2010 Dec 16 (154 days after the GRB),
for a total exposure time of 1128 s and 1208 s, respectively.
In the HST images, we detect a single, resolved object (F160W = 24.1 AB mag)
consistent with the Chandra X-ray position (Butler et al., GCN 10915
Not found
 
 
),
which we consider to be the GRB host galaxy. 

Its coordinates are (J2000):

RA = 11:48:49.34
Dec = -19:28:51.8

A spectrum of this source was taken on 2013 Mar 05 with the ESO VLT
equipped with the X-shooter spectrograph, covering the wavelength range
3000-20500 AA. The seeing was 0.8". In the VIS and NIR arm,
we detect several emission lines, interpreted as the doublet of
[O II](3726, 3729), [Ne III](3869),  [O III](4959), [O III](5007) and Halpha 
at a common redshift of z = 1.398.

At this redshift, the afterglow data imply a very high host exinction
along the GRB sight-line (A_V > 10-15, see D'Elia & Stratta 2011, A&A, 532, A48,
their Figure 4). The intrinsic soft X-ray absorption column measured
from Swift/XRT data (Margutti et al., GCN 10847
Not found
 
 
) at z = 1.398 is
N_{H,X} = 1.07 (+0.13, -0.12) x 10^23 cm^-2. This is similarly at the high end
of measured column densities, even among dark bursts
(e.g., Kruehler et al., 2012, ApJ, 758, 46, their Figure 5). In
contrast, the host of GRB 100615A is rather blue (F606W - F160W \sim 1
AB mag) and does not show obvious signs of heavy dust content.

We acknowledge excellent support from the observing staff at Paranal,
in particular Giacomo Beccari and Emanuela Pompei.