NICER detection of periodicity in the new X-ray transient MAXI J0901-531

Referred to by ATel #: 14564 Tweet Following the discovery of the new X-ray transient MAXI J0901-531 (ATel #14555), we triggered observations with the Neutron Star Interior Composition Explorer (NICER) that started on 2021 April 16 at 19:48 UT and obtained a total exposure of ~9.25 ks. The pointing direction was the Swift localization from ATel #14557. NICER's average measured count rate was 19.6 cts/s (0.5-8 keV), including a background contribution of < 1 c/s. The power spectrum exhibits a low-frequency noise component extending to about 0.1 Hz, plus a coherent periodicity with a fundamental frequency of 0.07 Hz with at least four significant harmonics. A pulse timing analysis gives a frequency of 0.0711477(2) Hz (P = 14.055 seconds). No significant frequency derivative is seen with a limit of |F1| < 4E-12 Hz/s. The frequency value is consistent with the Fermi/GBM measurement obtained by integrating data collected in the time interval April 13-16. We fitted the 0.5-10 keV spectrum with an absorbed power-law model. This model failed to describe the NICER energy continuum. To better fit the spectrum, an additional blackbody (bbodyrad in XSPEC) was added along with the power-law component. Following are preliminary spectral parameters derived from this analysis: absorption column density N_H = (4.1+/-0.2) x 10^21 cm^-2 (Wilms abundance), black body temperature kT = 0.61+/-0.02 keV, blackbody normalization BBnorm = 14+/-1, and photon index = 0.13+/-0.06. The best fit reduced chi-squared is 1.13 for 827 degrees of freedom. The 0.5-10 keV unabsorbed flux is 1.83x10^{-10} erg/s/cm^2. No iron line feature is observed between 6-7 keV though we see some excess around 7.5 keV. Further investigation is required. 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.