arXiv subjects

Santabrata Das

Publications and source records attributed to Santabrata Das.

At least 73 records · Page 4Linked to original sources

arXiv2016

Dynamical structure of magnetized dissipative accretion flow around black holes

We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several super-massive black hole sources and the observational implications of our present analysis are discussed.

astro-ph.HE↗

arXiv2015

Mass loss from advective accretion disc around rotating black holes

We examine the properties of the outflowing matter from an advective accretion disc around a spinning black hole. During accretion, rotating matter experiences centrifugal pressure supported shock transition that effectively produces a virtual barrier around the black hole in the form of post-shock corona (hereafter, PSC). Due to shock compression, PSC becomes hot and dense that eventually deflects a part of the inflowing matter as bipolar outflows because of the presence of extra thermal gradient force. In our approach, we study the outflow properties in terms of the inflow parameters, namely specific energy (${\mathcal E}$) and specific angular momentum ($λ$) considering the realistic outflow geometry around the rotating black holes. We find that spin of the black hole ($a_k$) plays an important role in deciding the outflow rate $R_{\dot m}$ (ratio of mass flux of outflow and inflow), in particular, $R_{\dot m}$ is directly correlated with $a_k$ for the same set of inflow parameters. It is found that a large range of the inflow parameters allows global accretion-ejection solutions and the effective area of the parameter space (${\mathcal E}$, $λ$) with and without outflow decreases with black hole spin ($a_k$). We compute the maximum outflow rate ($R^{max}_{\dot m}$) as function of black hole spin ($a_k$) and observe that $R^{max}_{\dot m}$ weakly depends on $a_k$ that lies in the range $\sim 10\%-18\%$ of the inflow rate for the adiabatic index $(γ)$ with $1.5 \ge γ\ge 4/3$. We present the observational implication of our approach while studying the steady/persistent Jet activities based on the accretion states of black holes. We discuss that our formalism seems to have the potential to explain the observed Jet kinetic power for several Galactic Black Hole sources (GBHs) and Active Galactic Nuclei (AGNs).

astro-ph.HE↗

arXiv2015

Unstable mass-outflows in geometrically thick accretion flows around black holes

Accretion flows around black holes generally result in mass-outflows that exhibit irregular behavior quite often. Using 2D time-dependent hydrodynamical calculations, we show that the mass-outflow is unstable in the cases of thick accretion flows such as the low angular momentum accretion flow and the advection-dominated accretion flow. For the low angular momentum flow, the inward accreting matter on the equatorial plane interacts with the outflowing gas along the rotational axis and the centrifugally supported oblique shock is formed at the interface of both the flows, when the viscosity parameter $α$ is as small as $α\le 10^{-3}$. The hot and rarefied blobs, which result in the eruptive mass-outflow, are generated in the inner shocked region and grow up toward the outer boundary. The advection-dominated accretion flow attains finally in the form of a torus disc with the inner edge of the disc at $3R_{\rm g} \le r \le 6R_{\rm g}$ and the center at $6R_ {\rm g} \le r \le 10R_{\rm g}$, and a series of hot blobs is intermittently formed near the inner edge of the torus and grows up along the outer surface of the torus. As a result, the luminosity and the mass-outflow rate are modulated irregularly where the luminosity is enhanced by 10-40% and the mass-outflow rate is increaed by a factor of few up to ten. We interpret the unstable nature of the outflow to be due to the Kelvin-Helmholtz instability, examining the Richardson number for the Kelvin-Helmholtz criterion in the inner region of the flow. We propose that the flare phenomena of Sgr A* may be induced by the unstable mass-outflow as is found in this work.

astro-ph.HE↗

arXiv2014

On the possibilities of mass loss from an advective accretion disc around stationary black holes

We study the coupled disc-jet system around the black hole where the outflow solutions are obtained in terms of the inflow parameters. We observe that an advective accretion disc can eject outflows/jets for wide range of viscosity parameter. However, such possibility is reduced if the cooling is active as the energy dissipative process inside the disc. For mass outflow, we obtain the parameter space spanned by the inflow angular momentum and the viscosity in terms of cooling and quantify the limits of viscosity parameter.

astro-ph.HE↗

arXiv2014

Formation of non-steady outflows and QPOs around black hole

We study the time dependent properties of sub-Keplerian viscous accretion flow around the black holes. We find that rotating matter feels centrifugal barrier on the way towards the black holes and eventually, shock transition is triggered allowing a part of the post-shock matter to eject out as bipolar outflow. This outflowing matters are supposed to be the precursor of relativistic jets. As viscosity is increased, shock becomes unsteady and start to oscillate when viscosity reached its critical value. This causes the inner part of the disk unsteady resulting periodic ejection of matter from the post-shock region. Also, the same hot and dense post-shock matter emits high energy radiation which modulates quasi-periodically. The power density spectra confirms this features as most of the power is concentrated at a narrow frequency range --- a characteristics (i. e. Quasi-Periodic Oscillation) commonly seen in several outbursting black hole candidates.

