Volume 598, February 2017
|Number of page(s)||7|
|Section||Stellar structure and evolution|
|Published online||27 January 2017|
Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808
1 Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy
2 INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00044 Monteporzio Catone (Roma), Italy
3 ISDC Data Centre for Astrophysics, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
4 Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
5 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
6 Institute of Space Sciences (CSIC-IEEC), Campus UAB, Carrer Can Magrans s/n, 08193 Barcelona, Spain
Received: 26 July 2016
Accepted: 8 November 2016
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 M⊙, which assumes a neutron star mass of 1.4 M⊙ 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 M⊙ 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.
Key words: X-rays: binaries / pulsars: general / stars: neutron / accretion, accretion disks / binaries: general
© ESO, 2017
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