GCN Circular 13348

N. R. Tanvir, K. Wiersema (U. Leicester), A. J. Levan (U. Warwick),
A. Cucchiara (UCSC/UCO Lick), D. Perley (Caltech), P. Hirst (Gemini),
T. Carroll, T. Kerr, W. Varricatt (JACH), C. Farina, M. Hrudkova (ING), 
L. Ker (Edinburgh) report on behalf of a larger collaboration:

We have combined observations of GRB 120521C (Baumgartner et al. GCN
13318
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) made using the WHT (Levan et al., GCN 13322
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), Keck (Perley et al. GCN
13332
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), UKIRT and Gemini-N telescopes across the optical-IR region in r,i,z,J,H,K.

Analysis of further WHT z-band imaging, obtained at 01:54 UT, 2 hours
after our initial epoch (Levan et al. GCN 13322
Not found
 
 
) shows a faint source
within the XRT error circle. This source is also seen at high confidence
in z-band imaging obtained with Gemini-N at 07:03 UT, and photometry 
shows that it brightened by 0.5 magnitudes over this period, suggesting 
it is the afterglow of GRB 120521C. This source is only very weakly visible in
contemporaneous I-band imaging obtained with Keck (Perley et al. GCN
13332
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), and this indicates a red colour i-z >2.3 . However, the source is not
detected in our initial reductions of the UKIRT WFCAM imaging in JHK,
suggesting that z-K < 1.3 (AB, corresponding to f(nu) ~ nu^-beta with
beta<1.3). This is indicative of a high-redshift, rather than a highly
reddened afterglow. We note that the position of the afterglow candidate
in our data matches the radio position reported by Zauderer et al (GCN 13344
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)
within our calibration uncertainty of ~0.3 arcseconds.

Gemini-North GMOS spectroscopy was obtained in poor conditions, using the
R400 grating centered at 8000 A, giving a wavelength range of
approximately 5850-10100 A. Observations (2 x 1800 s) started at 12:03 UT
on 23 May 2012. We used the GMOS Nod&Shuffle observing mode to improve
skyline subtraction.

In the resulting spectrum, a weak trace is seen at the expected location,
which is only detected redward of ~8700 A. The absence of any emission
bluewards of this wavelength is suggestive of a high redshift of this
source, z~6.0. The low signal of the trace precludes detection of
absorption lines.

Further analysis of the data is ongoing.