EDP Sciences
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Volume 493, Number 3, January III 2009
Page(s) L39 - L43
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200811401
Published online 11 December 2008

A&A 493, L39-L43 (2009)
DOI: 10.1051/0004-6361:200811401


XMM-Newton detects a relativistically broadened iron line in the spectrum of the ms X-ray pulsar SAX J1808.4-3658

A. Papitto1, 2, T. Di Salvo3, A. D'Aì3, R. Iaria3, L. Burderi4, A. Riggio4, M. T. Menna2, and N. R. Robba3

1  Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
    e-mail: papitto@oa-roma.inaf.it
2  INAF Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
3  Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
4  Dipertimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy

Received 22 November 2008 / Accepted 3 December 2008

We report on a 63-ks long XMM-Newton observation of the accreting millisecond pulsar SAX J1808.4-3658 during the latest X-ray outburst which started on September 21st 2008. The pn spectrum shows a highly significant emission line in the energy band where the iron K-$\alpha$ line is expected, and which we identify as emission from neutral (or mildly ionized) iron. The line profile appears to be quite broad (more than 1 keV FWHM) and asymmetric; the most probable explanation for this profile is Doppler and relativistic broadening from the inner accretion disc. From a fit with a diskline profile we find an inner radius of the disc of $8.7^{+3.7}_{-2.7}~R_{\rm g}$, corresponding to 18.0+7.6-5.6 km for a 1.4 $M_{\odot}$ neutron star. The disc therefore appears truncated inside the corotation radius (31 km for SAX J1808.4-3658) in agreement with the fact that the source was still showing pulsations during the XMM-Newton observation.

Key words: accretion, accretion disks -- line: profiles -- stars: pulsars: individual: SAX J1808.4-3658 -- relativity -- X-rays: binaries

© ESO 2009