astro-ph.HE↗

arXiv2014

Periodic massloss from viscous accretion flows around black holes

We investigate the behaviour of low angular momentum viscous accretion flows around black holes using Smooth Particle Hydrodynamics (SPH) method. Earlier, it has been observed that in a significant part of the energy and angular momentum parameter space, rotating transonic accretion flow undergoes shock transition before entering in to the black hole and a part of the post-shock matter is ejected as bipolar outflows, which are supposed to be the precursor of relativistic jets. In this work, we simulate accretion flows having injection parameters from the inviscid shock parameter space, and study the response of viscosity on them. With the increase of viscosity, shock becomes time dependent and starts to oscillate when the viscosity parameter crosses its critical value. As a result, the in falling matter inside the post-shock region exhibits quasi-periodic variations and causes periodic ejection of matter from the inner disc as outflows. In addition, the same hot and dense post-shock matter emits high energy radiation and the emanating photon flux also modulates quasi-periodically. Assuming a ten solar mass black hole, the corresponding power density spectrum peaks at the fundamental frequency of few Hz followed by multiple harmonics. This feature is very common in several outbursting black hole candidates. We discuss the implications of such periodic variations.

astro-ph.HE↗

arXiv2009

Intergalactic Magnetic Field and Arrival Direction of Ultra-High-Energy Protons

We studied how the intergalactic magnetic field (IGMF) affects the propagation of super-GZK protons that originate from extragalactic sources within the local GZK sphere. Toward this end, we set up hypothetical sources of ultra-high-energy cosmic-rays (UHECRs), virtual observers, and the magnetized cosmic web in a model universe constructed from cosmological structure formation simulations. We then arranged a set of reference objects mimicking active galactic nuclei (AGNs) in the local universe, with which correlations of simulated UHECR events are analyzed. With our model IGMF, the deflection angle between the arrival direction of super-GZK protons and the sky position of their actual sources is quite large with the mean value of $<θ> \sim 15^{\circ}$ and the median value of $\tilde θ\sim 7 - 10^{\circ}$. On the other hand, the separation angle between the arrival direction and the sky position of nearest reference objects is substantially smaller with $ \sim 3.5 - 4^{\circ}$, which is similar to the mean angular distance in the sky to nearest neighbors among the reference objects. This is a direct consequence of our model that the sources, observers, reference objects, and the IGMF all trace the matter distribution of the universe. The result implies that extragalactic objects lying closest to the arrival direction of UHECRs are not necessary their actual sources. With our model for the distribution of reference objects, the fraction of super-GZK proton events, whose closest AGNs are true sources, is less than 1/3. We discussed implications of our findings for correlation studies of real UHECR events.

astro-ph.HE↗

arXiv2009

Studies of dissipative standing shock waves around black holes

We investigate the dynamical structure of advective accretion flow around stationary as well as rotating black holes. For a suitable choice of input parameters, such as, accretion rate ($\dot {\cal M}$) and angular momentum ($λ$), global accretion solution may include a shock wave. The post shock flow is located at few tens of Schwarzchild radius and it is generally very hot and dense. This successfully mimics the so called Compton cloud which is believed to be responsible for emitting hard radiations. Due to the radiative loss, a significant energy from the accreting matter is removed and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of the parameter space ($\dot {\cal M} - λ$) which allows accretion flows to have some energy dissipation at the shock $(Δ{\cal E})$. As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation is reached its critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock whose oscillation causes Quasi-Periodic Oscillations (QPOs) in the emitted radiation, will produce oscillations of high frequency. Such an evolution of QPOs has been observed in several black hole candidates during their outbursts.

astro-ph.HE↗

arXiv2009

Particle Acceleration and the Formation of Relativistic Outflows in Viscous Accretion Disks with Shocks

In this Letter, we present a new self-consistent theory for the production of the relativistic outflows observed from radio-loud black hole candidates and active galaxies as a result of particle acceleration in hot, viscous accretion disks containing standing, centrifugally-supported isothermal shocks. This is the first work to obtain the structure of such disks for a relatively large value of the Shakura-Sunyaev viscosity parameter ($α=0.1$), and to consider the implications of the shock for the acceleration of relativistic particles in viscous disks. In our approach, the hydrodynamics and the particle acceleration are coupled and the solutions are obtained self-consistently based on a rigorous mathematical method. We find that particle acceleration in the vicinity of the shock can provide enough energy to power the observed relativistic jet in M87.

astro-ph.HE↗

arXiv2009

Dynamical Structure of Viscous Accretion Disks with Shocks

We develop and discuss global accretion solutions for viscous ADAF disks containing centrifugally supported isothermal shock waves. The fact that such shocks can exist at all in ADAF disks is a new result. Interestingly, we find that isothermal shocks can form even when the level of viscous dissipation is relatively high. In order to better understand this phenomenon, we explore all possible combinations of the fundamental flow parameters, such as specific energy, specific angular momentum, and viscosity, to obtain the complete family of global solutions. This procedure allows us to identify the region of the parameter space where isothermal shocks can exist in viscous ADAF disks. The allowed region is maximized in the inviscid case, and it shrinks as the level of viscous dissipation increases. Adopting the canonical value gamma=1.5 for the ratio of specific heats, we find that the shock region disappears completely when the Shakura-Sunyaev viscosity parameter alpha exceeds the critical value ~0.27. This establishes for the first time that steady ADAF disks containing shocks can exist even for relatively high levels of viscous dissipation. If an isothermal shock is present in the disk, it would have important implications for the acceleration of energetic particles that can escape to power the relativistic jets commonly observed around underfed, radio-loud black holes. In two specific applications, we confirm that the kinetic luminosity lost from the disk at the isothermal shock location is sufficient to power the observed relativistic outflows in M87 and Sgr A*.

