SANNA, ANDREAANDREASANNABahramian, A.A.BahramianBozzo, E.E.BozzoHeinke, C.C.HeinkeAltamirano, D.D.AltamiranoWijnands, R.R.WijnandsDegenaar, N.N.DegenaarMaccarone, T.T.MaccaroneRiggio, A.A.RiggioDi Salvo, T.T.Di SalvoIaria, R.R.IariaBURGAY, MARTAMARTABURGAYPOSSENTI, ANDREAANDREAPOSSENTIFerrigno, C.C.FerrignoPAPITTO, ALESSANDROALESSANDROPAPITTOSivakoff, G. R.G. R.SivakoffD'AMICO, NICOLO'NICOLO'D'AMICOBurderi, L.L.Burderi2020-10-142020-10-1420180004-6361http://hdl.handle.net/20.500.12386/27795We 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<SUP>-10</SUP> M<SUB>☉</SUB>, assuming a neutron star mass of 1.4 M<SUB>☉</SUB>, 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.STAMPAenArticle10.1051/0004-6361/2017322622-s2.0-85042191534000425360800001https://www.aanda.org/articles/aa/abs/2018/02/aa32262-17/aa32262-17.html2018A&A...610L...2SFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation