GCN Circular 15194

S. Schmidl (TLS Tautenburg), D. A. Kann (MPE Garching), S. Klose, S. 
Stecklum, F. Ludwig (all TLS Tautenburg), and J. Greiner (MPE Garching)
report:

We observed the field of the intense GRB 130907A (Swift trigger 569992,
Page et al., GCN #15183) with the 1.34m Tautenburg Schmidt telescope.

Starting at 22:02:18 UT (21 minutes after the trigger), we obtained in
total 3 x 120 sec Ic-band images and 12 Rc-band images of 120 sec and 60
sec exposure. Observations were obtained at high airmass of 2.6 - 3.2. The
afterglow is clearly detected on all frames.

Using the star at RA (J2000) = 14:23:28.93, Dec. (J2000) = +45:37:33.65,
for which we derive Rc = 15.10, Ic = 14.51 using SDSS magnitudes and the
transformations of Lupton (2005), we derive preliminary magnitudes for the
first image in each filter of:

Time (d)	Filter	mag	error
0.0152		Ic		15.26	0.03
0.0172		Rc		16.57	0.03

not corrected for the very small reddening of 0.01 mag along the line of
sight (Schlegel et al. 1998).

We note Swift was slewing when GRB 130907A started, the extended raw BAT
light curve (http://gcn.gsfc.nasa.gov/notices_s/sw00569992000msbx.gif)
shows the GRB began about 120 seconds before Swift T_0. Adding this time
to our observing times, we find the data is well-described by a single
power-
law decay with alpha = 1.39 +/- 0.02. The detection by Trotter et al. (GCN
#15191) agrees excellently with the extrapolation of our data, while the
RATIR detection (Lee et al., GCN #15192), after transforming to Vega mag,
indicates that the decay has slowed, which is confirmed by Trotter et al.
(GCN #15193). The very early detection by Gorbovskoy et al. (GCN #15184)
lies several magnitudes under a back-extrapolation of our data.

We derive a very large Rc-Ic color of about 1.2 mag from the fit, 
indicating possible strong reddening (see also the Skynet light curve, GCN
#15193, and the multicolor RATIR mags, GCN #15192), which would be in
accordance with the very strong absorption lines detected by de Ugarte
Postigo et al. (GCN #15187).

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