SANNA, ALBERTOALBERTOSANNAFerrigno, C.C.FerrignoRay, P. S.P. S.RayDucci, L.L.DucciJaisawal, G. K.G. K.JaisawalEnoto, T.T.EnotoBozzo, E.E.BozzoAltamirano, D.D.AltamiranoDi Salvo, T.T.Di SalvoStrohmayer, T. E.T. E.StrohmayerPAPITTO, ALESSANDROALESSANDROPAPITTORiggio, A.A.RiggioBurderi, L.L.BurderiBult, P. M.P. M.BultBogdanov, S.S.BogdanovGambino, A. F.A. F.GambinoMARINO, ALESSIOALESSIOMARINOIaria, R.R.IariaArzoumanian, Z.Z.ArzoumanianChakrabarty, D.D.ChakrabartyGendreau, K. C.K. C.GendreauGuillot, S.S.GuillotMarkwardt, C.C.MarkwardtWolff, M. T.M. T.Wolff2021-01-212021-01-2120180004-6361http://hdl.handle.net/20.500.12386/29937We report the discovery by the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron Star Interior Composition Explorer (NICER) of the accreting millisecond X-ray pulsar IGR J17591-2342. Coherent X-ray pulsations around 527.4 Hz (1.9 ms) with a clear Doppler modulation were detected. This implies an orbital period of ∼8.8 h and a projected semi-major axis of ∼1.23 lt-s. With the binary mass function, we estimate a minimum companion mass of 0.42 M☉, obtained assuming a neutron star mass of 1.4 M☉ and an inclination angle lower than 60°, as suggested by the absence of eclipses or dips in the light curve of the source. The broad-band energy spectrum, obtained by combining NuSTAR, swift and INTEGRAL observations, is dominated by Comptonisation of soft thermal seed photons with a temperature of ∼0.7 keV by electrons heated to 21 keV. We also detect black-body-like thermal direct emission that is compatible with an emission region of a few kilometers and a temperature compatible with the seed source of Comptonisation. A weak Gaussian line centred on the iron Kα complex can be interpreted as a signature of disc reflection. A similar spectrum characterises the NICER spectra, which was measured when the outburst faded.STAMPAenArticle10.1051/0004-6361/2018341602-s2.0-85055058916000445994700001https://www.aanda.org/articles/aa/abs/2018/09/aa34160-18/aa34160-18.html2018A&A...617L...8SFIS/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::PE9_10 High energy and particle astronomy – X-rays, cosmic rays, gamma rays, neutrinos