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 \sim 46 minutes and a projected semi-major axis of \sim 5 lt-ms , which makes IGR J16597 - 3704 an ultracompact X-ray binary system .
We estimated a minimum companion mass of 6.5 \times 10 ^ { -3 } M _ { \odot } , assuming a neutron star mass of 1.4 M _ { \odot } , and an inclination angle of < 75 ^ { \circ } ( 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 \sim 1.4 keV ) plus a comptonised power-law with photon index \sim 2.3 and an electron temperature of \sim 30 keV .
Radio pulsations from the source were searched for with the Parkes observatory and not detected .