Origin of the Soft Excess in X‐Ray Pulsars

Ryan C Hickox,Timothy R Kallman,Ramesh Narayan

doi:10.1086/423928

Ryan C Hickox, Timothy R Kallman + Show 1 more

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https://doi.org/10.1086/423928

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Abstract

The spectra of many X-ray pulsars show, in addition to a power law, a low-energy component that has often been modeled as a blackbody with kT ~ 0.1 keV. However the physical origin of this soft excess has remained a mystery. We examine a sample of well-studied, bright X-ray pulsars, which have been observed using ROSAT, ASCA, Ginga, RXTE, BeppoSAX, Chandra, and XMM-Newton. In particular we consider the Magellanic Cloud pulsars SMC X-1, LMC X-4, XTE J0111.2-7317, and RX J0059.2-7138 and the Galactic sources Her X-1, 4U 1626-67, Cen X-3, and Vela X-1. We show that the soft excess is a very common if not ubiquitous feature intrinsic to X-ray pulsars. We evaluate several possible mechanisms for the soft emission, using theoretical arguments as well as observational clues such as spectral shapes, eclipses, pulsations of the soft component, and superorbital modulation of the source flux. We find that reprocessing of hard X-rays from the neutron star by the inner region of the accretion disk is the only process that can explain the soft excess in all the pulsars with Lx > 10^38 ergs/s. Other mechanisms, such as emission from diffuse gas in the system, are important in less luminous objects.

Highlights

X-ray binary pulsars (XBPs) have provided a fruitful laboratory for high-energy astrophysics since their discovery in the 1970s
We find that reprocessing of hard X-rays from the neutron star by the inner region of the accretion disk is the only process that can explain the soft excess in all the pulsars with LX k 1038 ergs sÀ1
We find that none of the models are able to reproduce the observed emission from RX J0059.2À7138

Summary

Introduction

X-ray binary pulsars (XBPs) have provided a fruitful laboratory for high-energy astrophysics since their discovery in the 1970s (for a review, see Nagase 1989). Many XBPs have been observed extensively and are useful for studying a variety of astrophysical processes, including accretion onto compact objects, binary interactions, and the effects of strong magnetic fields. Most bright XBPs outside the Galaxy (for which soft X-rays are not heavily absorbed by Galactic gas) have shown a marked soft excess in their X-ray spectra above the standard power-law model (Nagase 2002). In this paper we seek to narrow these candidate mechanisms for the soft excess, by examining the physical processes in light of a wide range of observations

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Conclusion