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

A. Sanna, A. Papitto, L. Burderi, E. Bozzo, A. Riggio, T. Di Salvo, C. Ferrigno, N. Rea, R. Iaria

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    © ESO, 2017. 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 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 MȮ, which assumes a neutron star mass of 1.4 MȮ 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 × 106 K) pure helium white dwarf with a 0.028 MȮ 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 kTe ∼ 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.
    Original languageEnglish
    Article numberA34
    JournalAstronomy and Astrophysics
    Publication statusPublished - 1 Feb 2017


    • Accretion, accretion disks
    • Binaries: general
    • Pulsars: general
    • Stars: neutron
    • X-rays: binaries


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