Accurate position and spin-down measurement of the magnetar candidate EP J223759.5+531421

ATel #17909; F. Coti Zelati (ICE-CSIC), G. L. Israel (INAF), N. Rea, Y. L. Wang (ICE-CSIC), A. Borghese (ESA), H. Yang, X. Mao, C. Jin, H. Sun, W. Yuan (NAO, CAS), C. Dai (NJU), H. Feng, L. Tao (IHEP, CAS), L. Lin, Y. Fu (BNU), D. Gotz, E. Arrigoni (CEA, Saclay), A. Coleiro (APC), S. Guillot (IRAP), Z. Li (QUT), B. Zhang (HKU), J.-U. Ness (ESA), S. Mereghetti, A. Possenti, M. Burgay, M. Pilia (INAF), P. Esposito, A. Geminardi, A. Tiengo (IUSS Pavia), R. Taverna, R. Turolla (U. Padova), S. Zane (MSSL), M. Imbrogno, D. De Grandis, A. Marino (ICE-CSIC), A. K. H. Kong (NTHU), C. Maitra (IUCAA, MPE), C.-Y. Ng (HKU) on behalf of the Einstein Probe collaboration and a larger follow-up collaboration
on 20 Jul 2026; 08:27 UT
Credential Certification: Francesco Coti Zelati (cotizelati@ice.csic.es)

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We have been monitoring the magnetar candidate EP J223759.5+531421 (Yang et al., ATel #17859; Rea et al., ATel #17870; Gotz et al., #17885) with the Einstein Probe FXT (11 observations), NuSTAR (2 observations), XMM-Newton (one observation) and SVOM (3 observations) between 2026 June 29 and July 18 (MJD 61220.56 - 61239.68).

Our XMM-Newton observation provided an updated source position of:

RA (J2000.0) = 22:37:59.4

Dec (J2000.0)= +53:14:22.7

with an uncertainty of ~2'', ruling out all the near-infrared sources observed in the deep GTC field as possible counterparts (Coti Zelati et al., ATel #17885).

The X-ray flux is declining very slowly, decreasing from approximately 7e-11 to 4e-11 erg s^-1 cm^-2 over the 0.5-10 keV energy range between 2026 June 29 and July 18.

We also report a preliminary phase-coherent X-ray timing solution derived from observations acquired between 2026 June 29 and July 10 (MJD 61220.56 - 61231.65). The best-fitting phase-connected solution, referenced to the epoch 61220.0 BMJD(TDB), is:

spin period P = 5.9958723(4) s,

spin period derivative Pdot = 5(1) x 10^-12 s/s.

Uncertainties in parentheses correspond to the 1sigma confidence level. The fit yields a chi^2 value of 16.2 for 15 degrees of freedom and a post-fit rms residual of 13 ms. We note that our measured spin period derivative is approximately one order of magnitude larger than the upper limit reported in ATel #17908.

While the measured period derivative may not represent the long-term spin-down because of timing noise or torque variability, its nominal value implies an equatorial dipolar magnetic-field strength of approximately 2x10^14 G. This confidently places EP J223759.5+531421 among the population of highly magnetized neutron stars with a magnetar-like dipolar magnetic field. The recent detection of short X-ray bursts from the source (ATel #17908) further supports its classification as a magnetar.

Additional X-ray observations are planned over the next months to characterize the spin-down evolution and the source timing noise. Specifically, we will continue monitoring the source with Einstein Probe with a cadence of approximately one observation every other day; further NuSTAR observations from our anticipated GO program will be scheduled soon to follow the spectral evolution of the hard X-ray emission; XMM-Newton observations will be scheduled once the magnetar becomes too faint for Einstein Probe monitoring, allowing us to follow its evolution into quiescence.

Launched on 2024 January 9, Einstein Probe is a space-based X-ray observatory designed to monitor the soft X-ray sky and provide rapid X-ray follow-up observations (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). Einstein Probe is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES. We sincerely thank the NuSTAR and XMM-Newton science operation centres for the prompt scheduling of our observations and for their exceptional commitment, including work over weekends, to ensure the timely follow-up of this interesting transient.