Broad-band X-ray spectra of anomalous X-ray pulsars and soft $\gamma$-ray repeaters: pulsars in a weak-accretion regime ?

TL;DR

This study analyzes broad-band X-ray spectra of five AXPs and SGRs, fitting them with accretion flow models, and finds results consistent with the fall-back disk hypothesis, supporting the accreting pulsar interpretation.

Contribution

It demonstrates that the X-ray spectra of AXPs and SGRs can be explained by accretion models, providing evidence for their classification as accreting pulsars.

Findings

Accretion column radius ~150-350 m

Vertical Thomson optical depth ~50-400

Luminosity corresponds to accretion rates ~10^{15} g/s

Abstract

We present the results from the analysis of the broad-band X-ray spectra of 5 Anomalous X-ray Pulsars (AXPs) and Soft $\gamma$-ray Repeaters (SGRs). We fit their Suzaku and INTEGRAL spectra with models appropriate for the X-ray emission from the accretion flow onto a pulsar. We find that their X-ray spectra can be well described with this model. In particular we find that: (a) the radius of the accretion column is $\sim150-350$ m resulting in a transverse optical depth of $\sim 1$; (b) the vertical Thompson optical depth is $\approx 50-400$, and (c) their luminosity translates in accretion rates $\approx10^{15}\rm{g\, s^{-1}}$. These results are in good agreement with the predictions from the fall-back disk model, providing further support in the interpretation of AXPs and SGRs as accreting pulsars.