SRG/ART-XC confirmation of magnetar nature of EP J223759.5+531421 (EP260628c)

ATel #17908; S. Molkov (IKI RAS), A. Lutovinov (IKI RAS), A. Tkachenko (IKI RAS), Z. Li (QUT), I. Lapshov (IKI RAS)
on 20 Jul 2026; 05:10 UT
Credential Certification: Sergei Molkov (molkov@hea.iki.rssi.ru)

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The transient source EP260628C was discovered by the Einstein Probe Wide-field X-ray Telescope on June 28, 2026 (Yang et al., ATel #17859). Later, coherent X-ray pulsations with a period of 5.99 s were revealed by the Einstein Probe and NuSTAR observatories (Rea et al., ATel #17870). Based on its spectral and temporal properties, it was hypothesized that the source might belong to the class of Magnetars.

The Mikhail Pavlinsky ART-XC telescope on board the SRG observatory started a series of pointed observations of EP260628c. To date four observations have been conducted, starting on July 3, 6, 9 and 15 with exposures of 33, 105, 255 and 121 ks, respectively. During this period the source flux in the 4-12 keV energy band linearly decreased from 2.2e-11 erg/cm2/s (July 3) to 1.4e-11 erg/cm2/s (July 16). The averaged energy spectra in all four observations can be adequately approximated by a composite model consisting of two blackbody and power-law components (see also Rea et al., ATel #17870).

We clearly observe pulsations across the entire operating energy range of the ART-XC instrument (4-30 keV). Using a phase-connected analysis method, we determined the pulse period to be P = 5.995876(2) (1-sigma statistical error plus systematic uncertainty), which is consistent with the results obtained by Rea et al. (ATel #17870). We also estimated an upper limit for the first derivative of the pulsation period: modulus |Pdot|<4e-13 s/s.

During our observations, we detected seven short bursts from this source, lasting approximately 20-40 ms; this provides strong evidence that the object EPJ223759.5+531421/EP260628c is indeed a magnetar. The peak flux over a 10 ms interval during the bursts reached values 1.5e-8 -- 4.5e-8 ergs/s/cm2.

ART-XC will continue the monitoring campaign of this magnetar.

ART-XC is an X-ray telescope on board the SRG observatory. The telescope was designed and built by IKI (Russian Academy of Sciences) and RFNC, with participation from MSFC/NASA. The SRG observatory was launched from the Baikonur Cosmodrome on July 13, 2019 by a Proton rocket. The telescope was subsequently renamed the 'Mikhail Pavlinsky ART-XC telescope' in honor of its first PI, Dr. Mikhail Pavlinsky.