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

TL;DR

This paper reports the detection of a relativistically broadened iron line in the X-ray spectrum of the accreting millisecond pulsar SAX J1808.4-3658, indicating inner disc properties and truncation near the neutron star.

Contribution

First detection of a relativistically broadened iron line in SAX J1808.4-3658, revealing inner accretion disc characteristics during an outburst.

Findings

Iron line identified as neutral or mildly ionized iron

Inner disc radius estimated at ~18 km

Disc truncated inside the corotation radius

Abstract

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_g, corresponding to 18.0^(+7.6)_(-5.6) km for a 1.4 Msun 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.