SAX J1808.4-3658: high resolution spectroscopy and decrease of pulsed fraction at low energies

TL;DR

This study uses high-resolution spectroscopy and timing analysis of SAX J1808.4-3658 during its 2008 outburst to reveal a decrease in pulsed fraction at low energies and identify interstellar absorption features.

Contribution

It provides the first pulse profile analysis below 2 keV and detailed spectral analysis, including interstellar medium characterization, during an outburst.

Findings

Detection of a strong unpulsed component below 2 keV

Observation of double-peaked profiles at higher energies

Identification of interstellar absorption lines and edges

Abstract

XMM-Newton observed the accreting millisecond pulsar SAX J1808.4-3658 during its 2008 outburst. We present timing and spectral analyses of this observation, in particular the first pulse profile study below 2 keV, and the high-resolution spectral analysis of this source during the outburst. Combined spectral and pulse profile analyses suggest the presence of a strong unpulsed source below 2 keV that strongly reduces the pulsed fraction and a hard pulsed component that generates markedly double peaked profiles at higher energies. We also studied the high-resolution grating spectrum of SAX J1808.4-3658, and found several absorption edges and Oxygen absorption lines with whom we infer, in a model independent way, the interstellar column densities of several elements in the direction of SAX J1808.4-3658.