2011A&A...535L...4P

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)M _☉, allows to set a lower limit of 0.17M_☉ on the mass of the companion star, while an upper limit of 0.48M_☉ 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)x10^–14Hz/s. 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.