[vsnet-grb-info 4710] GRB070521: Second Epoch Gemini Imaging

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6457
SUBJECT: GRB070521: Second Epoch Gemini Imaging
DATE:    07/05/23 07:21:00 GMT
FROM:    S. Bradley Cenko at Caltech  <cenko at srl.caltech.edu>

S. B. Cenko, E. O. Ofek (Caltech) and P. A. Price (IfA) report on behalf
of a larger collaboration:

We have re-imaged the field of GRB070521 (Guidorzi et al., GCN 6431) with
the Gemini Multi-Imaging Spectrograph mounted on the Gemini North
telescope.  We obtained 5 x 180 s in the Sloan i' filter at a mean epoch
of 10:23:38 UT 22 May.

The host candidate (S1) identified by Hattori et al. (GCN 6444; see also
Perley et al., GCN 6451) is well detected in our second epoch.  In
comparison with our first epoch of i' imaging (GCN 6450), we find
marginal evidence for fading of this source.  Specifically, we measure the
following magnitudes, with calibration performed relative to SDSS field
stars (Cool et al., GCN 6432):

t-tb (hr)		Magnitude
----------------------------------------------------------------
1.21			24.07 +\- 0.17
27.54			24.30 +\- 0.09

However, PSF-matched image subtraction using the ISIS software package
(Alard & Lupton, ApJ, 503, 1998) reveals no variability at the location of
this source.

We do not find any new sources inside the revised XRT error circle
(Guidorzi et al., GCN 6452) in either epoch of imaging, to limits of i' >
24.0 (epoch 1) and i' > 25.0 (epoch 2).

The source S3 is marginally detected in our second epoch near the
detection limit.  This would represent a decline from measurements
reported previously (I = 24.39; Perley et al., GCN 6451).  However, we
caution our photometry at this location is affected by a variable
background from the nearby bright galaxy and is therefore quite
uncertain.  We encourage further observations to establish the nature of
this object.