e-MERLIN observations of Swift J1727.8-1613

ATel #16231; David Williams-Baldwin (JBCA/Manchester), Sara Motta (INAF), Lauren Rhodes (Oxford), Francesco Carotenuto (Oxford), Rob Fender (Oxford), Rob Beswick (JBCA/Manchester)
on 6 Sep 2023; 15:18 UT
Credential Certification: David Williams (david.williams-7@manchester.ac.uk)

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The recent discovery of Swift J1727.8-1613 (ATel #16205, #16206), initially reported as GRB 230824A (GCN #34536, #34537) has led to multiple follow-up observations with X-ray and optical facilities (ATel #16207, #16208, #16209, #16210, #16216, #16217, #16219, #16225). Radio observations with the VLA (ATel #16211) detected a radio source consistent with the optical counterpart which has increased in flux density with the ATA (ATel #16228) and recently been detected by the SMA (ATel #16230). Here, we present e-MERLIN radio observations of Swift J1727.8-1613 obtained the day prior to the ATA observations.

As part of a rapid response time (RRT) observation (RR16003, PI: Williams-Baldwin), e-MERLIN observed the field of Swift J1727.8-1613, centered on the VLA co-ordinates (ATel #16211). Six minute scans of the target source were interleaved with two minute scans of the nearby phase calibrator, J1724-1443, for a total of 6 hours. Scans of the flux calibrator 3C286, band pass calibrator OQ208 and point calibrator 3C84 book-ended the observing run. The observation was interleaved into another project and we use a central observing time of 2023-08-29 19:00 (MJD 60185.79), the middle point of the observations for Swift J1727.8-1613 for the observing run. Observations were performed at a central frequency of 5072.5 MHz, with a bandwidth of 512 MHz.

The data were reduced with version 1.1.19 of the e-MERLIN CASA Pipeline (Moldon 2021), using CASA version 5.6.2. The pipeline performs standard flagging and calibration of e-MERLIN radio data, producing quick-look images. A clear point source was visible in the images at the position noted in ATel #16211. To improve the image, we performed four rounds of phase-only self-calibration on the data. In our final phase self-calibrated image, we measure a source flux density of 120±12 mJy. We have included a conservative 10 per cent uncertainty in the flux calibration, which we have added in quadrature to the statistical fitting error. Due to the low declination of the source, our observations have a synthesised beam size of 0.25 x 0.02 arcsec, and a beam position angle of 13 degrees. Further observations are planned with e-MERLIN.

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.