Volume 535, November 2011
|Number of page(s)||4|
|Published online||17 November 2011|
Letter to the Editor
The discovery of the 401 Hz accreting millisecond pulsar IGR J17498-2921 in a 3.8 h orbit
1 Dipartimento di Fisica, Universitá degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
2 ISDC Science Data Center for Astrophysics of the University of Geneva, chemin d’Écogia 16, 1290 Versoix, Switzerland
3 INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
4 Dipartimento di Fisica, Universitá di Palermo, via Archirafi 36, 90123 Palermo, Italy
5 INAF – Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra, Italy
Received: 1 September 2011
Accepted: 7 November 2011
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 0.17 M⊙ on the mass of the companion star, while an upper limit of 0.48 M⊙ 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.
Key words: stars: neutron / stars: rotation / pulsars: individual: IGR J17498 − 2921 / X-rays: binaries
© ESO, 2011
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