{
  "bibcode": "2010GCN.11236....1K",
  "body": "D. A. Kann, U. Laux and B. Stecklum (TLS Tautenburg) report:\n\nWe obtained further observations at multiple epochs of the afterglow of\nGRB 100901A (Immler et al., GCN 11159) with the TLS 1.34m Schmidt\ntelescope. After another completely overcast night, we obtained data under\ngood to excellent conditions in the third and fourth nights.\n\nFor our first epoch (Kann et al., GCN 11187) we had used 5 USNO B1.0 R2\nstars. Now, we compare their magnitudes with those derived from the SDSS\ncatalog. We find that two of the stars we used, at\n\nRA = 01:49:05.595, Dec. = +22:44:51.96;\nRA = 01:48:59.497, Dec. = +22:46:18.56\n\nmust be variable, they yield strongly divergent zero points compared to\nthe SDSS values for the other three stars.\n\nFor the other three stars, which yield a stable zero point, we find:\n\nRA = 01:48:49.484, Dec. = +22:47:42.14; USNO R2 = 18.44; SDSS = 18.46\nRA = 01:48:53.990, Dec. = +22:48:30.07; USNO R2 = 17.73; SDSS = 17.91\nRA = 01:49:06.790, Dec. = +22:47:02.97; USNO R2 = 15.37; SDSS = 15.64\n\nNote that the last star has been used by many observers to calibrate the\nafterglow, we find that it is 0.27 mag fainter, this is in agreement with\nwhat was reported by Andreev et al., GCN 11191. Recalibrating all reported\nmagnitudes based on this calibration with this value strongly reduces the\nscatter in the light curve. Note the recently reported magnitudes by\nElenin et al. (GCN 11234) are about 0.4 magnitudes too bright.\n\nUsing these three stars, we re-calibrate our earlier data and add\nadditional observations as follows:\n\nMid-Time\tRc\tdRc\tExptime\n1.384629\t19.16\t0.03\t1 x 300\n1.388656\t19.24\t0.04\t1 x 300\n1.424663\t19.28\t0.03\t1 x 300\n1.428691\t19.36\t0.03\t1 x 300\n1.432719\t19.25\t0.04\t1 x 300\n1.436747\t19.18\t0.04\t1 x 300\n3.309522\t20.53\t0.07\t4 x 300\n3.549130\t20.70\t0.04\t1 x 600 + 2 x 450 + 4 x 300\n4.398521\t21.30\t0.05\t4 x 600\n4.424271\t21.31\t0.26\t1 x 120\n4.430357\t21.37\t0.13\t1 x 600\n4.559408\t21.27\t0.10\t1 x 600\n\nMidtime is in days after the trigger, exposure time in seconds.\n\nWe find that all observations after one day can be fit with a single power\nlaw with slope alpha = 1.46. This is steeper than the slope reported by\nPandey & Zheng (GCN 11179), in agreement with the break proposed by Kann\net al. (GCN 11187), though we caution the Delta alpha is shallow (and in\nagreement with the theoretical value for a cooling break) and there may\nstill be calibration issues.\n\nThis message may be cited.",
  "circularId": 11236,
  "createdOn": 1283799407000,
  "email": "kann@tls-tautenburg.de",
  "subject": "GRB 100901A: TLS observations, SDSS calibration, decay slope",
  "submitter": "Alexander Kann at TLS Tautenburg  <kann@tls-tautenburg.de>",
  "eventId": "GRB 100901A"
}