CENtrifugal Pressure Supported BOundary Layer Research Articles

Timing Properties of Shocked Accretion Flows around Neutron Stars in the Presence of Cooling

We carry out the first robust numerical simulation of accretion flows on a weakly magnetized neutron star using Smoothed Particle Hydrodynamics (SPH). We follow the Two-Component Advective Flow (TCAF) paradigm for black holes, and focus only on the advective component for the case of a neutron star. This low viscosity sub-Keplerian flow will create a normal boundary layer (or, NBOL) right on the star surface in addition to the centrifugal pressure supported boundary layer (or, CENBOL) present in a black hole accretion. These density jumps could give rise to standing or oscillating shock fronts. During a hard spectral state, the incoming flow has a negligible viscosity causing more sub-Keplerian component as compared to the Keplerian disc component. We show that our simulation of flows with a cooling and a negligible viscosity produces precisely two shocks and strong supersonic winds from these boundary layers. We find that the specific angular momentum of matter dictates the locations and the nature of oscillations of these shocks. For low angular momentum flows, the radial oscillation appears to be preferred. For flows with higher angular momentum, the vertical oscillation appears to become dominant. In all the cases, asymmetries w.r.t. the Z=0 plane are seen and instabilities set in due to the interaction of inflow and outgoing strong winds. Our results capture both the low and high-frequency quasi-periodic oscillations without invoking magnetic fields or any precession mechanism. Most importantly, these solutions directly corroborate observed features of wind dominated high-mass X-ray binaries, such as Cir X-1.

Images and spectra of time-dependent two-component advective flow in presence of outflows

Two Component Advective Flow (TCAF) successfully explains the spectral and tem- poral properties of outbursting or persistent sources. Images of static TCAF with Compton cloud or CENtrifugal pressure supported Boundary Layer (CENBOL) due to gravitational bending of photons have been studied before. In this paper, we study time dependent images of advective flows around a Schwarzschild black hole which include cooling effects due to Comptonization of soft photons from a Keplerian disks well as the self-consistently produced jets and outflows. We show the overall image of the disk-jet system after convolving with a typical beamwidth. A long exposure image with time dependent system need not show the black hole horizon conspicuously, un- less one is looking at a soft state with no jet or the system along the jet axis. Assuming these disk-jet configurations are relevant to radio emitting systems also, our results would be useful to look for event horizons in high accretion rate Supermassive Black Holes in Seyfert galaxies, RL Quasars.

Temporal evolution of photon energy emitted from two-component advective flows: origin of time lag

X-Ray time lag of black hole candidates contains important information regarding the emission geometry. Recently, study of time lags from observational data revealed very intriguing properties. To investigate the real cause of this lag behavior with energy and spectral states, we study photon paths inside a Two Component Advective Flow (TCAF) which appears to be a satisfactory model to explain the spectral and timing properties. We employ the Monte-Carlo simulation technique to carry out the Comptonization process. We use a relativistic thick disk in Schwarzschild geometry as the CENtrifugal pressure supported BOundary Layer (CENBOL) which is the Compton cloud. In TCAF, this is the post-shock region of the advective component. Keplerian disk on the equatorial plane which is truncated at the inner edge i.e., at the outer boundary of the CENBOL, acts as the soft photon source. Ray-tracing code is employed to track the photons to a distantly located observer. We compute the cumulative time taken by a photon during Comptonization, reflection and following the curved geometry on the way to the observer. Time lags between various hard and soft bands have been calculated. We study the variation of time lags with accretion rates, CENBOL size and inclination angle. Time lags for different energy channels are plotted for different inclination angles. The general trend of variation of time lag with QPO frequency and energy as observed in satellite data is reproduced.

Numerical simulation of vertical oscillations in an axisymmetric thick accretion flow around a black hole

We study time evolution of rotating, axisymmetric, two dimensional inviscid accretion flows around black holes using a grid based finite difference method. We do not use reflection symmetry on the equatorial plane in order to inspect if the disk along with the centrifugal barrier oscillated vertically. In the inviscid limit, we find that the CENtrifugal pressure supported BOundary Layer (CENBOL) is oscillating vertically, more so, when the specific angular momentum is higher. As a result, the rate of outflow produced from the CENBOL, also oscillates. Indeed, the outflow rates in the upper half and the lower half are found to be anti-correlated. We repeat the exercise for a series of specific angular momentum {\lambda} of the flow in order to demonstrate effects of the centrifugal force on this interesting behaviour. We find that, as predicted in theoretical models of disks in vertical equilibrium, the CENBOL is produced only when the centrifugal force is significant and more specifically, when {\lambda} > 1.5. Outflow rate itself is found to increase with {\lambda} as well and so is the oscillation amplitude. The cause of oscillation appears to be due to the interaction among the back flow from the centrifugal barrier, the outflowing winds and the inflow. For low angular momentum, the back flow as well as the oscillation are missing. To our knowledge, this is the first time that such an oscillating solution is found with an well-tested grid based finite difference code and such a solution could be yet another reason of why Quasi-Periodic Oscillations should be observed in black hole candidates which are accreting low angular momentum transonic flows.

