NICER / ISS Science Nugget
for February 29, 2024

Determining the orbit of a new pulsar

Following quickly on the heels of NICER's discovery of a new accreting millisecond X-ray pulsar (AMXP) last week, members of the NICER team used additional data to infer the system's orbital period through the Doppler shifts of the neutron star's pulsations, which amount to just 3% of the total X-ray emission from the binary system. A total of 2.4 hours of exposure collected in multiple 15-minute snapshots over nearly two days demonstrated that the pulsar is moving in a 5.22-hour orbit, with a companion star of minimum mass one-quarter that of our Sun. These results were reported this week by P. Ray (Naval Research Laboratory) and collaborators in a follow-up Astronomer's Telegram. Together, the detection of pulsations and subsequent orbit measurement have informed studies in progress with multiple space- and ground-based telescopes.

Measurement of a pulsar's orbital period enables characterization of the companion star, which in turn yields clues to the system's formation and its expected evolution over time. More than two dozen AMXPs are now known, with new ones turning up at the rate of approximately one per year; NICER participated in the discovery of the last five. With such a small sample, each new discovery adds to our understanding of the process that spins neutron stars up to remarkable rotation rates, approaching 1000 revolutions per second in the most extreme cases. This apparent upper limit to the spin rate - made more robust with each new discovery - suggests that some mechanism prevents higher spins from being attained. The leading hypothesis is that gravitational waves emitted by the neutron star at extreme rotation rates ultimately induce a braking torque. This raises the intriguing possibility that gravitational waves from such systems may be directly detectable with future Earth-based detectors.