SANNA, ALBERTOALBERTOSANNAPAPITTO, ALESSANDROALESSANDROPAPITTOBurderi, L.L.BurderiBozzo, E.E.BozzoRiggio, A.A.RiggioDi Salvo, T.T.Di SalvoFerrigno, C.C.FerrignoRea, N.N.ReaIaria, R.R.Iaria2021-01-212021-01-2120170004-6361http://hdl.handle.net/20.500.12386/29935We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of 339.97 Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of 44.3 min and a projected semi-major axis of 17.6 lt-ms. Based on the mass function, we estimate a minimum companion mass of 0.024 M<SUB>☉</SUB>, which assumes a neutron star mass of 1.4 M<SUB>☉</SUB> and a maximum inclination angle of 75° (derived from the lack of eclipses and dips in the light-curve of the source). We find that the Roche-lobe of the companion star could either be filled by a hot (5 × 10<SUP>6</SUP> K) pure helium white dwarf with a 0.028 M<SUB>☉</SUB> mass (implying I ≃ 58°) or an old (>5 Gyr) brown dwarf with metallicity abundances between solar/sub-solar and mass ranging in the interval 0.065 to 0.085 (16 < I < 21). During the outburst, the broad-band energy spectra are well described by a superposition of a weak black-body component (kT 0.5 keV) and a hard cut-off power-law with photon index Γ 1.7 and cut-off at a temperature kT<SUB>e</SUB> 130 keV. Up until the latest Swift-XRT observation performed on 19th July, 2016, the source had been observed in outburst for almost 150 days, which makes MAXI J0911-655 the second accreting millisecond X-ray pulsar with outburst duration longer than 100 days.STAMPAenDiscovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808Article10.1051/0004-6361/2016294062-s2.0-85011105076000394465000034https://www.aanda.org/articles/aa/abs/2017/02/aa29406-16/aa29406-16.html2017A&A...598A..34SFIS/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