Discovery of the radio counterpart of EP J171159.4-333253/EP250623a with MeerKAT

ATel #17258; F. J. Cowie (Oxford), A. K. Hughes (Oxford), R. P. Fender (Oxford, Cape Town), S. Motta (INAF-OAB) for the X-KAT collaboration
on 2 Jul 2025; 12:36 UT
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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ATel #17247 reported the discovery of a new X-ray transient first designated EP250623a. ATel #17255 re-designated the source as EP J171159.4-333253 based on further X-ray observations. ATel #17255 found eclipse-like features in the lightcurve, suggesting the source is an eclipsing X-ray binary system hosting a neutron star.

Following the X-ray trigger, we initiated follow-up radio observations with the MeerKAT radio telescope as part of the X-KAT programme. EP J171159.4-333253 was observed for 15 minutes between 18:19:09.8 and 18:34:05.4 UTC on June 30th, 2025 (MJD 60856.7). Observations were carried out at a central frequency of 1.28 GHz (L-band) with a total bandwidth of 856 MHz. J1939-6342 was used for flux and bandpass calibration, and J1830-3602 for phase calibration. We peeled a bright 0.2 Jy source to the south-west of the target to improve the image quality. The observation had an elliptical (Gaussian) synthesised beam with a full width at half maximum of 9'' x 6'' along the major and minor axes, respectively, and a position angle of 122 degrees east of north.

We detect one radio source within the X-ray uncertainty region of EP J171159.4-333253 (#17247). By fitting an elliptical Gaussian to the source, we measure a flux density of 108 \+/- 20 uJy (i.e., 5.4 sigma above the rms noise level) at 1.28 GHz using the imfit task in CASA. The uncertainty on the flux density was taken to be the RMS of a large emission-free region close to the source. We argue that this source is likely the radio counterpart of EP J171159.4-333253. Assuming the association is correct, we refine the target's position to:

RA(J2000) = 17:11:59.31 +/- 0.06s, Dec(J2000) = -33:32:52.5 +/- 0.6'',

where the uncertainties on position are 1 sigma statistical errors from the fitting results.

We will continue to monitor EP J171159.4-333253 using the MeerKAT radio telescope and Swift. Multi-wavelength observations are encouraged.

X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme contact Rob Fender and Sara Motta. The Swift follow-up of X-ray binaries is largely performed as part of the SwiftKAT program, which provides quasi simultaneous X-ray coverage of the X-KAT targets.