| dc.description.abstract | Black hole (BH) accretions are very powerful sources of energy in the universe. In accretion phenomena, the surrounding gas spirals down towards the central BH and forms a disc or quasi-spherical structure based on the outward transport of angular momentum. At the same time, the enormous gravitational potential energy of a BH is released as heat and radiation. However, the presence of strong magnetic fields can alter the underlying radiation mechanisms and thermodynamic properties. Theoretical and observational inferences indicate a signature of dynamically dominant magnetic fields in the vicinity of BHs. It was suggested that the externally generated large-scale magnetic fields could be captured from the environment, say, interstellar medium or companion star, and dragged inward by the accreting plasma. This magnetic field is significantly compressed and becomes dynamically dominant through flux freezing due to continued inward advection of the magnetic flux in the quasi-spherical accretion flow near the BH. This thesis work is on understanding the role of large-scale strong magnetic fields on optically thin and advective accretion flows in which magnetic fields influence the accretion dynamics, remove angular momentum from in-falling matter, help in the formation of strong outflows, and enhance the cooling mechanism through synchrotron and synchrotron self-Comptonization processes.
First, we discuss the importance of large-scale strong magnetic fields in the removal of angular momentum outward, as well as the possible origin of different kinds of magnetic barrier in optically thin, geometrically thick, sub-Keplerian, advective accretion flows around BHs. To treat the full set of magnetohydrodynamics equations, we solve the magnetic fields self-consistently along with the gas dynamics in the general advective paradigm. Here, we consider the pseudo-Newtonian framework with Paczyński \& Witta potential to mimic the space-time geometry around non-rotating BHs. In this simplest vertically averaged, 1.5-dimensional disc model, we choose the maximum upper limit of the magnetic field, which the disc around a BH can sustain. We have shown that in our Magnetically Arrested Advective Accretion Flow (MA-AAF) model, the accreting gas either decelerates or faces the magnetic barrier near the event horizon by the accumulated magnetic fields depending on its geometry. The magnetic barrier may knock the matter to infinity. We suggest that these types of flow are the building block to produce strong jets and outflows in the accreting system. We also find that in some cases, when the matter is trying to go back to infinity after knocking the barrier, the matter is prevented from being escaped by the cumulative action of strong gravity and the magnetic tension, hence by another barrier. In this way, the magnetic field can lock the matter in between these two barriers, and it might be a possible explanation for the formation of episodic jets.
Next, we construct a coupled disc-outflow model around BHs in the MA-AAF paradigm. As an immediate observational consequence, we have applied our disc-outflow symbiosis to explain some long-standing issues of ultraluminous X-ray sources (ULXs), which are very bright, off-nuclear, point sources with luminosity exceeding the standard Eddington limit for a stellar-mass BH. The existing physical scenarios to explain their unusual high luminosity and spectral nature are either the existence of the missing class of intermediate-mass BH (IMBH) or super-Eddington accretion around a stellar-mass BH. However, most ULXs with a steep power-law spectrum can be well explained through super-Eddington accretion, while the IMBH scenario has been disputed extensively. Nevertheless, the interpretation of ULXs with a hard power-law-dominated state remains mysterious. Here we show that the flow energetics of a 2.5-dimensional advective magnetized accretion disc-outflow system around a stellar-mass BH are sufficient to explain the power of ULXs in their hard states. Hence, there is neither need to incorporate the contentious IMBH scenarios nor super-Eddington accretions. We suggest that at least some ULXs are magnetically powered sub-Eddington accretors around a stellar-mass BH.
We further extend our disc-outflow symbiotic model to the general advective, two-temperature framework in the MA-AAF paradigm with explicit cooling. Here, we include the effects of magnetic fields, gas and radiation counterpart together in the entropy gradient based on the first law of thermodynamics to represent energy advection for ions and electrons separately, for the first time. The cooling process includes bremsstrahlung, synchrotron radiation, and inverse-Comptonization processes. Here, we consider the pseudo-Newtonian framework with Mukhopadhyay potential to mimic the space-time geometry around rotating BHs. Interestingly, in our optically thin MA-AAF solutions, the advection of both poloidal and toroidal magnetic fields is happening, unlike other magnetically dominated accretion models. Also, unlike the previous exploration in the framework of disc-outflow symbiosis, the dynamics here is primarily controlled by large-scale magnetic stress. The main objective here is to explain the bright, hard-state observations of accreting systems with stellar-mass to supermassive BHs. One of our main ventures is to find out the hidden nature of mysterious hard-state ULXs. Most importantly, the power-law photon index of these ULXs persists despite their X-ray luminosity varying by over an order of magnitude. Also, these ULXs are very rare in nature. We have shown that our magnetically dominated disc-outflow symbiosis around rapidly spinning stellar-mass BHs can achieve such large luminosity even for the sub-Eddington accretion rate. The magnetic field at the outer zone of the advective flow is more than the corresponding Eddington limit. Such a field becomes dynamically dominant near the BH through continuous accretion process due to flux freezing but maintaining its Eddington limit. This unique field configuration enhances the synchrotron and synchrotron self-Comptonization processes to achieve very large luminosity. However, such magnetic fields are not always possible to capture from the environment, say, companion stars, thus explaining such ULXs to be rare. Our solutions for supermassive BHs can explain the unusual large luminosity of ultra-luminous quasars through the same mechanism.
Finally, we have addressed a unified classification of blazars, a particular class of active galactic nucleus (AGN), by solving magnetized disc-outflow symbiosis self-consistently and compared with $Fermi$ blazar observations. Blazars are radio-loud AGNs with relativistic jets pointing close to our line of sight. Based on the equivalent width (EW) of the optical emission lines, blazars are classified into two subclasses: flat-spectrum radio quasars (FSRQs) with EW $\geq 5$ {\AA} and BL Lac objects with EW $<5$ {\AA}. The signature of strong emission lines suggests an efficient accretion process in FSRQs compared to BL Lacs. According to the $Fermi$ blazar observations, BL Lac objects are less luminous with harder spectra than FSRQs. We compute the jet intrinsic luminosities by beaming corrections determined by different cooling mechanisms. Observed $\gamma$-ray luminosities and spectroscopic measurements of broad emission lines suggest a correlation of the accretion disc luminosity with the jet intrinsic luminosity. Also, theoretical and observational inferences for these jetted sources indicate a signature of hot advective accretion flow and a dynamically dominant magnetic field at the jet footprint. Indeed it is difficult to imagine the powerful jet launching from a geometrically thin Keplerian disc. We propose a magnetized, advective disc-outflow symbiosis with explicit cooling to address a unified classification of blazars by controlling both the mass accretion rate and magnetic field strength. We suggest that the BL Lacs are more optically thin and magnetically dominated than FSRQs at the jet footprint to explain their spectral signatures and intrinsic $\gamma$-ray luminosities. | en_US |
