GCN Circular 6664

S. B. Cenko, A. Rau (Caltech), E. Berger (Carnegie/Princeton), P. A. Price
(IfA), and A. Cucchiara (Penn State) report on behalf of a larger
collaboration:

We have imaged the field of GRB070724 (Ziaeepour et al., GCN 6654
Not found
 
 
) on
the night of 2007 July 24 UT with the automated Palomar 60-inch telescope
(beginning approximately 2.5 minutes after the burst) and the Gemini
Multi-Object Spectrograph (GMOS) and Near-Infrared Imager (NIRI) mounted
on the 10-m Gemini North Telescope (beginning approximately 2 hours after
the burst).

The nearby galaxy first mentioned by Bloom (GCN 6658
Not found
 
 
) shows no sign of
variability, either from our P60 to our GMOS imaging (remaining at a
constant magnitude of i' ~ 20.3), or within our Gemini imaging (verified
with psf-matched digital image subtraction).

In addition the aforementioned galaxy, we find a faint source on the
outskirts of the revised XRT error circle (Page and Ziaeepour, GCN 6659
Not found
 
 
)
at coordinates (J2000.0):

   RA: 01:51:14.05
   Dec: -18:35:42.1

with an estimated astrometric uncertainty of approximately 0.3" in each
coordinate.  The object is only visible in our Gemini/GMOS i' imaging of
the field, with a magnitude of i' ~ 24.6 (using GMOS zeropoints from the
Gemini webpage).  This object is not present in our earlier P60 imaging of
the field; however it is well below our detection threshold (see below).
As our only two Gemini/GMOS i' images were taken back-to-back, we have no
information on the variability of this source at the current time.

No other object is detected in the revised XRT error circle in any other
filter.  Our limiting magnitudes, calculated with respect to the USNO-B
catalog (P60) or using zeropoints from the Gemini website (GMOS and NIRI),
are quoted below:

Instrument	Filter	UT (July 24)	dt (hr)	Exposure (s)	Magnitude
---------------------------------------------------------------------------
P60		i'	11:13		0.32	3x60 + 3x120	> 21.5
GMOS		i'	13:17		2.38	2x180		> 24.5
NIRI		Ks	13:44		2.83	15x60		> 20.0
NIRI		J	14:02		3.13	15x60		> 22.0
NIRI		H	14:19		3.42	15x60		> 21.0
NIRI		Ks	14:36		3.70	15x60		> 20.0