Einstein Probe discovery of a new magnetar candidate in outburst: EP J223759.5+531421 (EP260628c)

ATel #17870; N. Rea, F. Coti Zelati, Y. L. Wang (ICE-CSIC), H. Yang, X. Mao, C. Jin, H. Sun, W. Yuan (NAO, CAS), C. Dai (NJU), H. Feng, L. Tao (IHEP, CAS), L. Lin (BNU), Z. Li (QUT), B. Zhang (HKU), J.-U. Ness, A. Borghese (ESA), G. L. Israel, A. Possenti, M. Burgay, M. Pilia (INAF), P. Esposito, A. Geminardi (IUSS Pavia), M. Imbrogno, D. De Grandis, A. Marino (ICE-CSIC), A. K.H. Kong (NTHU), S. Guillot (IRAP), C. Maitra (IUCAA, MPE) on behalf of the Einstein Probe collaboration and a larger follow-up collaboration
on 3 Jul 2026; 12:47 UT
Credential Certification: Nanda Rea (rea@ice.csic.es)

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We report on follow-up X-ray observations of the newly discovered transient EP260628c (Yang et al., Atel #17859), first detected by the Einstein Probe mission. The source, now designated as EP J223759.5+531421, has been observed twice with the Einstein Probe FXT instrument (on 2026-06-29 for 2.5 ks and on 2026-06-30 for 2.2 ks) and once with NuSTAR (2026-06-30 for ~23ks).

Timing analysis of the combined data reveals coherent X-ray pulsations with a period of P = 5.995872(3) s (1-sigma error) referenced to EPOCH = 61222.125540838 MJD (TDB).

The pulse profile is double peaked, which explains the tentative 3s period reported earlier (Yang et al., Atel #17859), and it is highly energy dependent. Broadband spectral analysis of the NuSTAR data, together with the soft X-ray coverage provided by FXT, shows that the spectrum is well described by the sum of two blackbody components (kT_1~0.4keV and kT_2~1 keV) and a power-law tail (Gamma~1.4) extending to an energy of approximately 30 keV. The absorbed 0.3-10 keV flux, extrapolated from the best-fitting model, is ~9x10^-11 erg/cm^2/s. Serendipitous Swift XRT observations in 2017 did not detect the source with an observed flux upper limit of about < 5x10^-13 erg/cm^2/s (3-sigma).

The combination of a slow spin period, X-ray outburst, thermal emission with multiple blackbody components, and a hard X-ray power-law component is characteristic of magnetar emission, supporting the classification of EP J223759.5+531421 as a newly identified magnetar candidate. The position of the source at Galactic latitude b = -4 points possibly to an old magnetar; however, the possibility of a low-mass X-ray binary (i.e. an UCXB like 4U 1626-67) cannot be discarded yet.

Further observations are ongoing with EP, NuSTAR, XMM-Newton and GTC EMIR. Multi-band follow-up observations are highly encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the soft X-ray sky with rapid X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE, and CNES.