TITLE: GCN CIRCULAR NUMBER: 11187 SUBJECT: GRB 100901A: TLS Observations: Break? DATE: 10/09/03 01:34:04 GMT FROM: Alexander Kann at TLS Tautenburg D. A. Kann, S. Klose, U. Laux, and B. Stecklum (TLS Tautenburg) report: We observed the afterglow of the double-peaked Swift GRB 100901A (Immler et al., GCN 11159) with the TLS 1.34m Schmidt telescope through several holes in the persistent cloud cover. Otherwise, conditions were excellent. We clearly detect the afterglow in all single 300 sec frames at high signal-to-noise (statistical error 0.03 - 0.04 mag). In comparison to five USNOB1.0 (R2 magnitude) stars, we find: time Rc dRc 1.382892 18.93 0.13 1.386920 19.00 0.15 1.422927 19.04 0.13 1.426955 19.13 0.14 We compile all available GCN data (Klunko et al., GCN 11162; Gorbovsky et al., GCN 11163; Andreev et al., GCN 11166; Andreev et al., GCN 11168; De Cia et al., GCN 11170; Kuroda et al., GCN 11172; Hentunen et al., GCN 11173; Updike et al., GCN 11174; Sahu et al., GCN 11175; Pritchard & Immler, GCN 11176; Kopac et al., GCN 11177; Ivanov et al., GCN 11178; Sánchez-Ramírez et al., GCN 11180; Elenin et al., GCN 11184) and can make the following statements on the light curve evolution of this extraordinary burst: The light curve shows three peaks. The first is the prompt flare reported by Ivanov et al., GCN 11178 (it should also be seen strongly in the UVOT u band event mode finding chart). The second peak is at ~ 0.026 days, reported by Gorbovsky et al., GCN 11163. Then another rebrightening sets in (Kuroda et al., GCN 11172), which peaks at about 0.35 days (e.g.,Sahu et al., GCN 11175, also seen strongly in the X-rays, Page & Immler, GCN 11171) . The afterglow begins to decay after that, and we find that the decay seems to have become steeper between the latest reported data (Updike et al., GCN 11174; Elenin et al., GCN 11184) and our detections at ~1.4 days. This may possibly be a jet break. Note that the R1 magnitudes are systematically ~ 0.2 mag brighter and thus the significance of the break is reduced if this calibration is used instead. Observationally, the late rebrightening makes this one of the brightest afterglows ever detected beyond 0.35 days, on par with that of GRB 060729 and several others. Comparing the extraordinary light curve evolution with our afterglow data base (Kann et al. 2010, ApJ, 720, 1513), we find that the evolution is remarkably similar to that of two other afterglows, namely GRB 060906 (Cenko et al. 2009, ApJ, 693, 1484) and GRB 970508 (e.g., Garcia et al. 1998, ApJ, 500, L105). In the extinction-corrected z = 1 system (we assume beta = 0.6, A_V = 0.1 - typical values - for GRB 100901A, and use z = 1.408, Chornock et al., GCN 11164), we find that the large-amplitude optical flares of these three afterglows can be brought to a reasonable match if we shift the GRB 060906 light curve by (Delta t = x 4.8; Delta mag = + 0.4) (it is faster and brighter) and that of GRB 970508 by (Delta t = / 8.2; Delta mag = - 3.7) (it is slower and fainter). In log-log space, all three rebrightenings look remarkably similar. GRB 100901A and GRB 060906 match strongly except that the rebrightening of GRB 060906 has a lower total amplitude - the pre- and post-rebrightening decay slopes are strongly similar. For GRB 970508, the optical flare evolves faster, also, the pre-flare evolution is a flat plateau. The post-flare decay is again very similar. Pandey & Zheng (GCN 11179) report a post-flare decay of alpha ~ 1.2 for GRB 100901A, and Zeh et al. 2006, ApJ, 637, 889 indeed find alpha = 1.24 +/- 0.01 for GRB 970508! Finally, we point out that the strong, late optical flare may be an energy injection, and the shells which have produced this refreshed shock may be the same ones that created the second BAT flare at 390 seconds (Sakamoto et al., GCN 11169) which also was seen in the optical (Gorbovsky et al., GCN 11163). Such reverberation effects have been discussed by Vestrand et al. 2006, Nature, 442, 172 (GRB 050820A) as well as Guidorzi et al. 2007, A&A, 474, 793 (GRB 070311). Further optical and especially NIR follow-up is strongly encouraged. Monitoring in Tautenburg will continue in the next days if weather permits. This message may be cited.