GTC/EMIR near-infrared observations of the magnetar candidate EP J223759.5+531421

ATel #17877; F. Coti Zelati, N. Rea, Y. L. Wang, A. Marino, M. Imbrogno (ICE-CSIC), H. Yang, C. Jin, H. Sun, W. Yuan (NAO, CAS), H. Feng, L. Tao (IHEP, CAS), B. Zhang (HKU), J.-U. Ness, A. Borghese (ESA), S. Guillot (IRAP) on behalf of the Einstein Probe collaboration and a larger follow-up collaboration
on 6 Jul 2026; 13:36 UT
Credential Certification: Francesco Coti Zelati (cotizelati@ice.csic.es)

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Following the discovery of the X-ray transient EP260628c by Einstein Probe (Yang et al., ATel #17859) and its subsequent identification as a new magnetar candidate in outburst, EP J223759.5+531421 (Rea et al., ATel #17870), we performed near-infrared imaging observations of the field with EMIR on the 10.4-m Gran Telescopio Canarias.

The observations were carried out on 2026 July 4 between 03:49 and 04:46 UTC. Dithered J-, H-, and Ks-band images were acquired with total effective integration times of 195 s, 325 s, and 1200 s, respectively. The measured stellar FWHM in the final stacked images was 0.84 arcsec in J, 0.78 arcsec in H, and 0.77 arcsec in Ks.

Photometric calibration was performed using isolated UKIDSS DR11+ (Lawrence et al. 2007, MNRAS, 379, 1599) field stars, including corrections for both instrumental-magnitude and J-K color terms.

We detect three point sources within the 90% confidence X-ray localization region (radius 10 arcsec) reported in ATel #17859:

 
Source   Offset     J                 H                 Ks 
         (arcsec)   (mag)             (mag)             (mag) 
---------------------------------------------------------------- 
1        3.80       18.33 +/- 0.04    17.63 +/- 0.06    17.37 +/- 0.08 
2        4.65       19.47 +/- 0.07    18.85 +/- 0.07    18.51 +/- 0.09 
3        9.04       17.89 +/- 0.04    17.19 +/- 0.06    16.95 +/- 0.08 

All magnitudes are in the Vega system. Each source is positionally coincident with a previously catalogued UKIDSS DR11+ source. Sources 1 and 3 are also listed in Gaia DR3 and Pan-STARRS, with Gaia astrometric and photometric properties consistent with ordinary Galactic field stars. Comparison with the archival UKIDSS JHK photometry reveals no magnitude differences exceeding 2-sigma for any of the three sources. We therefore find no evidence that any of them is currently in outburst, making them unlikely to be the near-infrared counterpart of EP J223759.5+531421.

Within the X-ray localization region, the local 3-sigma limiting magnitudes are J > 21.5, H > 21.4, and Ks > 21.1 mag (Vega), estimated from blank apertures matched in size to those used for the source photometry. Combining these limits with the current unabsorbed X-ray flux of EP J223759.5+531421, we constrain its extinction-corrected X-ray-to-near-infrared flux ratio to be >10^4. This lower limit is consistent with the range of values reported for known magnetars.

We thank the GTC staff for their prompt support and execution of these observations.