Narrow Results

Low angular momentum accretion flows can have standing and oscillating shock waves. We study the region of the parameter space in which multiple sonic points occur in viscous flows in presence of various cooling effects such as bremsstrahlung and Comptonization. We also quantify the parameter space in which shocks are steady or oscillating. We find that cooling induces effects opposite to heating by viscosity even in modifying the topology of the solutions, though one can never be exactly balanced by the other due to their dissimilar dependence on dynamic and thermodynamic parameters. We show that beyond a critical value of cooling, the flow ceases to contain a shock wave.

Non-linearities such as shock waves are common in accretion flows around compact objects. Exact quantification of these non-linearities will help testing time-dependent numerical codes. In this paper, we study the detailed properties of these non-linear waves in a steady accretion or wind flows around a rotating black hole. We use a pseudo-Kerr geometry for this purpose. In the context of energy preserving standing shocks, we find that there are two shock locations for a given pair of conserved flow parameters, such as specific energy and angular momentum. We also show that as the Kerr parameter is increased, the shock location moves closer to the black hole. We discuss the astrophysical implications of such solutions.

We investigate the launching and stability of extragalactic jets through magnetohydrodynamic simulations of jet evolution. In these simulations, a small scale equilibrium magnetic corona is twisted by a differentially rotating accretion disk. Two-dimensional calculations show the formation of a collimated outflow. This outflow is divided into two regions by the Alfvén surface: a magnetically dominated Poynting region, and a kinetically dominated region. Three-dimensional calculations show that the outflow is unstable to the m = 1 kink instability, and that the growth rate of the kink instability decreases as the rotation rate of the accretion disk increases.

Microquasars (MQs) are X-ray binary systems that display relativistic radio jets. These objects constitute a suitable laboratory for testing high energy astrophysical processes still not well understood, such as those present when jets interact with the interstellar medium (ISM). Focusing on the study of the nonthermal contribution from cocoon and bow-shock regions, we explore, under different ISM densities and ages of the jet source, the possibility to detect MQ jet termination regions. We conclude that emission from these regions may be faint, but still detectable in the radio, X-ray, and gamma-ray bands.

We discuss the evidence for proton loading in relativistic jets from microquasars in light of recent constraints on the jet power. We argue that, both in the case of the Cygnus X-1 jet and the entire ensemble of Galactic microquasars, the evidence points towards a significant contribution to the total kinetic energy flux from cold protons. However, as with all other methods of constraining jet composition (except for the singular case of SS 433), a number of alternative, though maybe less plausible, explanations exist. In light of this continued elusiveness of a single slam-dunk argument for proton loading, the best we can hope for is a continuing accumulation of bits of evidence such as these which will, on the whole, form a preponderance of evidence against pure pair jets.

Solutions of black hole accretion flows with axisymmetric shocks are obtained self-consistently when the dissipation at the post-shock flow is taken into account. The Rankine–Hugoniot relationships had to be modified suitably to incorporate the energy loss as well as possible matter loss due to outflows in the post-shock region. The outflow rate from the post-shock region is also computed self-consistently. This was done by considering the quantities in the subsonic post-shock flow as the initial condition for the conical outflow. We have several major results: we find the analytical expression of the ratio of the outflow rate and the inflow rate R_ṁ. We find that R_ṁ strongly depends on the model assumptions which govern the flow geometry. It appears that, (a) the outflow rate is at most a few percent of the inflow rate, (b) the outflow is absent when the shock is relatively weak, (c) the outflow rate decreases with the increase in the energy loss at the post-shock region. These conclusions are very important as they have direct bearings on the observational effects. Since spectrally soft states are generally believed to be caused by the dominance of the soft photons and almost total loss of thermal energy of the Compton cloud by inverse Comptonization, a spectrally softer state should have less outflows. The opposite is generally true: A spectrally harder state will have a stronger outflow, but the result depends on the compression ratio and the adopted model. The other major result is that the model independence of the transonic properties of the flow does not hold in presence of the loss of the energy (radiation) and mass (outflow).

Accretion disks and astrophysical jets are used to model many active astrophysical objects, viz., young stars, relativistic stars, and active galactic nuclei.The problem of jet acceleration and collimation is central for understanding the physics of these objects. There is now a general consensus that jet acceleration is the result of an interplay between rotation and magnetic field. Global numerical simulations that include both the disk and jet physics have so far been limited to relatively short time scales and small ranges of viscosity and resistivity parameters that may be crucial to define the coupling of the inflow/outflow dynamics. Along these lines, we present in this paper self-consistent time-dependent simulations of supersonic jets launched from magnetized accretion disks, using high resolution numerical techniques. In particular we study the effects of the disk magnetic resistivity, parametrized through an α-presctiption, in determining the properties of the inflow/outflow system .We use the MHD FLASH code with adaptive mesh refinement, allowing us to follow the evolution of the structure for a time scale long enough to reach steady state.

We show that the removal of angular momentum is possible in the presence of large scale magnetic stresses, arisen by fields much stronger than that required for magnetorotational instability, in geometrically thick, advective, sub-Keplerian accretion flows around black holes in steady-state, in the complete absence of alpha-viscosity. The efficiency of such angular momentum transfer via Maxwell stress, with the field well below its equipartition value, could be equivalent to that of alpha-viscosity, arisen via Reynolds stress, with α = 0.01 − 0.08. We find in our simpler vertically averaged advective disk model that stronger the magnetic field and/or larger the vertical-gradient of azimuthal component of magnetic field, stronger the rate of angular momentum transfer is, which in turn may lead to a faster rate of outflowing matter, which has important implications to describe the hard spectral states of black hole sources. When the generic origin of alpha-viscosity is still being explored, mechanism of efficient angular momentum transfer via magnetic stresses alone is very interesting.

We search for low-dimensional chaotic signatures in the optical lightcurve of the Kepler field blazar W2R 1926+42. The frequently used correlation integral method is employed in our analysis. We find no apparent evidence for the presence of low-dimensional chaos in the lightcurve. If further confirmed, these results could be of importance for modeling the blazar emission mechanisms.