{
  "bibcode": "2009GCN.10076....1K",
  "body": "D. A. Kann, U. Laux, M. Roeder & H. Meusinger (TLS Tautenburg) report:\n\nWe observed the afterglow of the Swift GRB 091020 (Racusin et al., GCN \n10048) with the 1.34m Schmidt telescope of the Thueringer Landessternwarte \nTautenburg, Germany, as soon as it had risen to an airmass of\n\n2.2, starting 0.167 days after the GRB. We obtained 3 x 300 sec images in \nZ, Ic, Rc, V and B each. We detect the afterglow in all single images.\n\nAs a comparison star, we use the SDSS star at:\n\nRA (J2000) =    11:42:49.2 (175.7050129)\nDec. (J2000) = +51:00:52.42 (+51.01456163)\n\nFor this star, using the transformation equations of Lupton (2005) (taking \nthe mean of the two transformation equations each), and transforming the \nz' value to Vega mags following Fukugita (1995), we derive the\n\nmagnitudes:\n\nB =  19.109\nV =  17.740\nRc = 16.759\nIc = 15.927\nZ =  15.563\n\nWe derive the following afterglow magnitudes:\n\ndt\t\tFilter\tmag\tdmag\n____________________________________\n0.166947\tZ\t18.17\t0.13\n0.170987\tZ\t18.10\t0.13\n0.175038\tZ\t18.16\t0.13\n0.179181\tIc\t18.45\t0.06\n0.183221\tIc\t18.30\t0.05\n0.187260\tIc\t18.32\t0.05\n0.191404\tRc\t19.43\t0.04\n0.195444\tRc\t19.40\t0.04\n0.199483\tRc\t19.40\t0.04\n0.203627\tV\t19.90\t0.10\n0.207666\tV\t20.00\t0.10\n0.211706\tV\t20.06\t0.10\n0.215849\tB\t20.78\t0.10\n0.219889\tB\t20.71\t0.08\n0.223928\tB\t20.72\t0.08\n\nUsing further reported magnitudes (Oates et al., GCN 10054; Gorbovskoy et \nal., GCN 10052, Xu et al., GCN 10053, Perley, GCN 10058, GCN 10060), we \nfind that the Rc band light curve is very well described by a\n\nsingle power-law decay with alpha = 1.12 +/- 0.06, B and V light curves \nagree with an achromatic light curve evolution. We find a red afterglow \nwith spectral slope beta ~ 2, and note that the Ic - Z color is\n\nvery blue, this may be due to a filter mismatch between the SDSS z' filter \nand our Z filter.\n\nThis message may be cited.",
  "circularId": 10076,
  "createdOn": 1256450295000,
  "email": "kann@tls-tautenburg.de",
  "subject": "GRB 091020: Early TLS Multicolor detections, red afterglow",
  "submitter": "Alexander Kann at TLS Tautenburg  <kann@tls-tautenburg.de>",
  "eventId": "GRB 091020"
}