Short-Term Variability of X-rays from Accreting Neutron Star Vela X-1: II. Monte-Carlo Modeling

Abstract

We develop a Monte Carlo Comptonization model for the X-ray spectrum of accretion-powered pulsars. Simple, spherical, thermal Comptonization models give harder spectra for higher optical depth, while the observational data from Vela X-1 show that the spectra are harder at higher luminosity. This suggests a physical interpretation where the optical depth of the accreting plasma increases with mass accretion rate. We develop a detailed Monte-Carlo model of the accretion flow, including the effects of the strong magnetic field ($\sim 10^{12}$ G) both in geometrically constraining the flow into an accretion column, and in reducing the cross section. We treat bulk-motion Comptonization of the infalling material as well as thermal Comptonization. These model spectra can match the observed broad-band {\it Suzaku} data from Vela X-1 over a wide range of mass accretion rates. The model can also explain the so-called "low state", in which the uminosity decreases by an order of magnitude. Here, thermal Comptonization should be negligible, so the spectrum instead is dominated by bulk-motion Comptonization.

13 pages, 11 figures, accepted for publication in ApJ

References in corpus (5)

• Thermal and Bulk Comptonization in Accretion-Powered X-Ray Pulsars
• X-ray Spectral Study of the Photoionized Stellar Wind in Vela~X-1
• High variability in Vela X-1: giant flares and off states
• X-ray Diagnostics of Giant Molecular Clouds in the Galactic Center Region and Past Activity of Sgr A*
• Short-Term Variability of X-rays from Accreting Neutron Star Vela X-1: I. Suzaku Observations

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• X-ray polarisation properties of thermal-radiative disc winds in binary systems
• Orbital- and spin-phase variability in the X-ray emission from the accreting pulsar Cen X-3
• Discovery of Red-shifted He-like Iron Absorption Line from Luminous Accreting Neutron Star SMC X-1
• Generalized Flows around Neutron Stars