Discovery of 105 Hz coherent pulsations in the ultracompact binary IGR J16597-3704
Altamirano, D.; Sanna, A.; Degenaar, N.; Wijnands, R.; Possenti, A.; Burgay, M.; Ferrigno, C.; Maccarone, T.; Papitto, A.; Di Salvo, T.; Burderi, L.; Bozzo, E.; Iaria, R.; Riggio, A.; Sivakoff, G. R.; D'Amico, N.; Bahramian, A.; Heinke, C.
Italy, United States, Switzerland, Canada, United Kingdom, Netherlands
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 that implies an orbital period of 46 min and a projected semi-major axis of 5 lt-ms, which makes IGR J16597-3704 an ultracompact X-ray binary system. We estimated a minimum companion mass of 6.5 × 10-10 M⊙, assuming a neutron star mass of 1.4 M⊙, 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 1.4 keV) plus a comptonised power-law with photon index 2.3 and an electron temperature of 30 keV. Radio pulsations from the source were unsuccessfully searched for with the Parkes Observatory.