GCN Circular 13257

S. Schulze (Univ. Iceland), A. J. Levan (Univ. Warwick), D. Malesani 
(DARK/NBI), J. P. U. Fynbo (DARK/NBI), N. R. Tanvir (Univ. Leicester), 
B. Milvang-Jensen (DARK/NBI), A. de Ugarte Postigo (IAA/CSIC and 
DARK/NBI), V. D'Elia (ASI/ASDC and INAF/OAR), J. Sollerman (OKC, 
Stockholm), J. Hjorth (DARK/NBI), report on behalf of the X-shooter GRB 
GTO collaboration:

We observed the optical counterpart of GRB 120422A (Troja et al., GCN 
13243
Not found
 
 
; Cucchiara et al., GCN 13245
Not found
 
 
) with the ESO VLT equipped with the 
X-shooter spectrograph. Observations started on 2012 April 22.99 UT 
(16.5 hr after the GRB), for a total exposure time of 80 min in each of 
the UVB, VIS, and NIR arms, covering the wavelength range 3000-25000 AA. 
The slit was aligned so to cover both the galaxy at z = 0.28 present in 
the SDSS (Cucchiara et al., GCN 13245
Not found
 
 
; Tanvir et al., GCN 13251
Not found
 
 
) and the 
GRB optical counterpart (Cucchiara et al., GCN 13245
Not found
 
 
).

We detect a plethora of nebular emission lines from the candidate host 
galaxy, including [O II], [O III], Balmer lines up to Hzeta, [N II], [S 
II], at a common redshift z = 0.283, thus confirming the results of 
Tanvir et al. (GCN 13251
Not found
 
 
).

The trace of the GRB counterpart is rather blue (see also Nardini et 
al., GCN 13256
Not found
 
 
), and we detect Mg II at z = 0.283 in absorption. No 
further absorption features are visible. We also note that the spatial 
extent of the brightest emission lines from the SDSS galaxy encompasses 
the position of the GRB counterpart, indicating nebular emission at z = 
0.283 at the GRB position.

While a chance superposition cannot be excluded, we note that the lack 
of absorption features at z > 0.283 would require a very tenuous 
environment at the GRB location. Furthermore, the UVOT detection in all 
the UV filters (Kuin & Troja, GCN 13248
Not found
 
 
) implies a low redshift z <~ 
1.3. Last, we note that the UVOT photometry at t ~ 5 ks after the burst 
reveals a very blue spectrum, which is not typical of GRB afterglows, 
but is reminiscent of the very blue early emission of GRB 060218 
(Campana et al. 2006, Nature, 442, 1008). We thus conclude that the GRB 
is also likely at z = 0.283.

Using the gamma-ray fluence reported by Barthelmy et al. (GCN 13246
Not found
 
 
), at 
z = 0.283 the isotropic-equivalent energy release of GRB 120422A is 
~4.4*10^49 erg, which is typical of local GRBs, often associated with 
supernovae. We encourage photometric and spectroscopic follow-up to 
detect the emergence of a supernova, or lack thereof.

We acknowledge the excellent support of the ESO observing staff in 
Paranal, in particular Henri Boffin and Giovanni Carraro. We thank Nino 
Cucchiara for providing us with a finding chart from the Gemini 
observation (GCN 13245
Not found
 
 
).