Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808

We 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 similar to 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 similar to 17.6 lt-ms. Based on the mass function, we estimate a minimum companion mass of 0.024 M-circle dot, which assumes a neutron star mass of 1.4 M-circle dot and a maximum inclination angle of 75 degrees (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 x 10(6) K) pure helium white dwarf with a 0.028 M-circle dot mass (implying i similar or equal to 58 degrees) 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 similar to 0.5 keV) and a hard cut-off power-law with photon index Gamma similar to 1.7 and cut-off at a temperature kT(e) similar to 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.