XMM-Newton detection of the 2.1 ms coherent pulsations from IGR J17379-3747

Abstract

We report the detection of X-ray pulsations at 2.1 ms from the known X-ray burster IGR J17379-3747 using XMM-Newton. The coherent signal shows a clear Doppler modulation from which we estimate an orbital period of 1.9 h and a projected semi-major axis of 8 lt-ms. Taking into account the lack of eclipses (inclination angle of <75°) and assuming a neutron star mass of 1.4 M☉, we have estimated a minimum companion star of 0.06 M☉. Considerations on the probability distribution of the binary inclination angle make the hypothesis of a main-sequence companion star less likely. On the other hand, the close correspondence with the orbital parameters of the accreting millisecond pulsar SAX J1808.4-3658 suggests the presence of a bloated brown dwarf. The energy spectrum of the source is well described by a soft disk black-body component (kT 0.45 keV) plus a Comptonisation spectrum with photon index 1.9. No sign of emission lines or reflection components are significantly detected. Finally, combining the source ephemerides estimated from the observed outbursts, we obtained a first constraint on the long-term orbital evolution of the order of Ṗ_orb = (-2.5 ± 2.3) × 10^-12 s s^-1.