GCN Circular 10247: "GRB 091202: GTC & CAHA optical/nIR afterglow candidate "

A. de Ugarte Postigo (INAF-OAB), A.J. Castro-Tirado, J. Gorosabel, M. Jelinek, S. Guziy (IAA-CSIC), A. Cabrera-Lavers, D. García, G. Gomez (GTC/IAC), G. Bergond (CAHA), D. Perez-Ramirez (Univ. de Jaen), R. Cuniffe (IAA-CSIC) and P. Kubanek (IAA-CSIC, U. Valencia) report on behalf of a larger collaboration:

"We have observed the field of the INTEGRAL GRB 091202 (Mereghetti et al. GCN 10234) using OSIRIS at the 10.4m Gran Telescopio Canarias (GTC) at the Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the island of La Palma. Observations were carried out between 04:55 and 05:23 UT on 4 December 2009 (~1.25 days after the burst). The observation consisted of 5x30s exposures in r, i and z bands. Addtional near-IR observations were obtained in with Omega2000 at the 3.5m telescope at the German-Spanish Calar Alto Observatory in J, H and K' bands between 01:06 and 04:46 UT.

Within the XRT error circle (Vetere et al. GCN 10239) we detect two sources, one point-like and one apparently extended in all three GTC frames. Only one source (point-like) shows up in the near-IR frame, which appears to be superimposed to the extended one seen in the optical. This will imply a strong red colour (r-H >= 4) typical of GRB afterglows (Gorosabel et al. 2002, A&A 384, 11). Hence, we identify this source as the likely afterglow to GRB 091202, at coordinates (J2000.0):

R.A.: 09 15 19.75

Dec.: +62 32 59.0

with a 0".5 astrometry error.

At a mean epoch of 1.25 days after the burst, we measure for it a magnitude of r = 23.7 ± 0.1. From the nIR observations we measure H = 19.7 ± 0.2. at a mean epoch 1.21 days after the burst. For the optical photometry we used the SDSS star located at coordinates R.A.: 09 15 21.23, Dec: +62 32 33.4 (J2000.0) for which we assume a magnitude of r = 21.27. The nIR images were calibrated against a 2MASS star at coordinates R.A.: 09 15 14.41, Dec: +62:31:55.5 (J2000.0) with H = 13.94.

Based on the optical GTC image, at this moment we cannot rule out that the optical flux arises from an underlying somehow extended blue galaxy whereas the highly extincted afterglow is only observed in the nIR. A figure showing the optical & nIR images of the afterglow candidate can be found at: http://www.iaa.es/~gss/GRB091202

This Circular can be quoted.