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
e-mail: andrea.sanna@dsf.unica.it
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

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_⊙, 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 kT_e ~ 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.