Pulse profile changes in Swift J0243.6+6124, MXB 0656-072 and GRO J1008-57

Date

2025-6-18

Author

Dönmez, Çağatay Kerem

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This thesis investigates pulse profile variations in three Be-type high-mass X-ray binary pulsars, Swift J0243.6+6124, MXB 0656-072, and GRO J1008-57, focusing on their spin frequency evolution and accretion-related transitions. Using data from NICER/XTI, RXTE/PCA, Fermi/GBM and Swift/BAT, timing analyses are performed to identify pulse profile structure changes across different luminosity levels. Theoretical models of accretion column geometry (the Becker model) and accretion torque (Ghosh & Lamb model) are tested against observations. For Swift J0243.6+6124, a previously unreported transition in the pulse profile is discovered at ~7 * 10^36 erg s^-1, consistent with the theoretical luminosity threshold L_coul between pure pencil beam and hybrid beam regimes. At higher luminosities, the data support the predicted L_crit transition to fan beam geometry, allowing the neutron star's magnetic field strength to be constrained to 4.7-5.3 * 10^12 G. MXB 0656-072 shows consistent single-peaked profiles across its 2007-2008 Type I outbursts. Torque-luminosity modelling of the spin frequencies shows that the Ghosh & Lamb model successfully reproduces the spin frequency evolution, and resulting a magnetic field estimate of ~4.2 * 10^12 G, in line with earlier cyclotron line detections. Pulse profiles of GRO J1008-57 remain double-peaked and stable despite flux variation by a factor of 75. The absence of transitions in pulse profiles challenges the Becker model unless implausibly high magnetic fields or neutron star parameters are assumed. As a whole, the results demonstrate that analysing pulse profiles and spin evolution provides key insights into the accretion mechanisms in Be-type X-ray binaries.

Subject Keywords

Neutron stars, Accreting pulsars, X-ray binaries, Accretion columns

URI

https://hdl.handle.net/11511/115196

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Graduate School of Natural and Applied Sciences, Thesis