GCN Circular 6505

D. A. Kann, A. C. Wilson, S. Schulze, S. Klose, M. Henze, F. Ludwig, U. 
Laux (TLS Tautenburg) and J. Greiner (MPE Garching) report:

We observed the XRT position (Pagani & Kennea, GCN 6490
Not found
 
 
) of Swift GRB 
070610 (Pagani et al. GCN 6489
Not found
 
 
) with the Tautenburg 1.34m Schmidt 
Telescope in Rapid Response Mode (Klose et al., GCN 3609
Not found
 
 
). Observations 
started 640 seconds after the trigger, at an initial airmass of 1.945. 
Weather conditions were good and the airmass decreased during 
observations. We obtained two sequences of images with 120 seconds 
exposure time each, consisting of 6 Ic, 3 Rc and 3 V images per sequence.

The optical transient first detected by Stefanescu et al. (GCN 6492
Not found
 
 
) and 
confirmed by de Ugarte Postigo et al. (GCN 6501
Not found
 
 
) is detected in only three 
images, thus showing a rapid, short-lived flaring behaviour. Stacks of 
images were created, but we do not detect the transient in these stacks. 
We report the following upper limits and magnitudes assuming the USNO B1.0 
star at RA = 19:55:03.2, Dec. = +26:14:14.5 has R2 = 15.8 and I = 15.32:

tmid (days)	filter	Exp.	Limit	OT mag
0.008407	Ic	1x120	19.7	
0.012122	Ic	6x120	20.2	
0.022194	Rc	3x120	21.2	
0.033376	Ic	2x120	20.4	
0.036369	Ic	1x120	20.2	19.36 +\- 0.15
0.040228	Ic	3x120	20.5	
0.044356	Rc	1x120	21.3	
0.046336	Rc	1x120	21.3	20.89 +\- 0.20
0.048304	Rc	1x120	21.3	21.78 +\- 0.50

The final detection is marginal.

We note that this flaring behaviour is very uncommon for the afterglow of 
a GRB. Furthermore, the maps of Schlegel et al. 1998 give E(B-V) = 3.256, 
tranlating to A_R = 8.7 and A_I = 6.3, at the position of the transient. 
This would imply the flare in the I band reached 13th magnitude, very 
unusual at such a late time. Furthermore, the X-ray afterglow as seen on 
the Swift XRT repository webpage 
(http://www.swift.ac.uk/xrt_curves/00281993/index.php) shows a more or 
less constant behaviour, with small flares superposed.

The behaviour of the transient in both the optical and the X-ray regime as 
well as the location in the Galactic plane strongly suggest that this is a 
new Galactic transient source and not a GRB and its associated afterglow.

This message may be cited.

[GCN OPS NOTE(12jun07): Per author's request, "A_V" was changed to "A_I".]