[53.02] The Origin of the Soft Excess in X-ray Pulsars

AAS 203rd Meeting, January 2004
Session 53 Pulsars and Other Neutron Stars
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

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[53.02] The Origin of the Soft Excess in X-ray Pulsars

R.C. Hickox, R. Narayan (Harvard-Smithsonian Center for Astrophysics)

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 (L_x > 10³⁷ erg s^-1), using archival observations from XMM-Newton, Chandra, RXTE, ASCA, and ROSAT. In particular we consider the Magellenic Cloud pulsars SMC X-1, LMC X-4, RX J0059-7138, and XTE J1101-7317, and the Galactic sources Her X-1 and 4U 1626-67. We show that the soft excess is a very common if not ubiquitous feature.

We evaluate several possible mechanisms for the soft emission in terms of theoretical considerations and observational data. Observed behaviors such as eclipses, pulsations of the soft component, and superorbital modulation of the source flux help constrain these mechanisms. We find that optically thick reprocessing from the inner region of the accretion disk is a promising possibility for explaining the soft excess. Such a mechanism may be important for understanding the process of accretion onto magnetized objects.

The author(s) of this abstract have provided an email address for comments about the abstract: rhickox@cfa.harvard.edu

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