* Astronomy

Title: Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808
Author: A. Sanna, A. Papitto, L. Burderi, E. Bozzo, A. Riggio, T. Di Salvo, C. Ferrigno, N. Rea, R. Iaria

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 ~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 minutes 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 solar masses, which assumes a neutron star mass of 1.4 solar masses 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 companion star's Roche-Lobe could either be filled by a hot (5 x 10^6 K) pure helium white dwarf with a 0.028 solar masses (implying i \simeq 58°) or an old (>5 Gyr) brown dwarf with metallicity abundances between solar/sub-solar and mass ranging in the interval 0.065-0.085 solar masses (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 cutoff power-law with photon index Gamma ~ 1.7 and cut-off at a temperature kTe~ 130 keV. Up to the latest Swift-XRT observation performed on 2016 July 19 the source has 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.

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