Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17511–3057

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 School of Physics and School of Mathematical Sciences, Monash University, VIC 3800, Australia
⁵ Department of Physics and Yale Center for Astronomy and Astrophysics, Yale University, PO Box 208121, New Haven, CT 06520-8121, USA
⁶ ISDC, Data centre for astrophysics, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁷ Service dAstrophysique (SAp), IRFU/DSM/CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁸ Unité mixte de recherche Astroparticule et Cosmologie, 10 rue Alice Domon et Leonie Duquet, 75205 Paris, France
⁹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
¹⁰ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Roma, Italy

Received: 1 December 2010
Accepted: 23 January 2011

Abstract

Context. IGR J17511–3057 is the second X-ray transient accreting millisecond pulsar discovered by INTEGRAL. It was in outburst for about a month beginning on September 13, 2009.

Aims. We analyze the spectral and timing properties of the object and the characteristics of X-ray bursts with the aim 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 0.8–300 keV energy band using simultaneous INTEGRAL, RXTE and Swift data obtained in September 2009. We also describe the timing properties in the 2–120 keV energy range such as the outburst light curve, pulse profile, pulsed fraction, pulsed emission, and time lags, and moreover study the properties of X-ray bursts discovered by RXTE 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 ~ 25 keV, soft seed photons of kT_bb ~ 0.6 keV, and Thomson optical depth τ_T ~ 2 in a slab geometry. During the outburst the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth being constant within errors. We fitted the outburst profile with the exponential model, and using the disk instability model we inferred the outer disk radius to be (4.8−5.4) × 10¹⁰ cm. The INTEGRAL and RXTE data reveal the X-ray pulsation at a period of 4.08 milliseconds up to  ~120 keV. The pulsed fraction is shown to decrease from  ~22% at 3 keV to a constant pulsed fraction of  ~17–18% between 7–30 keV, and then to decrease again to  ~13% at 60 keV. The nearly sinusoidal pulses show soft lags that monotonically increase with energy to about 0.2 ms at 10–20 keV similar to those observed in other accreting pulsars. The short burst profiles indicate 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 2 times solar. However, the variation of the burst recurrence time as a function of ṁ (inferred from the X-ray flux) is much smaller than predicted by helium-ignition models.