2011ATel.3363....1H

Following the MAXI/GSC detection of a recent flux increase and two X-ray bursts from the position of the globular cluster M15 (Atel #3356), a Swift XRT observation was conducted May 18 at 03:44 UT for 1.0 ks. The XRT data are heavily piled up, and affected by a bad column. We find the position by fitting a circle to the pileup ring, estimating a center at RA, Dec (J2000)=21:29:57.9, +12:10:06 with a 90% conf. positional uncertainty of about 5". The position is consistent with the bright LMXBs AC 211 or M15 X-2 (White Angelini 2001, ApJ, 561, L101), which are located in the core of M15; however, it is inconsistent with the very faint transient M15 X-3 (Heinke et al. 2009, ApJ, 692, 584).

We extracted a spectrum from a 8-30 pixel annulus. It is reasonably fit (reduced chi-squared of 1.08 for 61 degrees of freedom) by an absorbed power-law and multicolor disk blackbody (diskbb). We find N_H=2.2{+0.7}_{-0.4}*10^{21} cm^{-2}, photon index 2.0^{+0.1}_{-0.2}, and 0.5-10 keV L_X=1.4{+0.3}_{-0.2}*10^{37} ergs/s (assuming a 10.3 kpc distance). The disk has kT=0.23^{+0.07}_{-0.05} keV, with inner disk radius 100^{+150}_{-50} km/sqrt(cos(theta)), and composes 13-32% of the total 0.5-10 keV flux. A partial covering absorber (as commonly used to fit AC 211's accretion-disk-corona spectrum) does not improve the fit. This source is now over ten times more luminous than White et al. (2001) reported for either M15 source. RXTE/ASM detected only one similarly bright outburst from M15, in 2007.

It is not immediately obvious from this X-ray spectrum which X-ray binary is responsible for this outburst. An increased rate of X-ray bursts from M15 (Atel #3356) coincident with the flux tentatively rules out AC 211. The photosphere of the neutron star in AC 211, from which X-ray bursts arise, is thought to be obscured by the accretion disc. A factor of >10 flux variation arising from M15 X-2 would be surprising, as ultracompact X-ray binaries such as M15 X-2 (22 minute period, Dieball et al. 2005, ApJ, 634, L105) are generally not expected to show such large variations. Maccarone et al. (2010, MNRAS, 406, 2087) describe a counterexample in the globular cluster NGC 1851. A new transient X-ray binary in the core of M15 is also a possibility. We encourage follow-up observations at all wavelengths.

We thank J. Kennea and the Swift team for their quick scheduling.