Discovery of 105 Hz coherent pulsations in the ultracompact binary IGR J16597-3704

TL;DR

This paper reports the discovery of 105 Hz X-ray pulsations in the ultracompact binary IGR J16597-3704, revealing its orbital parameters and spectral characteristics, and noting the absence of radio pulsations.

Contribution

First detection of coherent X-ray pulsations in IGR J16597-3704, establishing it as an ultracompact binary with detailed spectral and orbital properties.

Findings

Pulsations at 105.2 Hz detected with NuSTAR and Swift.

Orbital period of approximately 46 minutes inferred.

Spectral analysis shows a disk blackbody plus Comptonized power-law.

Abstract

We report the discovery of X-ray pulsations at 105.2 Hz (9.5 ms) from the transient X-ray binary IGR J16597-3704 using NuSTAR and Swift. The source was discovered by INTEGRAL in the globular cluster NGC 6256 at a distance of 9.1 kpc. The X-ray pulsations show a clear Doppler modulation implying an orbital period of ~46 minutes and a projected semi-major axis of ~5 lt-ms, which makes IGR J16597-3704 an ultra-compact X-ray binary system. We estimated a minimum companion mass of 0.0065 solar masses, assuming a neutron star mass of 1.4 solar masses, and an inclination angle of <75 degrees (suggested by the absence of eclipses or dips in its light-curve). The broad-band energy spectrum of the source is well described by a disk blackbody component (kT ~1.4 keV) plus a comptonised power-law with photon index ~2.3 and an electron temperature of ~30 keV. Radio pulsations from the source were searched for with the Parkes observatory and not detected.