Einstein Probe detects significant pulsations from AMXP MAXI J1957+032 during its 2025 outburst

ATel #17279; Zhaosheng Li (Xiangtan University), L. Kuiper (SRON), Y. Y. Pan (Xiangtan University), R. X. Xu (PKU), L. Zhang (IHEP), S. N. Zhang (IHEP), S. Zhang (IHEP), J. L. Qu (IHEP), L. M. Song (IHEP), Y. P. Chen (IHEP), S. M. Jia (IHEP), M. Y. Ge (IHEP), X. B. Li (IHEP), S. J. Zheng (IHEP), Y. Huang (IHEP), Y. Chen (IHEP), L. Tao (IHEP), H. Feng (IHEP), W. M. Yuan (NAOC)
on 9 Jul 2025; 01:39 UT
Credential Certification: Zhaosheng Li (lizhaosheng@xtu.edu.cn)

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Einstein Probe (EP) reported a new outburst from the accreting millisecond X-ray pulsar (AMXP) MAXI J1957+032 on May 6, 2025 (GCN #40375; see also ATEL #17170), which lasted for about one week. An EP target of opportunity (ToO) observation of MAXI J1957+032 was promptly triggered, consisting of six observations executed between MJD UTC 60801.88 and 60807.26. We performed a timing analysis of the EP data taken in timing mode (time resolution of 23.68 micro s; 13.79 ks total exposure time) during the time window MJD UTC 60802.73- 60805.47. We used cleaned timing mode events with energies in the range 0.5-10 keV extracted from a 35" radius region. These events were barycenter corrected using the position RA=299.16295 and Dec=+3.44546 (ATEL # 9591) and the JPL DE421 solar system ephemeris. Starting from the orbital parameters and constant spin frequency as given in Sanna et al. (2022) (MNRAS 516(1), L76-L80), we applied a 4d optimization scheme based on a SIMPLEX algorithm (see e.g. De Falco et al. 2017; Li et al. 2021 for more details) using the Z22-test statistics. The orbital solution assuming a circular orbit is described by an orbital period of 3652.95(43) s, a projected semi-major axis ax*sin(i) of 0.013 85(12) lt-s, a time of ascending node Tasc of 60802.746 344(60) MJD (TDB), while a spin frequency of 313.643 736 8(7) Hz is obtained. Uncertainties are all given at 1 sigma confidence level. After correction for the orbital motion we folded the 0.5-10 keV events upon the optimum spin frequency and obtained an asymmetric double peaked pulse profile (similar to the NICER profile for the 2022 outburst) at a significance of 12.9 sigma. We also searched for the timing signal in NICER data taken between May 11-19, 2025, but we could not detect the pulsed signal in these NICER observations, very likely, because the source was already in the OFF state (ATEL #17187) during these observations. To our knowledge, EP is the only instrument that detects the pulsations from MAXI J1957+032 during its 2025 outburst, highlighting its potential timing capabilities during its operation.