Accretion Flows in Magnetic Cataclysmic Variables

Abstract

Magnetic Cataclysmic Variables (MCVs) are a unique category of semi-detached binary star systems consisting of a high magnetic field (B >1 MG) white dwarf (WD) and a companion star, usually a late-type main sequence star that fills its Roche lobe. These MCV systems offer a wide range of timing behaviours, including periodic and quasi-periodic variations, along with a wide range of spectral properties. These characteristics serve as valuable tools for unravelling and gaining insights into the intricate accretion processes within these systems. Only a small subset of known MCVs currently limits our grasp of the intricacies surrounding their accretion and evolutionary processes. Consequently, it is imperative to identify and characterize the potential MCV candidates, thereby expanding our database of these systems. This will deepen our understanding of their evolutionary paths and the accretion mechanism. Among the MCVs, the appropriate classification of new sources is also essential to comprehend these systems thoroughly regarding the accretion mechanism. Furthermore, the variable nature of accretion flow represents a pivotal characteristic, which has been observed in only a handful of intermediate polars (IPs). In light of these considerations, this thesis focuses on the study of a diverse sample of MCVs in various stages of evolution with spin-to-orbital period ratios (P! /P ) ranging from 0.1 to 1.0 to gain insights into the various accretion flow scenarios these systems manifest. To accomplish our goal, the sample of candidate MCVs was observed using various 1- 2 m class telescopes from India, Uzbekistan, and South Africa. To complete our analysis, we used archival optical and X-ray data from multiple sources, e.g., Transiting Exoplanet Survey Satellite (TESS), XMM-Newton, and Swift satellites. The time-resolved timing analyses of three IPs, TX Col, V902 Mon, and Swift J0746.3- 1608, all exhibiting P! /P <0.1, reveal the variable disc overflow accretion with disc dominance in the majority of the time. The long baseline, continuous, and high cadence data indicate that the variable disc-overflow accretion is the true nature of these systems. It becomes apparent that previous limitations in observational capabilities hindered our ability to detect this phenomenon in the majority of IPs. Swift J0503.7ð€€€2819 is identified as the first nearly synchronous IP below the period with synchronicity of 80 per cent and spin period of 65 min. It is found that this system is predominantly accreting via stream. Further, the energy-dependent X-ray spin pulsations in Swift J0503.7ð€€€2819 are due to the photo-electric absorption in the accretion flow. Such sources play a pivotal role in advancing our understanding of the potential connection between nearly synchronous IPs and polar systems, particularly in the context of the stream-fed accretion scenario. The timing and spectral analyses of the sources 1RXSJ174320.1ð€€€042953, YY Sex, and RBS 0490 confirmed that these sources belong to the category of synchronous MCVs known as polars. The presence of strong hydrogen Balmer lines with strong He II 4686 Ã… and H in the optical spectra of 1RXS J174320.1ð€€€042953 and YY Sex confirms the magnetic nature of accretion flow. However, the characteristic features seen in the optical spectra of RBS 0490 either weaken its chance to be magnetic or may imply the low magnetic strength of the WD. In addition, only the negative value of circular polarization (between 0 and ð€€€20 per cent) modulated on the orbital period unambiguously identifies YY Sex as a one-pole accretor polar. The source LS Cam is identified as a superhumping CV with a superorbital period of 4 d and the source SDSS J075939.79+191417.3 is identified as a non-magnetic CV.