The Energy Spectrum of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters

TL;DR

This paper proposes a model where accretion from fallback disks explains the X-ray spectra and radio properties of AXPs and SGRs, providing a unified understanding of their emission characteristics.

Contribution

It introduces a bulk-motion Comptonization model for accretion flow that accounts for both soft and hard X-ray emissions in AXPs and SGRs, aligning with observed spectra and radio behavior.

Findings

Successfully reproduces spectra of AXP 4U 0142+61 using the model

Explains the absence or presence of radio emission in these sources

Suggests the model cannot account for giant X-ray outbursts in SGRs

Abstract

Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt to explain both the soft and the hard X-ray emission as the result of the accretion process. We also attempt to explain their radio emission or the lack of it. We test the hypothesis that the power-law, hard X-ray spectra are produced in the accretion flow mainly by bulk-motion Comptonization of soft photons emitted at the neutron star surface. Fallback disk models invoke surface dipole magnetic fields of $10^{12} - 10^{13}$ G, which is what we assume here. Unlike normal X-ray pulsars, for which the accretion rate is highly super-Eddington, the accretion rate is approximately Eddington in AXPs and SGRs and thus the bulk-motion Comptonization operates efficiently. As an illustrative example we reproduce both the hard and the soft X-ray spectra of AXP 4U 0142+61 well using the XSPEC package compTB. Our model seems to explain both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their radio emission or the lack of it, in a natural way. It might also explain the short bursts observed in these sources. On the other hand, it cannot explain the giant X-ray outbursts observed in SGRs, which may result from the conversion of magnetic energy in local multipole fields.