The discovery of the 401 Hz accreting millisecond pulsar IGR J17498-2921 in a 3.8 h orbit

We report on the detection of a 400.99018734(1) Hz coherent signal in the Rossi X-ray Timing Explorer (RXTE) light curves of the recently discovered X-ray transient, IGR J17498 - 2921. By analysing the frequency modulation caused by the orbital motion observed between August 13 and September 8, 2011, we derive an orbital solution for the binary system with a period of 3.8432275(3) h. The measured mass function, f(M2,M1,i) = 0.00203807(8) Msun, allows to set a lower limit of 0.17 Msun on the mass of the companion star, while an upper limit of 0.48 Msun is set by imposing that the companion star does not overfill its Roche lobe. We observe a marginally significant evolution of the signal frequency at an average rate of - (6.3 ± 1.9) × 10-14 Hz s-1. The low statistical significance of this measurement and the possible presence of timing noise hampers a firm detection of any evolution of the neutron star spin. We also present an analysis of the spectral properties of IGR J17498 - 2921 based on the observations performed by the Swift-X-ray Telescope and the RXTE-Proportional Counter Array between August 12 and September 22, 2011. During most of the outburst, the spectra are modeled by a power-law with an index Γ ≈ 1.7 - 2, while values of ≈ 3 are observed as the source fades into quiescence.