Origin of the Soft Excess in X‐Ray Pulsars

The spectra of many X-ray pulsars show, in addition to a power law, alow-energy component that has often been modeled as a blackbody with kT ~ 0.1keV. However the physical origin of this soft excess has remained a mystery. Weexamine a sample of well-studied, bright X-ray pulsars, which have beenobserved using ROSAT, ASCA, Ginga, RXTE, BeppoSAX, Chandra, and XMM-Newton. Inparticular we consider the Magellanic Cloud pulsars SMC X-1, LMC X-4, XTEJ0111.2-7317, 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 notubiquitous feature intrinsic to X-ray pulsars. We evaluate several possiblemechanisms for the soft emission, using theoretical arguments as well asobservational clues such as spectral shapes, eclipses, pulsations of the softcomponent, and superorbital modulation of the source flux. We find thatreprocessing of hard X-rays from the neutron star by the inner region of theaccretion disk is the only process that can explain the soft excess in all thepulsars with Lx > 10^38 ergs/s. Other mechanisms, such as emission from diffusegas in the system, are important in less luminous objects.Comment: 17 pages, 9 figures, accepted for publication in ApJ. Minor corrections; final version as in ApJ (note correction to equation in sec. 7.1