NICER / ISS Science Nugget for July 26, 2018

Rarely visible accreting pulsar observed in 2018 outburst

Accreting millisecond X-ray pulsars are fast-spinning neutron stars; their pulsations are visible thanks to episodes of infalling plasma (from a companion star) that is magnetically channeled to the neutron star's magnetic poles. Swift J1756.9-2508 is one such pulsar. First discovered in June, 2007, this neutron star rotates once around every 5.5 milliseconds.

Swift J1756.9-2508 was only visible for a few days in 2007, and another few days again in 2009. Its April 2018 X-ray outburst provided a rare opportunity for NICER to observe this neutron star and study its long-term evolution.

Thanks to excellent sampling of the 2018 outburst, the NICER data improved our knowledge of the binary orbit and, importantly, enabled the use of pulse timing to reduce uncertainty in the source coordinates on the sky (left panel) over those measured using the X-Ray Telescope (XRT) on-board NASA's Swift observatory. In the right panel (right panel), we see that, after applying this position correction, the pulsar's spin frequency shows a significant long-term trend, approximately 250 nHz over 10 years. This amounts to a rate of change in the neutron star spin period of about 2 x 10^-20.

This work by P. Bult et al., accepted for publication in The Astrophysical Journal, represents NICER's 10th peer-reviewed paper.