Investigating the Effects of Outbursts on Pulsars, The Case of 2S 1417-624

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2022-6-15

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Özüdoğru, Özgür Can

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This thesis examines the relationship with physical properties of neutron stars and their disturbances due to an outburst, focusing on particular case of 2S 1417-624 which exhibited a giant outburst in 2018.The entire outburst duration is monitored with The Neutron Star Inner Composition Explorer, which carried out Target of Opportunity observations of 2S 1417-624 between April 1 and September 2018. The extracted lightcurve of all the NICER observations demonstrates the flux evolution of the source during the outburst. Additionally in the case of periodogram analysis, the spin period of the source is measured to be around 17.4 s which is clearly consistent with the frequency signal in the power spectrum and high signal-to-noise ratio pulse profiles generated with epoch folding. While individual power spectra for each observation are investigated, one of them provides a marginal signal around 1 Hz which is interpreted as a possible quasi-periodic oscillation. Pulse profiles demonstrates variability with the change in luminosity due to the outburst. This implies that there is a luminosity dependent change in the accretion geometry. Therefore, this thesis aims to understand the variations of geometry in terms of spectral evolution, using a partial covering absorber model to reveal the accretion geometry along the line of sight. Time resolved spectral parameter changes show a contribution of an external partial covering component hydrogen column density of which are found to be in similar evolution with the flux. Photon index of the power law and covering fraction of the partial covering model provides very similar evolutionary structures and these relational configurations are mutually confined to change as the accretion regime transition takes place (i.e. from sub-critical to super critical and vice versa.) However, the evolution of the spectral parameters do not show any sign for transition from gas dominated disc to radiation dominated disc at supercritical regime below 12 keV. However, flux evolution of the spectral parameters show consistent results in photon index with previous outbursts and reveals physical information regarding the partially covering material and the transition in the accretion geometry due to the increasing flux values.

Subject Keywords

neutron stars, X-ray binaries, accretion discs, 2S 1417-624

URI

https://hdl.handle.net/11511/98104

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

Ö. C. Özüdoğru, “Investigating the Effects of Outbursts on Pulsars, The Case of 2S 1417-624,” M.S. - Master of Science, Middle East Technical University, 2022.