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Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921
Falanga M., Kuiper L., Poutanen J., Galloway D.K., Bozzo E. et al
Astronomy and Astrophysics 545 (2012) A26 - http://hal.archives-ouvertes.fr/hal-00736062
Physique/Astrophysique/Phénomènes cosmiques de haute energie
Planète et Univers/Astrophysique/Phénomènes cosmiques de haute energie
Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921
M. Falanga, L. Kuiper, J. Poutanen, D. K. Galloway, E. Bozzo, A. Goldwurm1, 2, W. Hermsen, L. Stella
1 :  IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers (ex DAPNIA)
2 :  APC - UMR 7164 - AstroParticule et Cosmologie
CNRS : UMR7164 – IN2P3 – Observatoire de Paris – Université Paris VII - Paris Diderot – CEA : DSM/IRFU
APC - UMR 7164, Université Paris Diderot, 10 rue Alice Domon et Léonie Duquet, case postale 7020, F-75205 Paris Cedex 13
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 kTe ~ 50 keV, soft seed photons of kTbb ~ 1 keV, and Thomson optical depth τT ~ 1 in a slab geometry. The slab area corresponds to a black body radius of Rbb ~ 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.

Astronomy and Astrophysics (Astron. Astrophys.)
Publisher EDP Sciences
ISSN 0004-6361 (eISSN : 1432-0756)
Articles dans des revues avec comité de lecture

pulsars: individual: IGR J17498-2921 – stars: neutron – X-rays: binaries – X-rays: bursts
9 pages, 8 figures, accepted for publication in A&A. arXiv admin note: text overlap with arXiv:1012.0229

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