{
  "bibcode": "2012GCN.13348....1T",
  "body": "N. R. Tanvir, K. Wiersema (U. Leicester), A. J. Levan (U. Warwick),\nA. Cucchiara (UCSC/UCO Lick), D. Perley (Caltech), P. Hirst (Gemini),\nT. Carroll, T. Kerr, W. Varricatt (JACH), C. Farina, M. Hrudkova (ING), \nL. Ker (Edinburgh) report on behalf of a larger collaboration:\n\nWe have combined observations of GRB 120521C (Baumgartner et al. GCN\n13318) made using the WHT (Levan et al., GCN 13322), Keck (Perley et al. GCN\n13332), UKIRT and Gemini-N telescopes across the optical-IR region in r,i,z,J,H,K.\n\nAnalysis of further WHT z-band imaging, obtained at 01:54 UT, 2 hours\nafter our initial epoch (Levan et al. GCN 13322) shows a faint source\nwithin the XRT error circle. This source is also seen at high confidence\nin z-band imaging obtained with Gemini-N at 07:03 UT, and photometry \nshows that it brightened by 0.5 magnitudes over this period, suggesting \nit is the afterglow of GRB 120521C. This source is only very weakly visible in\ncontemporaneous I-band imaging obtained with Keck (Perley et al. GCN\n13332), and this indicates a red colour i-z >2.3 . However, the source is not\ndetected in our initial reductions of the UKIRT WFCAM imaging in JHK,\nsuggesting that z-K < 1.3 (AB, corresponding to f(nu) ~ nu^-beta with\nbeta<1.3). This is indicative of a high-redshift, rather than a highly\nreddened afterglow. We note that the position of the afterglow candidate\nin our data matches the radio position reported by Zauderer et al (GCN 13344)\nwithin our calibration uncertainty of ~0.3 arcseconds.\n\nGemini-North GMOS spectroscopy was obtained in poor conditions, using the\nR400 grating centered at 8000 A, giving a wavelength range of\napproximately 5850-10100 A. Observations (2 x 1800 s) started at 12:03 UT\non 23 May 2012. We used the GMOS Nod&Shuffle observing mode to improve\nskyline subtraction.\n\nIn the resulting spectrum, a weak trace is seen at the expected location,\nwhich is only detected redward of ~8700 A. The absence of any emission\nbluewards of this wavelength is suggestive of a high redshift of this\nsource, z~6.0. The low signal of the trace precludes detection of\nabsorption lines.\n\nFurther analysis of the data is ongoing.",
  "circularId": 13348,
  "createdOn": 1338330151000,
  "email": "nrt3@star.le.ac.uk",
  "subject": "GRB 120521C redshift estimate",
  "submitter": "Nial Tanvir at U.Leicester  <nrt3@star.le.ac.uk>",
  "eventId": "GRB 120521C"
}