NICER measures the binary orbit of MAXI J1816-195

Following the discovery of the new accreting millisecond X-ray pulsar MAXI J1816-195 (ATel #15418, #15421, #15425), we have commenced a regular monitoring campaign of this source with NICER. Between 2022-06-08 19:00 UTC and 2022-06-10 03:30 UTC, the source was observed to steadily increase in count-rate, rising from ~500 ct/s up to ~820 ct/s (in the 0.5-10 keV energy range). Since then, the source intensity has gradually declined. At the time of our most recent observation, at 2022-06-13 13:20 UTC, the source count rate was 680 ct/s. During this interval, we observed four more thermonuclear X-ray bursts, suggesting that MAXI J1816-195 is in an active bursting state.

A coherent timing analysis of these data consistently recovers the 528 Hz pulse signal. In an effort to measure the orbital parameters, we divided the data into ~500 second duration segments and measured the pulse frequency in each. The resulting frequency measurements showed a clear sinusoidal evolution over time. Modeling this modulation with a circular binary orbit, we find a best-fit orbital period of P_b = 17402.27(12) s, a projected semi-major axis of the pulsar of a_x sin(i) = 0.262948(18) light-s and a time of passage through the ascending node of T_asc = MJD 59738.875669(12) TDB. The barycentric pulse frequency is found at 528.6111152(3) Hz with a 2-10 keV sinusoidal fractional pulse amplitude of 2.7(1)%. The mass function for the pulsar is 4.8 × 10^-4 Msol which implies a companion mass of 0.10 < M2 < 0.75 Msol, assuming a 1.4 Msol neutron star mass and a binary inclination of 10 < i < 90 degrees.

NICER will continue regular monitoring of this source.

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.