TITLE:   GCN CIRCULAR
NUMBER:  6762
SUBJECT: GRB070810b: optical candidate
DATE:    07/08/21 17:05:01 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

V. Rumyantsev (CrAO) , V.Biryukov (SAI, MSU and CrAO), A. Pozanenko (IKI)
report on behalf of larger GRB follow-up collaboration:

We observed the error box of short GRB070810b  (Marshall et al., GCN 6743)
with Shajn 2.6m telescope of CrAO on Aug. 10.  Series of images were
obtained between (UT) 2007-08-10 22:05:23 and 2007-08-10 23:25:50 in R
filter. The combined image of a total exposure of 4260 s (71x60 s) covers
entire of the error box of GRB 070810b and can be found at
http://grb.rssi.ru/GRB070810b/GRB070810b_ZTSh.gif. The limiting magnitude (3
sigma) calibrated against USNO-A2.0 of the combined image is 24.8 mag.

We investigate  the S1, S2 XRT positions of GRB70810b (GCN 6754 , Starling
et al.) and S3 position  (Thoene et al., GCN 6756).

There is marginally detected object in a place of the source S1  (see
http://grb.rssi.ru/GRB070810b/GRB070810b_s1_ZTSh.gif).

Both optical sources around S3 (RA = 00:35:46.892 Dec = +08:51:05.96) are
presented in our and Keck's images, see
http://grb.rssi.ru/GRB070810b/GRB070810b_s3_ZTSh.gif.

Comparison of objects around source S2  ( RA =  00h 35m 50.0s Dec = +08d 48'
45.0'' ) in our image ( see
http://grb.rssi.ru/GRB070810b/GRB070810b_s2_ZTSh.gif) with the image
obtained on Aug. 11 (Thoene et al., GCN 6756) reveals a source between   two
nearby galaxies. We consider this source as a candidate of the optical
counterpart of  GRB070810b and a redshift  z~0.49 obtained for the galaxies
(Thoene et al., CN 6756) is quite natural for short GRB detected by Swift.
However more detailed comparison of the images (Keck I and ZTSh) and
additional observations are necessary to secure the nature of the candidate.

The massage can be cited.