NICER discovers the ultracompact binary orbit of MAXI J1957+032

ATel #15456; P. M. Bult (NASA/GSFC), P. S. Ray (NRL), A. Sanna (University of Cagliari), M. Ng, D. Chakrabarty (MIT), K. C. Gendreau, Z. Arzoumanian, T. E. Strohmayer (NASA/GSFC), H. Negoro (Nihon U.), W. Iwakiri (Chuo U.), T. Mihara (RIKEN), S. Guillot (IRAP/CNRS)
on 23 Jun 2022; 01:30 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)

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Following up on the discovery of a 314 Hz coherent pulsation in the new outburst from the faint X-ray transient MAXI J1957+032 (ATel #15440, #15444), NICER has continued to regularly observe this target. Between 2022-06-19 01:20 UTC and 2022-06-21 13:30 UTC we have collected 11.2 ks of exposure. We observed a rapid decline in source intensity, dropping from an initial 0.5-10 keV rate of 140 ct/s down to ~3 ct/s at the time of the latest observation.

After correcting the photon arrival times to the solar system barycenter based on source coordinates as measured with Chandra (ATel #9591), we divided the data into ~100 second segments and measured the pulse frequency in each. The resulting frequencies show a clear sinusoidal oscillation in time, pointing to an approximately one-hour ultracompact binary orbit. Modeling the precise pulse arrival times using a circular orbit yielded a binary period of Pb = 3653.47(10) s, a projected semi-major axis of a1 sin i = 0.01381(3) light-s, and a time of passage through the ascending node of Tasc = MJD 59749.633118(18) TDB. The barycentric pulse frequency was found at 313.643740(1) Hz. All uncertainties are statistical one sigma errors.

While the source was bright, its pulse waveform had a first harmonic amplitude of 8.9 ± 0.4%. A second and third harmonic could also be detected, with amplitudes of 3.2 ± 0.4% and 2.4 ± 0.5%, respectively. A notable feature of the obtained timing solution is that it reveals a ~0.25 cycle phase jump similar to those frequently seen in outbursts from SAX J1808.4-3658 (e.g., Burderi et al. 2006, ApJL 653, 133), suggesting that the accretion geometry of MAXI J1957+032 may be evolving with time. The presence of this phase jump does imply that systematic errors may be present due to pulse waveform changes. The binary mass function is 1.58 x 10^-6 Msol, implying a companion mass of 0.015 < M2 < 0.087 Msol, if we assume a binary inclination angle of >10 degrees.

NICER will continue to monitor this source until it can no longer be detected. We acknowledge the essential contributions of the MAXI team in triggering the early NICER observations that resulted in the discovery of millisecond pulsations as reported in ATel #15444.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.