[vsnet-grb-info 4371] GRB 920925C: optical afterglow candidate

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6155
SUBJECT: GRB 920925C: optical afterglow candidate
DATE:    07/02/27 21:22:28 GMT
FROM:    Denis Denissenko at IKI, Moscow  <denis at hea.iki.rssi.ru>

GRB 920925C: optical afterglow candidate

D. Denisenko, O. Terekhov (Space Research Institute, Moscow, Russia) report:

The entire error circle of bright long GRB 920925C detected by WATCH
instrument onboard GRANAT satellite was serendipitously observed by
1.2-m Oschin Schmidt camera in the course of the second Palomar
Observatory Sky Survey (POSS-II) 0.25 day after the burst.  The event
triggered WATCH detector at 22:46:24 UT.  It was localized to the circle
with the center at R.A.=330.80, Dec=25.48 deg (22h03m12s, +25o28'48",
Epoch J2000.0) and 3-sigma error radius 0.34 deg, or 0.39 deg including
systematic error (Sazonov et al., 1998).

Blue Palomar plate SJ04837 (A1F7, region name XJ532) with 55-min
exposure was taken on 1992 Sep. 26 between 04:51-05:46 UT (6.1-7.0 hr
after the burst).  The WATCH position of GRB 920925C lies 0.31 deg from
the plate center.

Inside the error circle we found a star-like object of blue magnitude
17.8 at the following coordinates (J2000.0):

R.A. = 22 03 31.26, Dec = +25 25 01.4 (330.8802 deg, +25.4171 deg)

This is 0.096 deg from the center of WATCH error circle.  The optical
transient also falls inside the Ulysses-WATCH IPN annulus (Hurley et
al., 2000) within 0.058 deg from its center.  Half-width of IPN annulus
is 0.086 deg (3-sigma).

No stellar object is present at this position on blue and red Palomar
plates taken with the same telescope in August 1950, photovisual plate
of October 1983, red plate of October 1990 and IR plate of July 1995.
However, the extended object northwest of the transient position can be
seen at the detection limit on the infrared plate.

The images showing the possible optical transient are available at
http://hea.iki.rssi.ru/~denis/GRB920925C.html

No objects are present at the position of the optical transient in
Simbad or NED databases.  No known minor planets were located nearby at
the time the plate was taken.  Nothing can be seen on the 2MASS images
of November 1999 and on NEAT images of August, September and November
both in 2000 and 2001.

We conclude that the object on Palomar plate is most probably the
optical afterglow of GRB 920925C.  The follow-up observations are
necessary to confirm the presense of an extended object and to measure
its redshift if this is indeed the host galaxy of GRB 920925C.  The
object is currently near conjunction with the Sun and will become
visible later in the spring.

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