[vsnet-grb-info 10709] GRB 110709B: Gemini-South additional analysis and photometry

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12132
SUBJECT: GRB 110709B: Gemini-South additional analysis and photometry
DATE:    11/07/10 02:10:22 GMT
FROM:    Edo Berger at Harvard  <eberger at cfa.harvard.edu>

E. Berger (Harvard) reports on behalf of a larger collaboration:

"Additional inspection and photometry of our Gemini-South r-band image (GCN
12128) reveals a second, fainter source (S2) at the following coordinates
(J2000):

RA = 10:58:36.955
DEC = -23:27:20.62

with an uncertainty of about 0.8" (due to centroid uncertainty).  This
position is consistent with the faint source detected by GROND (GCN 12129).

Photometry of the two sources using the Gemini-South/GMOS standard
photometric zero-point yields the following magnitudes:

S1:  r = 21.28+/-0.05 mag ("DSS" source)
S2:  r = 22.18+/-0.12 mag

Taken at face value, the brightness of S2 is consistent with the GROND value
(r=22.15+/-0.15 mag; GCN 12129), suggesting that S1 has actually faded by
about 0.3+/-0.15 mag between 1 and 3.2 hours after the initial trigger.

Alternatively, if the offset relative to the GROND photometry of S1
(r=20.9+/-0.1 mag) is due to a zero-point offset, the resulting brightness
of S2 is r = 21.9+/-0.1 mag.  This is marginally consistent with the GROND
value (r=22.15+/-0.15 mag) which was obtained about 2.2 hours earlier than
our observations.

To determine which scenario is correct relative to the GROND photometry, we
measure the brightness of the star at:

RA = 10:58:38.053
DEC = -23:26:31.78

to be r = 18.25 mag."