A broad-band X-ray study of magnetic accretion in neutron star X-ray binaries
Abstract
Neutron star X-ray binaries represent some of the most extreme environments in the Universe, with gravitational and magnetic fields trillions of times stronger than those found on Earth. In these binary systems, the stellar companion transfers gas to a neutron star through Roche lobe overflow or a stellar outflow. The gas spirals towards the pulsar and forms an accretion disk, which is truncated at the pulsar's magnetosphere where the magnetic pressure exceeds the disk ram pressure. The magnetic field funnels gas along field lines and directly onto the pulsar's magnetic poles, forming a structure known as a magnetized accretion stream. While theoretical models can predict accretion stream structures, observations are needed to constrain these models and investigate the behavior of matter in these extreme environments. My dissertation provides observational constraints on magnetic accretion streams. I have investigated the geometry and kinematics of gas within the magnetosphere of three pulsars that display periodic variability in their luminosities: LMC X-4, SMC X-1, and Her X-1. This variability, called superorbital because the time scale is longer than the binary orbit, is believed to be caused by a warped inner accretion disk. As the disk precesses, it will partially obscure the pulsar and cause regular changes in brightness. The geometry of these warped disk systems is such that hard pulses, directly from the pulsar beam, and soft pulses, reprocessed by the accretion disk, can be disentangled with broad-band X-ray coverage. I use carefully timed 0.2-79 keV X-ray observations that span a complete superorbital cycle to perform pulse-phase spectroscopy and tomography. With this analysis, I find that the spectral and pulse profile shape are periodic with superorbital phase, proving that these changes are caused by the precessing inner disk. I reproduce the observed changes in pulse profile shape by modeling the geometry of the magnetic accretion disk with a simple reprocessing disk model. The resulting disk geometries provide insights into the structure and kinematics of warped inner accretion disks. My work provides the most complete observational picture to date of pulsar magnetic accretion disk structure.
- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23540803B