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GCN Circular 13348

Subject
GRB 120521C redshift estimate
Date
2012-05-29T22:22:31Z (12 years ago)
From
Nial Tanvir at U.Leicester <nrt3@star.le.ac.uk>
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) made using the WHT (Levan et al., GCN 13322), Keck (Perley et al. GCN
13332), 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) 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), 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)
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.
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