{
  "bibcode": "2010GCN.10399....1P",
  "body": "D. A. Perley, J. S. Bloom, S. B. Cenko, B. E. Cobb, A. N. Morgan, A. A. \nMiller, and C. R. Klein (UC Berkeley) report:\n\nWe conducted a deep optical imaging integration on the field of GRB \n100205A (Racusin et al., GCN 10361) using LRIS on the Keck I 10m \ntelescope starting at 09:48 UT on 2010 Feb 7.  We integrated for a total \nexposure time of 4000 seconds in g-band and 3770 seconds in R-band \nsimultaneously under clear skies but variable and generally poor seeing \nconditions (1.5 arcsec average).  The mid-point of the integration was \nat 10:28 UT.\n\nWe detect no source at the position of the putative infrared afterglow \n(Tanvir et al., GCN 10366).  Calibrating relative to USNOB1.0, we \nmeasure limiting magnitudes (3-sigma) of:\n\nR > 26.7\ng > 26.6\n\nas measured at the afterglow location.  A faint source is marginally \ndetected (<2 sigma) in R-band only at the northwestern edge of the XRT \nerror circle (Evans et al., GCN 10367), not consistent with the position \nof the infrared afterglow.\n\nWe also stacked the R-band and g-band images together to create a \nwhite-light composite image.  There is no significant detection in this \nimage, with an approximate limiting magnitude of White > 27 mag.\n\nAll properties of this GRB are consistent with it being at high redshift:\n\n* Lack of optical detection starting at early times (e.g. Malesani et \nal., GCN 10362; Updike et al., GCN 10364; Cobb et al, GCN 10365)\n* An apparently fading afterglow candidate detected only in H- and \nK-bands (Cucchiara et al., GCN 10374; Im et al., GCN 10398)\n* Limited X-ray absorption: excess N_H(z=0) = 7(+6/-5) x 10^20 cm^-2 \n(Starling and Racusin, GCN 10369), generally suggestive of higher \nredshift (Grupe et al. 2007, AJ 133:2216G) and inconsistent with the \nalternative hypothesis of a large extinction column at low redshift for \ntypical values of A_V/N_H.\n* Lack of host galaxy to deep limits (Perley et al. 2009, AJ 138:1690)\n\nAs mentioned by Cucchiara et al., the implied redshift of this GRB if \nthe red H-K color is due to Lyman-alpha absorption would be z ~ 11-13. \nHowever, significant dust absorption at intermediate redshift (e.g., \nA_V~3 mag at z~4) is still generally consistent with the available data, \nand unfortunately the X-ray afterglow is too faint to impose meaningful \nconstraints on extinction in the K-band during the Gemini observation.\n\nAn image of the field is posted to:\nhttp://lyra.berkeley.edu/~dperley/100205a/100205a_keck_gR.png\n\nWe encourage continued deep infrared follow-up of the field.",
  "circularId": 10399,
  "createdOn": 1265807170000,
  "email": "dperley@astro.berkeley.edu",
  "subject": "GRB 100205A: Keck limits on an underlying host galaxy",
  "submitter": "Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>",
  "eventId": "GRB 100205A"
}