NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062-6143

TL;DR

NICER observations reveal that the accreting millisecond pulsar IGR J17062-6143 is in an ultracompact binary with the shortest known orbital period of about 38 minutes, providing new insights into such systems.

Contribution

This study presents the first detailed measurement of the ultracompact orbit of IGR J17062-6143 using NICER data, establishing it as the shortest known orbital period for an AMXP.

Findings

Confirmed pulsations at 163.656 Hz with NICER

Discovered a 38-minute orbital period for the binary

Determined the smallest known mass function for such a system

Abstract

We present results of recent Neutron Star Interior Composition Explorer observations of the accreting millisecond X-ray pulsar IGR J17062-6143 that show that it resides in a circular, ultracompact binary with a 38 minute orbital period. NICER observed the source for approximately 26 ksec over a 5.3 day span in 2017 August, and again for 14 and 11 ksec in 2017 October and November, respectively. A power spectral analysis of the August exposure confirms the previous detection of pulsations at 163.656 Hz in Rossi X-ray Timing Explorer data, and reveals phase modulation due to orbital motion of the neutron star. A coherent search for the orbital solution using the Z^2 method finds a best-fitting circular orbit with a period of 2278.21 s (37.97 min), a projected semi-major axis of 0.00390 lt-sec, and a barycentric pulsar frequency of 163.6561105 Hz. This is currently the shortest known orbital period for an AMXP. The mass function is 9.12 e-8} solar masses, presently the smallest known for a stellar binary. The minimum donor mass ranges from about 0.005 - 0.007 solar masses, for a neutron star mass from 1.2 - 2 solar masses. Assuming mass transfer is driven by gravitational radiation, we find donor mass and binary inclination bounds of 0.0175 - 0.0155 solar masses and 19 deg < i < 27.5 deg, where the lower and upper bounds correspond to 1.4 and 2 solar mass neutron stars, respectively. Folding the data accounting for the orbital modulation reveals a sinusoidal profile with fractional amplitude 2.04 +- 0.11 % (0.3 - 3.2 keV).