GCN Circular 12991

K. Wiersema (Leicester), P. Goldoni (APC/Univ. Paris 7 and SAp/CEA), J. P.
U. Fynbo (DARK/NBI), N. R. Tanvir (U. Leicester), D. Malesani (DARK/NBI),
A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), S. D. Vergani (INAF/OAB), S.
Covino (INAF/OAB), H. Flores (Obs. Paris) report on behalf of the
X-shooter GTO GRB collaboration:

We observed the position of the afterglow of GRB 120224A (Saxton et al.,
GCN 12980
Not found
 
 
) using the X-shooter spectrograph mounted on the VLT.
Observations started on 00:30 UT on 25 February 2012, i.e. 19.83 hours
after burst.
In the acquisition images, 5 x 150 seconds in R filter, we find a faint
source, consistent with the candidate afterglow reported by Elliott et al
(GCN 12988
Not found
 
 
). We find a preliminary magnitude of R=22.3 using the USNO-B
star 0722-0043411 (R=16.65). The detection of the afterglow in the R band
places a limit on the redshift of approximately z < 5.

We performed spectroscopy of this source, under poor seeing and high
airmass. We detect a faint continuum in the near-infrared arm over the
whole wavelength range, at low signal to noise.
In the visual arm data we detect a very faint continuum at the red end of
the wavelength range. No emission lines are detected. Because of the low
signal to noise we are not able to identify absorption features. The non-
detection of continuum emission below ~600 nm may be indicative of a
moderately high redshift for this burst, though alternatives, such as a
reddened spectrum (note that the X-ray afterglow shows considerable
absorption; Osborne et al. GCN 12986
Not found
 
 
), can not be excluded at present.

We are grateful for the excellent support from the Paranal Observatory
staff, in particular Myriam Rodrigues and Ivo Saviane.