Viscosity parameter in dissipative accretion flows with mass outflow around black holes

Numerical hydrodynamic simulation of inviscid and viscous flows have shown that significant outflows could be produced from the CENtrifugal pressure supported BOundary Layer or CENBOL of an advective disk. However, this barrier is weakened in presence of viscosity, more so, if there are explicit energy dissipations at the boundary layer itself. We study effects of viscosity and energy dissipation theoretically on the outflow rate and show that as the viscosity or energy dissipation (or both) rises, the prospect of formation of outflows is greatly reduced, thereby verifying results obtained through observations and numerical simulations. Indeed, we find that in a dissipative viscous flow, shocks in presence of outflows can be produced only if the Shakura-Sunyaev viscosity parameter {\alpha} is less than 0.2. This is a direct consequence of modification of the Rankine-Hugoniot relation across the shock in a viscous flow, when the energy dissipation and mass loss in the form of outflows from the post-shock region are included. If we ignore the effects of mass loss altogether, the standing dissipative shocks in viscous flows may occur only if {\alpha} < 0.27. These limits are tighter than the absolute limit of {\alpha} = 0.3 valid for a situation when the shock itself neither dissipates energy nor any outflow is formed. We compute typical viscosity parameters required to understand spectral and temporal properties of several black hole candidates such as GX399-4, MAXI J1659-152 and MAXI J1836-194 and find that required {\alpha} are indeed well within our prescribed limit.

MONTE CARLO SIMULATIONS OF THE THERMAL COMPTONIZATION PROCESS IN A TWO-COMPONENT ACCRETION FLOW AROUND A BLACK HOLE IN THE PRESENCE OF AN OUTFLOW

A black hole accretion may have both the Keplerian and the sub-Keplerian component. In the so-called Chakrabarti–Titarchuk scenario, the Keplerian component supplies low-energy (soft) photons while the sub-Keplerian component supplies hot electrons which exchange their energy with the soft photons through Comptonization or inverse Comptonization processes. In the sub-Keplerian component, a shock is generally produced due to the centrifugal force. The postshock region is known as the CENtrifugal pressure–supported BOundary Layer (CENBOL). In this paper, we compute the effects of the thermal and the bulk motion Comptonization on the soft photons emitted from a Keplerian disk by the CENBOL, the preshock sub-Keplerian disk and the outflowing jet. We study the emerging spectrum when the converging inflow and the diverging outflow (generated from the CENBOL) are simultaneously present. From the strength of the shock, we calculate the percentage of matter being carried away by the outflow and determine how the emerging spectrum depends on the outflow rate. The preshock sub-Keplerian flow is also found to Comptonize the soft photons significantly. The interplay between the up-scattering and down-scattering effects determines the effective shape of the emerging spectrum. By simulating several cases with various inflow parameters, we conclude that whether the preshock flow, or the postshock CENBOL or the emerging jet is dominant in shaping the emerging spectrum depends strongly on the geometry of the flow and the strength of the shock in the sub-Keplerian flow.

Spectral Signature of Mass Loss from (and Mass Gain by) an Accretion Disk around a Black Hole

Accretion and outflows are common in systems that include black holes. Assuming that outflows are primarily produced at the centrifugal pressure-supported boundary layer at the inner disk, the hard X-ray spectrum is softened in the presence of winds. Conversely, the spectrum would be hardened when mass gain takes place at the inner disk. We first demonstrate these changes in the spectrum from theoretical consideration. We then give examples of the spectra of a few black hole candidates that indicate that a significant mass loss or mass gain may be taking place in short timescales, causing changes is spectral slopes.

Computation of outflow rates from accretion disks around black holes

We self-consistently estimate the outflow rate from the accretion rates of an accretion disk around a black hole in which both the Keplerian and the sub-Keplerian matter flows simultaneously. While Keplerian matter supplies soft-photons, hot sub-Keplerian matter supplies thermal electrons. The temperature of the hot electrons is decided by the degree of inverse Comptonization of the soft photons. If we consider only thermally-driven flows from the centrifugal pressure-supported boundary layer around a black hole, we find that when the thermal electrons are cooled down, either because of the absence of the boundary layer (low compression ratio), or when the surface of the boundary layer is formed very far away, the outflow rate is negligible. For an intermediate size of this boundary layer the outflow rate is maximal. Since the temperature of the thermal electrons also decides the spectral state of a black hole, we predict that the outflow rate should be directly related to the spectral state.