High-precision polarimetric, broad-band spectroscopic and temporal studies of black hole X-ray binaries

Abstract

Black holes are among the most unusual objects in the Universe. Powered through accretion, they are strong sources of radiation in a broad energy range, from radio to hard X-rays. Stellar-mass black holes manifest themselves in binary systems, when their companion star – main sequence or giant – starts to lose matter, which is then captured and accreted by the black hole.

Accreting black hole binaries are complex systems. We observe emission from multiple components, such as accretion disc, hot accretion flow (corona), jet, and even donor star. A large number of accreting black holes are transient sources – they undergo periods of violent activity, increasing luminosity by several orders of magnitude. Throughout the course of an active phase – an outburst – the spectral energy distribution and relative contribution of each component can change dramatically, resulting in a gradual evolution of the observed spectra of the black hole binary transient.

In the first part of the thesis I describe the nature of the black hole binaries, focusing on the emission mechanisms. Using the archival photometric data of GX 339−4 I demonstrate how observed optical and infrared spectral properties of the source can be explained with a three-component jet, hot flow and accretion disc model. In the second part I discuss polarization mechanisms. Polarization is a fundamental property of light and it carries information about the geometrical structure of the source and scattering or polarizing media. I review processes that can produce polarized (or polarize unpolarized) radiation in the accreting black hole binaries and interstellar medium.

In the third part I introduce the novel optical polarimeter (DIPol-UF), which was built in Tuorla Observatory as part of an international collaboration. I outline the challenges of remotely operating high-precision polarimeter and describe the control software that I developed specifically for this instrument.

Finally, I discuss the properties of intrinsic polarization of low-mass X-ray binaries based on polarimetric data of V404 Cyg and MAXI J1820+070. Both objects showed small and variable intrinsic polarization during an outburst, with polarization angle coinciding with jet position angle. I demonstrate how high-precision polarimetry can augment photometric and timing studies of X-ray binaries, shedding more light onto the nature of optical emission in these objects.