2018A&A...610L...2S

We report the discovery of X-ray pulsations at 105.2Hz (9.5ms) 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 9.1kpc. The X-ray pulsations show a clear Doppler modulation that implies an orbital period of ∼46min and a projected semi-major axis of ∼5lt-ms, which makes IGR J16597-3704 an ultracompact X-ray binary system. We estimated a minimum companion mass of 6.5x10^–10M_☉, assuming a neutron star mass of 1.4M_☉, and an inclination angle of <75° (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∼1.4keV) plus a comptonised power-law with photon index ∼2.3 and an electron temperature of ∼30keV. Radio pulsations from the source were unsuccessfully searched for with the Parkes Observatory.