Limit on the brightness of the optical counterpart of the new LMXB in the globular cluster NGC 6440
ATel #2144; Weidong Li, Ryan Chornock, and Alexei V. Filippenko (University of California at Berkeley)
on 4 Aug 2009; 04:52 UT
Credential Certification: Weidong Li (weidong@astron.berkeley.edu)
Subjects: Optical, Globular Cluster, Neutron Star, Transient, Variables
We report on optical observations of the new LMXB in the globular
cluster NGC 6440 announced by Heinke & Budac (2009, Atel
2139) and Heinke et al. (2009, Atel 2143). Observations were performed
with the 0.76-m Katzman Automatic Imaging Telescope (KAIT)
at Lick Observatory on Jul 30 and 31 UT in the unfiltered mode.
A series of 30 s exposures was obtained and stacked, yielding
a total exposure of 180 s on Jul 30 and 300 s on Jul 31.
As part of our search for transients in globular clusters, we have
obtained unfiltered images of NGC 6440 in the years 2007 and 2008.
We have selected and stacked the best images to generate a template
image (with a total exposure time of 144 s). We used an image subtraction
technique in an attempt to identify the optical counterpart of the
new LMXB. The results on the stacked Jul 30 image can be viewed at http://astro.berkeley.edu/~bait/lmxb-ngc6440.jpg.
386 stars in the USNO B1 catalog were used to secure an astrometric
solution to our images with an uncertainty of about 0.4". The
reported position of the LMXB in Atel 2139 is marked as red circles
with a radius of 4".
No apparent optical transient is detected at the nominal position
of the LMXB on either the Jul 30 or the Jul 31 stacked images.
The typical limiting magnitude of these stacked unfiltered KAIT
images is about 20.5, but in a crowded environment such as NGC 6440,
the limiting magnitude is probably around 19.5-20.0.
The subtracted image shows several stars with positive/negative
residuals, likely caused by RR Lyrae or other kinds of variable
stars in NGC 6440. The brightest such residuals are marked as
"1" and "2" in the finder chart, with the following positions:
Star 1, R.A. = 17:48:52.63, Decl. = -20:20:56.5 (J2000), mag 12.5
Star 2, R.A. = 17:48:52.40, Decl. = -20:21:38.9 (J2000), mag 13.5
