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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; 4:52 UT
Password Certification: Weidong Li (weidong@astron.berkeley.edu)

Subjects: Optical, Globular Clusters, Neutron Stars, Transients, 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


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