Chandra HRC-I detection of a possible very faint X-ray transient in the globular cluster M15

ATel #1211; Craig O. Heinke (U. Virginia / Northwestern U.), Haldan N. Cohn and Phyllis M. Lugger (Indiana U.)
on 11 Sep 2007; 13:06 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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We have recently identified an X-ray source at L_X=6*10³³ ergs/s in archival (2004) Chandra HETGS zeroth-order images of the globular cluster M15. Through June and July 2007, the RXTE ASM has observed M15 to reach luminosities of 50 mCrab (~4 times normal), which is the highest observed by RXTE since 1996 (from ASM/RXTE teams at MIT and GSFC). We obtained a 2.1 ks Chandra HRC-I observation of M15 on Sept. 5, 2007 to attempt to identify whether another source (besides the two known bright LMXBs) was responsible for the outburst, and if so, to catch it as it faded.

We see the new transient, hereafter M15 X-3, at
RA=21:29:58.16,
Dec=+12:09:40.1
The position is boresighted to Kulkarni et al.'s (1990, ApJ, 363, L5) position for AC211, 23" away. The count rate is 0.015 counts/s, corresponding to L_X(0.5-10 keV)=4.5*10³³ ergs/s for a similar spectrum as seen in 2004 (power-law with index 1.5, column density N_H=6*10²⁰ cm^-2), and a distance of 10 kpc. This object is barely detected in combined Chandra HRC-I imaging in 2001 at L_X~2-6*10³¹ ergs/s. We identify a faint optical counterpart in archival 1994 (U₃₃₆) and 2002 (V₅₅₅) HST WFPC2 imaging, at V₅₅₅~22 and U₃₃₆~21.

The faintness of the optical counterpart to M15 X-3 and its 100-fold variation in X-ray flux suggest a neutron star, rather than cataclysmic variable, interpretation. We are not certain whether this object is responsible for the recent increase in M15's ASM count rates; if it is not, it is likely a member of the class of very faint X-ray transients (e.g. Wijnands et al., 2006, A&A, 449, 1117). We encourage follow-up observations of this source at other wavelengths. We thank the Chandra DDT committee for granting time for this observation.