Planned NuSTAR monitoring campaign of the magnetar candidate EP J223759.5+531421 (EP260628c)

ATel #17904; G. Younes (UMBC/NASA-GSFC)
on 17 Jul 2026; 19:05 UT
Credential Certification: George Younes (gyounes@umbc.edu)

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We report the approval of a NuSTAR Director Discretionary Time monitoring campaign targeting the newly discovered magnetar candidate EP J223759.5+531421 (EP260628c).

EP260628c was discovered in outburst by the Einstein Probe Wide-field X-ray Telescope on 2026 June 28 (Yang et al., ATel #17859). Subsequent Einstein Probe and NuSTAR observations revealed coherent X-ray pulsations at a period of 5.99 s, a strongly energy-dependent double-peaked pulse profile, and a broadband spectrum comprising two thermal components and a hard power-law tail extending to approximately 30 keV (Rea et al., ATel #17870). A search with the upgraded GMRT detected neither periodic radio emission nor isolated radio bursts (Maan et al., ATel #17872).

The approved NuSTAR program consists of eleven approximately 5 ks observations, for a total exposure of 55 ks. The observations are planned at a cadence of approximately one epoch every two weeks, beginning in the coming week and continuing through approximately December 2026, subject to NuSTAR scheduling constraints.

The primary objective is to measure pulse times of arrival and construct a phase-coherent timing solution over the campaign. These observations will enable measurements of the source spin evolution and timing noise and a search for any outburst-associated glitch and subsequent recovery. The source relatively high Galactic latitude may indicate an older magnetar, making its post-outburst timing evolution a particularly valuable comparison with the generally younger magnetars for which dense timing campaigns have previously been obtained. The data will also track the evolution of the hard X-ray flux and energy-dependent pulse morphology, including searches for pulse-peak migration on week-to-month timescales.

High-cadence magnetar timing campaigns have historically relied on short Swift and NICER observations. With both facilities currently unavailable for science observations, NuSTAR provides the only present opportunity to the broader community to establish this timing baseline. This program makes particular use of the NuSTAR newly available capability to execute short observations efficiently, hence, enabling such a monitoring campaign.

We encourage interested members of the community to make use of this unique data set and to pursue complementary multiwavelength monitoring.

We thank the NuSTAR PI and Science Operations Center for approving and scheduling this campaign.