Low frequency upper limits on continuum radio emission from EP J223759.5+531421 (EP260628c)

ATel #17892; Pulak Mohapatra (NCRA - TIFR, India), Yogesh Maan (NCRA - TIFR), Yash Bhusare (NCRA - TIFR), Chandreyee Maitra (IUCAA, MPE), Joeri van Leeuwen (ASTRON), Banshi Lal (NCRA - TIFR)
on 15 Jul 2026; 09:37 UT
Credential Certification: Yogesh Maan (maan@astron.nl)

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EP J223759.5+531421 (EP260628c) was recently discovered with the Wide-field X-ray Telescope which was further localized to 10 arcseconds precision by the Follow-up X-ray Telescope, both on board the Einstein Probe, where a tentative 3 second periodicity was seen (Yang et al., Atel #17859). Further follow up observations revealed a 5.995872(2)s periodic pulsed X-ray emission, which, along with the X-ray outburst, pulse-profile and spectral properties, confirmed it to be a magnetar candidate (Rea et al., Atel #17870).

We conducted observations of EP260628c using Band-4 (550 - 750 MHz) and Band-5 (1060- 1460 MHz) of the upgraded Giant Metrewave Radio Telescope (uGMRT). We previously reported non-detection and subsequent upper limits on the pulsed emission at 650 MHz (Maan et al., Atel #17872). Here we present the upper limits on the continuum emission from both the bands and on the pulsed emission from Band-5 observation.

We had a total of 1 hr 10 minutes and 1 hr 23 minutes of on-source observing time in Band-4 and Band-5 respectively. We carried out the radio imaging by using a combination of automated imaging using CAPTURE (R. Kale & C. H. Ishwara-Chandra 2020) with manual intervention, wherever necessary. Since the source is near the galactic plane, and there are multiple bright extended sources in the field, the image has some low-level systematic artifacts. However, the upper limits presented here are unlikely to be affected by these low-level systematic artifacts. We also cross matched the positions of the bright sources with the available data from NASA/IPAC Extragalactic Database (NED), and find that the positions are consistent within the synthesized beam size (i.e., the resolution of the radio image). We used Briggs weighting with a robust parameter of 0.5 so that the image has optimum sensitivity along with minimal artifacts.

We obtain RMS noise of 24.0 micro-Jy/beam and 16.9 micro-Jy/beam in Band-4 and Band-5, respectively. Given the low-level systematics in the image, we place conservative, 5-sigma upper limits of 120 micro-Jy/beam and 84.5 micro-Jy/beam, at Band-4 and Band-5, respectively.

The final images are accessible at the link provided below. The image in the left subplot is the Band-5 image and the one on the right is the Band-4 image. The yellow dashed circle represents the 10" localization uncertainty region. The closest detected source is ~55" away from the target's best position. The synthesized beam size is denoted by the gray ellipse at the bottom left of both the images. The colorbar represents the flux density in units of Jy/beam.

We also conducted the search for any pulsed radio signals at or around the period of the magnetar candidate as well as for any bright, isolated bursts, using the time-series data. The procedure followed for these searches was similar to that used for the Band-4 data (Atel #17872). No genuine pulsed signal was found, and we place 10-sigma upper limits of 65 micro-Jy on periodic emission (assuming 10% pulse duty cycle) and 100 mJy on any 10 ms wide bright pulses.

Given these upper limits, the pulsed radio emission from the magnetar candidate is expected to be either very faint, or the onset of radio activity has not started yet. Our upper limits on the continuum emission also constrain the presence of any persistent emission counterpart, e.g., an associated magnetar wind nebula or radio emission from large plasma outflows.

We would like to thank the scheduling team and the observatory for the prompt scheduling of our observations. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.

References:

Camilo, F., et al. 2006, Nature, 442, 892.

Kale, R., & Ishwara-Chandra, C. H. 2020, ExA, 51, 95.

NASA/IPAC Extragalactic Database (NED) 2019, NASA/IPAC Extragalactic Database (NED), IPAC, doi:10.26132/NED1