TITLE: GCN CIRCULAR NUMBER: 11236 SUBJECT: GRB 100901A: TLS observations, SDSS calibration, decay slope DATE: 10/09/06 18:56:47 GMT FROM: Alexander Kann at TLS Tautenburg D. A. Kann, U. Laux and B. Stecklum (TLS Tautenburg) report: We obtained further observations at multiple epochs of the afterglow of GRB 100901A (Immler et al., GCN 11159) with the TLS 1.34m Schmidt telescope. After another completely overcast night, we obtained data under good to excellent conditions in the third and fourth nights. For our first epoch (Kann et al., GCN 11187) we had used 5 USNO B1.0 R2 stars. Now, we compare their magnitudes with those derived from the SDSS catalog. We find that two of the stars we used, at RA = 01:49:05.595, Dec. = +22:44:51.96; RA = 01:48:59.497, Dec. = +22:46:18.56 must be variable, they yield strongly divergent zero points compared to the SDSS values for the other three stars. For the other three stars, which yield a stable zero point, we find: RA = 01:48:49.484, Dec. = +22:47:42.14; USNO R2 = 18.44; SDSS = 18.46 RA = 01:48:53.990, Dec. = +22:48:30.07; USNO R2 = 17.73; SDSS = 17.91 RA = 01:49:06.790, Dec. = +22:47:02.97; USNO R2 = 15.37; SDSS = 15.64 Note that the last star has been used by many observers to calibrate the afterglow, we find that it is 0.27 mag fainter, this is in agreement with what was reported by Andreev et al., GCN 11191. Recalibrating all reported magnitudes based on this calibration with this value strongly reduces the scatter in the light curve. Note the recently reported magnitudes by Elenin et al. (GCN 11234) are about 0.4 magnitudes too bright. Using these three stars, we re-calibrate our earlier data and add additional observations as follows: Mid-Time Rc dRc Exptime 1.384629 19.16 0.03 1 x 300 1.388656 19.24 0.04 1 x 300 1.424663 19.28 0.03 1 x 300 1.428691 19.36 0.03 1 x 300 1.432719 19.25 0.04 1 x 300 1.436747 19.18 0.04 1 x 300 3.309522 20.53 0.07 4 x 300 3.549130 20.70 0.04 1 x 600 + 2 x 450 + 4 x 300 4.398521 21.30 0.05 4 x 600 4.424271 21.31 0.26 1 x 120 4.430357 21.37 0.13 1 x 600 4.559408 21.27 0.10 1 x 600 Midtime is in days after the trigger, exposure time in seconds. We find that all observations after one day can be fit with a single power law with slope alpha = 1.46. This is steeper than the slope reported by Pandey & Zheng (GCN 11179), in agreement with the break proposed by Kann et al. (GCN 11187), though we caution the Delta alpha is shallow (and in agreement with the theoretical value for a cooling break) and there may still be calibration issues. This message may be cited.