[vsnet-grb-info 7657] GRB090418A Rc-band photometry

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9211
SUBJECT: GRB090418A Rc-band photometry
DATE:    09/04/23 19:03:44 GMT
FROM:    Arne A. Henden at AAVSO  <arne at aavso.org>

A. Henden (AAVSO), J. Gross (SRO), B. Denny (DC-3),
D. Terrell (SwRI), and W. Cooney (SRO) report:

We obtained photometry of the GRB090418A afterglow
reported by Chornock et al. (GCN 9148) using the Sonoita
Research Observatory (SRO) 35cm telescope in southern
Arizona, utilizing an automatic VOevent trigger.  The
Rc-band exposures began about 2 minutes after the burst
and continued until twilight.  Ten 60-second and five
180-second exposures were acquired.

Astrometry of the afterglow from several images and
using UCAC as the reference catalog yields coordinates:
17:57:15.151 +33:24:20.93 J2000 (+/- 50mas)

Photometry, assuming that the star at
17:57:42.53 +33:25:47.2
has an R magnitude of 13.12

UT(mid)  delT  exp  Rc      err
11.1728   162  60  15.802  0.044
11.1919   231  60  16.185  0.048
11.2111   300  60  16.500  0.043
11.2303   369  60  16.818  0.058
11.2494   438  60  17.019  0.066
11.2686   507  60  17.249  0.067
11.2878   576  60  17.440  0.063
11.3069   645  60  17.605  0.105
11.3261   714  60  17.680  0.089
11.3450   782  60  17.855  0.115
11.3958   965 180  18.007  0.104
11.6522  1888 180  18.778  0.129
11.7067  2084 180  18.919  0.138
11.7592  2273 180  18.960  0.142
11.8114  2461 180  18.891  0.196

Where delT is the time in seconds from the burst (Mangano et al.,
GCN 9149), and the exposure is in seconds. The optical afterglow
candidate is in the filter reflection halo from HD163948,
a 6.9mag star about 2arcmin distant, so the last
few magnitudes may have a systematic bias.

We note that this field was not calibrated by SDSS through DR6.
We will perform a BVRI calibration over the next few nights.