{
  "bibcode": "2007GCN..6457....1C",
  "body": "S. B. Cenko, E. O. Ofek (Caltech) and P. A. Price (IfA) report on behalf\nof a larger collaboration:\n\nWe have re-imaged the field of GRB070521 (Guidorzi et al., GCN 6431) with\nthe Gemini Multi-Imaging Spectrograph mounted on the Gemini North\ntelescope.  We obtained 5 x 180 s in the Sloan i' filter at a mean epoch\nof 10:23:38 UT 22 May.\n\nThe host candidate (S1) identified by Hattori et al. (GCN 6444; see also\nPerley et al., GCN 6451) is well detected in our second epoch.  In\ncomparison with our first epoch of i' imaging (GCN 6450), we find\nmarginal evidence for fading of this source.  Specifically, we measure the\nfollowing magnitudes, with calibration performed relative to SDSS field\nstars (Cool et al., GCN 6432):\n\nt-tb (hr)\t\tMagnitude\n----------------------------------------------------------------\n1.21\t\t\t24.07 +\\- 0.17\n27.54\t\t\t24.30 +\\- 0.09\n\nHowever, PSF-matched image subtraction using the ISIS software package\n(Alard & Lupton, ApJ, 503, 1998) reveals no variability at the location of\nthis source.\n\nWe do not find any new sources inside the revised XRT error circle\n(Guidorzi et al., GCN 6452) in either epoch of imaging, to limits of i' >\n24.0 (epoch 1) and i' > 25.0 (epoch 2).\n\nThe source S3 is marginally detected in our second epoch near the\ndetection limit.  This would represent a decline from measurements\nreported previously (I = 24.39; Perley et al., GCN 6451).  However, we\ncaution our photometry at this location is affected by a variable\nbackground from the nearby bright galaxy and is therefore quite\nuncertain.  We encourage further observations to establish the nature of\nthis object.",
  "circularId": 6457,
  "createdOn": 1179904860000,
  "email": "cenko@srl.caltech.edu",
  "subject": "GRB070521: Second Epoch Gemini Imaging",
  "submitter": "S. Bradley Cenko at Caltech  <cenko@srl.caltech.edu>",
  "eventId": "GRB 070521"
}