D. B. Fox (Penn State) and S. B. Cenko (Caltech) report on behalf of a
"The robotic Palomar 60-inch telescope (P60) has imaged the
localization region of GRB060501 (Swift trigger 208050; La Parola et
al., GCN 5040) in a series of exposures beginning 08:48 UT (33 minutes
after the burst). We have compared our i-band images to the
second-epoch digitized sky survey (XDSS) in a search for new sources
consistent with the XRT localization (La Parola et al., GCN 5042). We
estimate the XDSS limit in this region, by reference to USNO-B1.0
catalog photometry, as I~19.5 mag. We note that due to the location
of this field close to the Galactic plane, the background stellar
density is large and the estimated Galactic extinction (E_(B-V)~0.3
mag) is substantial. In particular, there are three star-like objects
in XDSS consistent with the XRT localization.
We find no new i-band sources within or near the XRT localization, to
a limit of I~19 mag in a coaddition of 4x60s images with mean epoch
08:58 UT (43 minutes after the burs).
A deeper coaddition of 5x150s images with mean epoch 09:47 UT (92
minutes after the burst) reveals no new source within the XRT
localization to I~21 mag. However, we do identify a single new
point-like source with I~19.3 mag, 8.1" from the center of the XRT
localization, at coordinates:
R.A. 21:53:29.52, Dec. +43:59:49.9 (J2000)
(estimated uncertainty of 0.3" in each coordinate). There is no
evidence of this source in XDSS survey images.
The source is not resolved from a neighboring star in our earlier
images, which have poorer seeing; however, these images do constrain
the object to not be significantly brighter at 43 minutes post-burst
than at our first detection, 92 minutes post-burst.
Given the absence of evidence for variability, and the distance of the
source from the XRT position, we consider the association of this
source with GRB060501 tentative at this point."