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2011ATel.3363....1H - The Astronomer's Telegram, 3363, 1 (2011/May-0)
Swift/XRT measurement of position and X-ray spectrum of M 15 X-ray binary.
HEINKE C.O., SIVAKOFF G.R., MORII M. and KUULKERS E.
Abstract (from CDS):
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 NH=2.2{+0.7}_{-0.4}*10^{21} cm^{-2}, photon index 2.0^{+0.1}_{-0.2}, and 0.5-10 keV LX=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.
Abstract Copyright: ∼
Journal keyword(s): Request for Observations - Binary - Globular Cluster - Neutron Star - Transient
Simbad objects: 5
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