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 similar to 1.9 h and a projected semi-major axis of similar to 8 lt-ms. Taking into account the lack of eclipses (inclination angle of <75 degrees) and assuming a neutron star mass of 1.4 M-circle dot, we have estimated a minimum companion star of similar to 0.06 M-circle dot. 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 similar to 0.45 keV) plus a Comptonisation spectrum with photon index similar to 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) x 10(-12) s s(-1).