Chandra identifies M15 X-2 as the source currently in outburst in M15
ATel #15640; J. Homan (Eureka Scientific), M. van den Berg (Harvard/CfA), C. Heinke (University of Alberta), D. Pooley (Trinity University, Eureka Scientific)
on 2 Oct 2022; 07:27 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)
Subjects: X-ray, Globular Cluster, Transient
X-ray emission from the globular cluster M15 has increased by a factor of 3 over the average, since late August 2022 (ATel #15586). M15 hosts two persistent X-ray binaries (AC 211 and M15 X-2; White & Angelini 2001, ApJ, 561, 101) and at least one transient X-ray source (M15 X-3; Heinke et al. 2009, ApJ, 692, 584). Observations with Swift/XRT on September 3 yielded an X-ray position close to M15 X-2 (2.3 arcsec offset) and AC 211 (2.6 arcsec offset), but the large error radius (2.4 arcsec, 90% confidence) meant that the source of the outburst could not be identified unambiguously (ATel #15589). NICER observations of the current outburst revealed a type-I X-ray burst (ATel #15593), which suggests that M15 X-2 is the source of the outburst, since AC 211 has never shown such bursts before. On the other hand, radio observations of M15 on September 6 only detected a single source, consistent with AC 211 (ATel #15604), suggesting that AC 211 might be the source of the X-ray outburst.
To investigate the source of the current X-ray outburst we observed M15 with Chandra/ACIS-S for 9.3 ks, starting on September 27 at 05:06 UTC. Since NICER observations indicated that the source was still bright, the High Energy Transmission Grating was inserted to reduce the effects of pile-up. Two sources are detected in the zeroth order image, with positions consistent with those of AC 211 and M15 X-2. The broader point-spread-function wings of M15 X-2 suggest that this source is the brighter of the two, but the effects of pile-up result in a lower peak-count rate in the center of the point-spread-function. Inspection of the first order HEG spectra, which are not affected by pile-up, shows that M15 X-2 has a ~3.3 times higher count rate than AC 211. We therefore conclude that M15 X-2 is the source that is currently in outburst.
We thank the Chandra staff for their help in executing this observation.
