NICER / ISS Science Nugget
for August 11, 2022

NICER reveals the anomalous fast orbital expansion of the eclipsing pulsar SWIFT J1749.4-2807

SWIFT J1749.4-2807 is a known rapidly rotating neutron star that forms a tight binary system with a sub-Solar companion star. The system was discovered by SWIFT-BAT in 2006, while coherent brightness oscillations at a period of 1.9 milliseconds were revealed for the first time by the Rossi X-ray Timing Explorer in 2010. The occultation of the neutron star X-ray emission by its companion every 8.8 hours makes SWIFT J1749.4-2807 the only known eclipsing accreting millisecond X-ray pulsar (AMXP, hereafter).

On March 1, 2021, NICER started monitoring the X-ray activity of SWIFT J1749.4-2807. Quickly after the trigger, the NICER team confirmed the source's new outburst by detecting the characteristic millisecond pulsations (Astronomer's Telegram #14428). Thanks to NICER's uniquely precise timing capability, it was possible to accurately determine an updated orbital ephemeris, which, combined with the previous outburst, enabled high-precision timing of this pulsar over more than a decade.

The long-term evolution of the orbital parameters of SWIFT J1749.4-2807 suggests, for the first time in these systems, a rapid increase in both the orbital period and the separation between the two stars in the binary system. Moreover, at odds with the other AMXPs, SWIFT J1749.4-2807 showed marginally significant evidence for a non-circular orbit. These results challenge the standard interpretation of mass and orbital momentum transfer in AMXPs. Possible scenarios have been proposed, including, e.g., i) a highly non-conservative mass-transfer process in which the Sub-Solar companion transfers enormous quantities of matter with only a tiny fraction (<0.5%) accreted onto the neutron star; ii) periodic redistribution of the mass in the Sub-Solar companion star that induces changes in the binary orbit by gravitational coupling. Finally, an intriguing, although highly speculative scenario, proposed to explain coherently all the peculiar results obtained from the timing analysis of the pulsar, invokes the presence of another object gravitationally bound to the binary system, forming a hierarchical triple system.

This analysis was led by Dr. Andrea Sanna of the University of Cagliari (Italy), was recently accepted for publication in the astronomical journal Monthly Notices of the Royal Astronomical Society.