NICER follow-up observations of the accreting millisecond X-ray pulsar MAXI J0911-655

ATel #12869; P. M. Bult, K. C. Gendreau, T. E. Strohmayer, Z. Arzoumanian (NASA/GSFC), D. Altamirano (Univ. of Southampton), S. Guillot (IRAP/CNRS/CNES)
on 14 Jun 2019; 12:49 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)

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The accreting millisecond X-ray pulsar MAXI J0911-655 (also known as Swift J0911.9-6452) has shown intermittent pulsations only during the first month of its 3.2 year outburst history (Sanna et al. 2017, A&A, 598, 34). Long-term monitoring with Swift/XRT indicated that the source recently underwent an anomalous swing in luminosity, with the flux dropping by a factor of >60 for a ~2 week period, before returning to a high flux level (ATel #12831, ATel #12846).

Following up on these reports, NICER performed pointed observations of MAXI J0911-655 to verify if pulsations are once again visible. NICER observed the source starting on 2019 June 10 at 16:50 UTC, collecting 1.7 ks of exposure, and again on 2019 June 12 at 15:21 UTC, collecting 1 ks of good exposure. The source is clearly detected at ~45 ct/s (0.2-12 keV) in both observations. This rate, however, is still lower than the ~70 ct/s NICER observed from MAXI J0911-655 during observations collected in 2017 and 2018.

After barycenter-correcting the event times, we computed a power spectrum by averaging the Fourier transforms of 32 second segments of the light curve, but did not observe pulsations at the known 339.975 Hz spin frequency. We then attempted to recover pulsations by folding the data on the spin period after correcting the photon arrival times for Doppler delays due to the binary orbit using the ephemeris of Sanna et al (2017). We attempted this recovery over a grid of values for the time of ascending node, using steps of 10 seconds in T_asc. Again, we did not detect coherent pulsations for any of the trials, to a collective 95% confidence upper limit of 2.3% fractional sinusoidal amplitude.

Further NICER monitoring of this source is underway.

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.