e-MERLIN radio observations of M15

ATel #15604; David Williams (JBCA/Manchester), Nick Wrigley (JBCA/Manchester), Rob Beswick (JBCA/Manchester), Arash Bahramian (Curtin)
on 9 Sep 2022; 15:43 UT
Credential Certification: David Williams (david.williams-7@manchester.ac.uk)

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A new outburst in M15 was reported by MAXI/GSC on 2022 September 3 (ATel #15586), and subsequently followed up with Swift (ATel #15589) and NICER (ATel #15593). In particular, the Swift observations showed that the X-ray emission was likely coming from one of the sources AC 211 or M15 X-2 and the NICER observations detected a Type I X-ray burst from the M15 field, though it is not possible to pinpoint which source it was emitted from.

Upon the discovery of the outburst, we observed the M15 field, centred on the pointing given in ATel #15586, while some antennas in the array were undergoing maintenance or unable to take part. This test observation (TS14001) was performed at a central frequency of 1.5 GHz, had a bandwidth of 512 MHz and lasted from 2022 September 06 14:30:00 UT until 2022 September 07 08:30:00, including 8.4 hours of on source time. 3C286 and OQ208 were used as flux and band pass calibrators, with 2123+1007 used as the interleaved phase reference calibrator. The e-MERLIN CASA Pipeline (Moldon 2021) was used to perform standard flagging, calibration and imaging of the target field, using CASA version 5.6.2. Our observations achieved a synthesised beam size of 0.27 x 0.10 arcsec, with a beam position angle of 24 degrees.

The nearby source NVSS J213000+120609 (3.9 arcmin away) was used as an in-beam calibrator to perform self calibration to improve the image quality of the data. After three rounds of self calibration, we produced a large image approximately 5.5'x5.5' in size, with a cell size of 0.02". This is well within the primary beam of e-MERLIN, which is ~30' at 1.5 GHz.

Our final image achieved an r.m.s. sensitivity of 50 microJy/beam. In the central region of the image, we detect a radio source at position of 21:29:58.311 +012.10.02.64, with an error of 0.03", conservatively estimated as 10% of the synthesised beam. The source has an integrated flux of 0.78+-0.10 mJy, which aligns well with the position of AC 211 (White and Angelini, 2001). This radio flux, at a distance of 10.9 kpc (ATel #15589) would indicate a radio luminosity of ~1.7E29 erg/s at 1.5 GHz. We do not detect a source coincident with M15 X-2, to a 3 sigma noise level of 0.15 mJy/beam.

The previous two outbursts of the central region of M15 were localised to M15 X-2. Radio observations at the time detected both AC 211 and M15 X-2 at 5 GHz flux densities of 276 and 199 microJy/beam, respectively (ATel #3378). As we have not detected M15 X-2 above our 3 sigma 0.15 mJy/beam limit during this outburst but AC 211 is detected at ~3 times the aforementioned radio detection, we suggest that AC 211 may be the source of the X-ray outburst this time. However, higher resolution X-ray observations would need to be made to confirm this.

We thank the e-MERLIN scheduling team for scheduling these observations. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC, part of UK Research and Innovation.