GCN Circular 10076
Subject
GRB 091020: Early TLS Multicolor detections, red afterglow
Event
Date
2009-10-25T05:58:15Z (16 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
D. A. Kann, U. Laux, M. Roeder & H. Meusinger (TLS Tautenburg) report:
We observed the afterglow of the Swift GRB 091020 (Racusin et al., GCN
10048) with the 1.34m Schmidt telescope of the Thueringer Landessternwarte
Tautenburg, Germany, as soon as it had risen to an airmass of
2.2, starting 0.167 days after the GRB. We obtained 3 x 300 sec images in
Z, Ic, Rc, V and B each. We detect the afterglow in all single images.
As a comparison star, we use the SDSS star at:
RA (J2000) = 11:42:49.2 (175.7050129)
Dec. (J2000) = +51:00:52.42 (+51.01456163)
For this star, using the transformation equations of Lupton (2005) (taking
the mean of the two transformation equations each), and transforming the
z' value to Vega mags following Fukugita (1995), we derive the
magnitudes:
B = 19.109
V = 17.740
Rc = 16.759
Ic = 15.927
Z = 15.563
We derive the following afterglow magnitudes:
dt Filter mag dmag
____________________________________
0.166947 Z 18.17 0.13
0.170987 Z 18.10 0.13
0.175038 Z 18.16 0.13
0.179181 Ic 18.45 0.06
0.183221 Ic 18.30 0.05
0.187260 Ic 18.32 0.05
0.191404 Rc 19.43 0.04
0.195444 Rc 19.40 0.04
0.199483 Rc 19.40 0.04
0.203627 V 19.90 0.10
0.207666 V 20.00 0.10
0.211706 V 20.06 0.10
0.215849 B 20.78 0.10
0.219889 B 20.71 0.08
0.223928 B 20.72 0.08
Using further reported magnitudes (Oates et al., GCN 10054; Gorbovskoy et
al., GCN 10052, Xu et al., GCN 10053, Perley, GCN 10058, GCN 10060), we
find that the Rc band light curve is very well described by a
single power-law decay with alpha = 1.12 +/- 0.06, B and V light curves
agree with an achromatic light curve evolution. We find a red afterglow
with spectral slope beta ~ 2, and note that the Ic - Z color is
very blue, this may be due to a filter mismatch between the SDSS z' filter
and our Z filter.
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