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

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 P orb = (−2.5 ± 2.3) × 10− 12 s s −1 .