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THE ASTRONOMER Electronic Circular No 2385      2007 Oct 15 22.10UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
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GRB 071010B POSSIBLE OPTICAL TRANSIENT
Arto Oksanen (Nyrola Observatory, Jyvaskyla, Finland) reports on GCN 
6873 the following optical observations of GRB071010B (GCN #6871, 
Markwardt et al.):
Arto Oksanen reports the detection of an optical transient located at RA 
10:02:9.26, Dec +45:43:50.3 (J2000) at an initial unfiltered magnitude 
of 17.538 calibrated relative to USNOB R-band magnitudes. Midpoint of 
the exposures is 2007 Oct 10, 21:11:05 UTC, approximately 25 minutes 
after the trigger.
There are no comparably bright stars within 0.5 arcminutes, and no minor 
planets in the area.
Subsequent unfiltered images place the star fainter than 18th magnitude 
(unfiltered) indicating the object has faded since observations began.

On GCN 6903 A. Oksanen (Hankasalmi Observatory, Hankasalmi, Finland), M. 
Templeton (AAVSO) and A. Henden (AAVSO) report further details on 
optical observations of GRB071010B (GCN #6871, Markwardt et al.; GCN 
#6873 Oksanen; GCN #6878 Kennea et al.; GCN #6884 Kann et al.):
Further to the initial report given in GCN #6873, Arto Oksanen reports 
unfiltered photometry of the GRB 071010B afterglow spanning 2007 Oct 10 
21:02:33 UT to 2007 Oct 11 03:24:27 UT (T0 + 17 minutes to T0 + 6 hours 
39 minutes post-burst).  The data are reasonably well fit by a single 
power law with index -0.482 +/- 0.064.  The measured unfiltered 
magnitudes are approximately 0.3 magnitudes fainter than the Rc 
magnitudes reported by Kann et al (GCN #6884) taken at the same time, 
which is consistent with differences in zero-point calibration and 
bandpass.  It should be noted that this field is covered by the SDSS, 
and that the AAVSO will obtain a BVRI calibration with the Sonoita 
Research Observatory, weather permitting.
Oksanen also reports additional preliminary Ic and V-band magnitudes:
2007 Oct 10, 21:32 UT   Ic = 17.64 +/- 0.05
2007 Oct 10, 21:52 UT   V  = 18.30 +/- 0.05
Both magnitudes are calibrated relative to the transformed SDSS ugriz 
magnitudes for the object at RA 10:01:54.62, Dec +45:40:29.0 (J2000).

Editor: We offer our sincere congratulations to Arto on this outstanding 
piece of observing and for discovery of an optical transient so early 
after trigger.

Guy M Hurst