Broad relativistic iron emission line observed in SAX J1808.4-3658

TL;DR

This study reports the detection of a broad relativistic iron emission line in SAX J1808.4-3658, providing insights into the inner accretion disk radius and magnetic field strength during an outburst.

Contribution

First detection of a broad relativistic Fe K-alpha line in SAX J1808.4-3658, constraining the inner disk radius and magnetic field in this neutron star system.

Findings

Inner disk radius estimated at 13.2 +/- 2.5 GM/c^2

Inner disk not very receded in the island state

Magnetic field strength around 3 x 10^8 G

Abstract

During the September-October 2008 outburst of the accreting millisecond pulsar SAX J1808.4-3658, the source was observed by both Suzaku and XMM-Newton approximately 1 day apart. Spectral analysis reveals a broad relativistic Fe K-alpha emission line which is present in both data-sets, as has recently been reported for other neutron star low-mass X-ray binaries. The properties of the Fe K line observed during each observation are very similar. From modeling the Fe line, we determine the inner accretion disk radius to be 13.2 +/- 2.5 GM/c^2. The inner disk radius measured from the Fe K line suggests that the accretion disk is not very receded in the island state. If the inner disk (as measured by the Fe line) is truncated at the magnetospheric radius this implies a magnetic field strength of ~3E8 G at the magnetic poles, consistent with other independent estimates.