Browsing by Author "Donmez, C. K."

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    Comprehensive analysis of the transient X-ray pulsar MAXI J1409 619
    (2020) Donmez, C. K.; Serim, M. M.; Inam, S. C.; Sahiner, S.; Serim, D.; Baykal, A.
    We probe the properties of the transient X-ray pulsar MAXI J1409-619 through RXTE and Swift follow-up observations of the outburst in 2010. We are able to phase-connect the pulse arrival times for the 25 d episode during the outburst. We suggest that either an orbital model (with P-orb similar or equal to 14.7(4) d) or a noise process due to random torque fluctuations (with S-r approximate to 1.3 x 10(-18) Hz(2) S-2 Hz(-1)) is plausible to describe the residuals of the timing solution. The frequency derivatives indicate a positive torque-luminosity correlation, which implies temporary accretion disc formation during the outburst. We also discover several quasiperiodic oscillations in company with their harmonics whose centroid frequencies decrease as the source flux decays. The variation of the pulsed fraction and spectral power-law index of the source with X-ray flux is interpreted as the sign of transition from a critical to a subcritical accretion regime at the critical luminosity within the range of 6 x 10(37)-1.2 x 10(38) erg s(-1). Using pulse-phase-resolved spectroscopy, we show that the phases with higher flux tend to have lower photon indices, indicating that the polar regions produce spectrally harder emission.
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    Timing and spectral analysis of 2S 1417-624 during its 2018 outburst
    (2021) Serim, M. M.; Ozudogru, O. C.; Donmez, C. K.; Sahiner, S.; Serim, D.; Baykal, A.; Inam, S. C.; 0000-0003-0820-9186
    We investigate timing and spectral characteristics of the transient X-ray pulsar 2S 1417-624 during its 2018 outburst with NICER follow up observations. We describe the spectra with high-energy cut-off and partial covering fraction absorption (PCFA) model and present flux-dependent spectral changes of the source during the 2018 outburst. Utilizing the correlation-mode switching of the spectral model parameters, we confirm the previously reported sub-critical to critical regime transitions and we argue that secondary transition from the gas-dominated to the radiation pressure-dominated disc do not lead us to significant spectral changes below 12 keV. Using the existing accretion theories, we model the spin frequency evolution of 2S 1417-624 and investigate the noise processes of a transient X-ray pulsar for the first time using both polynomial and luminosity-dependent models for the spin frequency evolution. For the first model, the power density spectrum of the torque fluctuations indicate that the source exhibits red noise component (Gamma similar to -2) within the time-scales of outburst duration which is typical for disc-fed systems. On the other hand, the noise spectrum tends to be white on longer time-scales with high timing noise level that indicates an ongoing accretion process in between outburst episodes. For the second model, most of the red noise component is eliminated and the noise spectrum is found to be consistent with a white noise structure observed in wind-fed systems.