Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498–2921

¹ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
e-mail: mfalanga@issibern.ch
² SRON – Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Astronomy Division, Department of Physics, PO Box 3000, 90014 University of Oulu, Finland
⁴ Monash Center for Astrophysics, School of Physics, and School of Mathematical Sciences, Monash University, VIC 3800, Australia
⁵ ISDC, Data centre for astrophysics, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁶ Service d’Astrophysique (SAp), IRFU/DSM/CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁷ Unité mixte de recherche Astroparticule et Cosmologie, 10 rue Alice Domon et Léonie Duquet, 75205 Paris, France
⁸ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
⁹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy

Received: 11 May 2012
Accepted: 26 July 2012

Abstract

Context. IGR J17498–2921 is the third X-ray transient accreting millisecond pulsar discovered by INTEGRAL. It was in outburst for about 40 days beginning on August 08, 2011.

Aims. We analyze the spectral and timing properties of the object and the characteristics of X-ray bursts to constrain the physical processes responsible for the X-ray production in this class of sources.

Methods. We studied the broad-band spectrum of the persistent emission in the 0.6–300 keV energy band using simultaneous INTEGRAL, RXTE, and Swift data obtained in August–September 2011. We also describe the timing properties in the 2–100 keV energy range such as the outburst lightcurve, pulse profile, pulsed fraction, pulsed emission, time lags, and study the properties of X-ray bursts discovered by RXTE, Swift, and INTEGRAL and the recurrence time.

Results. The broad-band average spectrum is well-described by thermal Comptonization with an electron temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson optical depth τ_T ~ 1 in a slab geometry. The slab area corresponds to a black body radius of R_bb ~ 9 km. During the outburst, the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth that are constant within the errors. This behavior has been interpreted as indicating that the X-ray emission originates above the neutron star (NS) surface in a hot slab (either the heated NS surface or the accretion shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a period of 2.5 ms up to  ~65 keV. The pulsed fraction is consistent with being constant, i.e. energy independent and has a typical value of 6–7%. The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV at a rather small value of  ~−60   μs with those observed in other accreting pulsars. The short burst profiles indicate that there is a hydrogen-poor material at ignition, which suggests either that the accreted material is hydrogen-deficient, or that the CNO metallicity is up to a factor of about two times solar. However, the variation in the burst recurrence time as a function of ṁ (inferred from the X-ray flux) is much smaller than predicted by helium-ignition models.