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|Title:||Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808||Authors:||SANNA, ALBERTO
Di Salvo, T.
|Issue Date:||2017||Journal:||ASTRONOMY & ASTROPHYSICS||Number:||598||First Page:||A34||Abstract:||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<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.||URI:||http://hdl.handle.net/20.500.12386/29935||URL:||https://www.aanda.org/articles/aa/abs/2017/02/aa29406-16/aa29406-16.html||ISSN:||0004-6361||DOI:||10.1051/0004-6361/201629406||Bibcode ADS:||2017A&A...598A..34S||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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