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NICER discovers 314 Hz pulsations from MAXI J1957+032

ATel #15444; M. Ng (MIT), P. M. Bult, K. C. Gendreau, Z. Arzoumanian (NASA/GSFC), A. Sanna (University of Cagliari), W. C.G. Ho (Haverford), S. Guillot (IRAP/CNRS), G. K. Jaisawal (DTU Space), T. Guver (Istanbul Univ.), D. Chakrabarty (MIT), on behalf of the NICER team
on 19 Jun 2022; 20:19 UT
Credential Certification: Mason Ng (masonng@mit.edu)

Subjects: X-ray, Neutron Star, Transient, Pulsar

Referred to by ATel #: 15446, 15448, 15456, 15457, 15460, 15462, 15466

MAXI/GSC reported the detection of a faint X-ray transient in the direction of and likely associated with MAXI J1957+032 (ATel #15440). NICER proceeded to begin raster scan observations near the MAXI coordinates, finding a source position that is consistent with the Chandra coordinates of MAXI J1957+032 obtained during the previous outburst of the source in 2016 (ATel #9591): R.A. = 299.162950 deg. and Dec. = 3.445461 deg. (J2000). NICER collected a 1.3 ks pointed observation at these coordinates starting on 2022-06-19 14:54:14 UTC.

Coherent pulsation searches over 0.3-10.0 keV with a Fourier power spectrum and acceleration searches yielded a significant (over 10 sigma, single-trial) signal at 313.6 Hz. The acceleration search also yielded a weaker (2 sigma) signal at the second harmonic frequency of 627.2 Hz.

A broadband 0.4-10.0 keV spectral analysis found that an absorbed power law and disk blackbody provided a good fit, with a chi-squared value of 158 for 109 degrees of freedom. We find an absorption column density of nH = 0.072 (-0.011, +0.013) × 10²²cm^-2, a photon index of 1.32 (-0.16, +0.13), and an inner disk temperature of 0.79 (-0.02, +0.02) keV. This translates to a 0.3-10.0 keV absorbed flux of 2.57 (-0.07, +0.02) × 10^-10 erg/s/cm^-2 and an unabsorbed flux of about 2.8 × 10^-10 erg/s/cm^-2. Uncertainties are reported at 90% confidence.

NICER will continue to observe MAXI J1957+032. We encourage further observations of this source with other observatories.

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.