NICER Detects Pulsations from Swift J0243.6+6124

MAXI/GSC detected an increase in X-ray flux at 13:23 UT on April 8, 2023, in the field containing the transient Be X-ray pulsar Swift J0243.6+6124 as well as the high-mass gamma-ray binary LS I +61 303 (ATel #15983). Swift/XRT performed follow-up observations of the MAXI error region and reported that Swift J0243.6+6124 is likely the source of the MAXI/GSC trigger (ATel #15984). NICER observed Swift J0243.6+6124 for a total exposure time of 6.2 ks starting from 15:05 UT on April 10, 2023. The 0.6-10.0 keV count rate is 18.4(1) c/s.

NICER detects pulsations at around 9.8 s with a Z2 search at the 32σ significance level. In what follows, the timing and spectroscopy analyses were done over 0.6-10.0 keV, given the absorption along the line of sight (see below). The final pulsation frequency estimate is 0.1020(2) Hz. The folded pulse profile is sinusoidal, and the corresponding fractional sinusoidal amplitude is ~18%. No orbital corrections were applied.

A broadband 0.6-10.0 keV spectral analysis found that an absorbed cutoff power-law model fits the data with a chi-squared value of 121 for 114 degrees of freedom. We find an absorption column density of nH = 0.99 (-0.04, +0.05)× 1022 cm-2, a photon index of -0.16 (-0.09, +0.09), and a cutoff energy of 4.3 (-0.4, +0.5) keV. The corresponding 0.6-10.0 keV absorbed flux is ~1.7 × 10-10 erg/s/cm2; and the 0.6-10.0 keV unabsorbed flux is 1.875 (-0.012, +0.013) × 10-10 erg/s/cm2, which suggests a decrease in flux by about a factor of 4 compared with the Swift/XRT observations. Uncertainties are reported at 90% confidence.

NICER will continue to observe Swift J0243.6+6124. We encourage further observations with other facilities.

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.