astro-ph.HE↗

arXiv2009

A study of the Correlation of Arrival Directions of UHECRs with the Large Scale Structure of the Universe

Ultrahigh energy cosmic rays (UHECRs) are believed to originate from astrophysical sources, which should trace the large scale structure (LSS) of the universe. On the other hand, the magnetic field in the intergalactic space (IGMF), which also traces the LSS of the universe, deflects the trajectories of the charged UHECRs and spoils the positional correlation of the observed UHECR events with their true sources. To explore this problem, we studied a simulation of the propagation of UHE protons through the magnetized LSS of the universe, reported earlier in Das et al. (2008), in which the IGMF was estimated based on a turbulence dynamo model (Ryu et al. 2008). Hypothetical sources were placed inside clusters and groups of galaxies in the simulated universe, while observers were located inside groups of galaxies that have similar properties as the Local Group. We calculated the statistics of the angular distance between the arrival directions of simulated UHE proton events and the positions of candidate sources in our simulation. We compared the statistics from our simulation with those calculated with the Auger data. We discussed the implication of our works on the nature of the sources of UHECRs.

astro-ph.HE↗

arXiv2008

Dissipative accretion flows around a rotating black hole

We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely, the specific energy (${\cal E}$), the specific angular momentum ($λ$) and the accretion rate (${\dot m}$) of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the ${\cal E}-λ$ plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine-Hugoniot shocks are possible is maximum when the flow is dissipation free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate (${\dot m}$), the closer is the shock location. We believe that some of the high frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes.

astro-ph↗

arXiv2008

Propagation of UHE Protons through Magnetized Cosmic Web

If ultra-high-energy cosmic rays (UHECRs) originate from extragalactic sources, understanding the propagation of charged particles through the magnetized large scale structure (LSS) of the universe is crucial in the search for the astrophysical accelerators. Based on a novel model of the turbulence dynamo, we estimate the intergalactic magnetic fields (IGMFs) in cosmological simulations of the formation of the LSS. Under the premise that the sources of UHECRs are strongly associated with the LSS, we consider a model in which protons with E >10^{19} eV are injected by sources that represent active galactic nuclei located inside clusters of galaxies. With the model IGMFs, we then follow the trajectories of the protons, while taking into account the energy losses due to interactions with the cosmic background radiation. For observers located inside groups of galaxies like ours, about 70% and 35% of UHECR events above 60 EeV arrive within ~15 degree and ~5 degree, respectively, of the source position with time delays of less than ~10^7 yr. This implies that the arrival direction of super-GZK protons might exhibit a correlation with the distribution of cosmological sources on the sky. In this model, nearby sources (within 10 - 20 Mpc) should contribute significantly to the particle flux above ~10^{20} eV.

astro-ph↗

arXiv2008

Turbulence and Magnetic Fields in the Large Scale Structure of the Universe

The nature and origin of turbulence and magnetic fields in the intergalactic space are important problems that are yet to be understood. We propose a scenario in which turbulent flow motions are induced via the cascade of the vorticity generated at cosmological shocks during the formation of the large scale structure. The turbulence in turn amplifies weak seed magnetic fields of any origin. Supercomputer simulations show that the turbulence is subsonic inside clusters/groups of galaxies, whereas it is transonic or mildly supersonic in filaments. Based on a turbulence dynamo model, we then estimate that the average magnetic field strength would be a few microgauss inside clusters/groups, approximately 0.1 microgauss around clusters/groups, and approximately 10 nanogauss in filaments. Our model presents a physical mechanism that transfers the gravitation energy to the turbulence and magnetic field energies in the large scale structure of the universe.

astro-ph↗

arXiv2008

Computation of mass loss from viscous accretion disc in presence of cooling

Rotating accretion flow may undergo centrifugal pressure mediated shock transition even in presence of various dissipative processes, such as viscosity and cooling mechanism. The extra thermal gradient force along the vertical direction in the post shock flow drives a part of the accreting matter as bipolar outflows which are believed to be the precursor of relativistic jets. We compute mass loss rates from a viscous accretion disc in presence of synchrotron cooling in terms of the inflow parameters. We show cooling significantly affects the mass outflow rate, to the extent that, jets may be generated from flows with higher viscosity. We discuss that our formalism may be employed to explain observed jet power for a couple of black hole candidates. We also indicate that using our formalism, it is possible to connect the spectral properties of the disc with the rate of mass loss.

astro-ph↗

arXiv2007

Propagation of UHE protons through a magnetized large scale structure

The propagation of UHECRs is affected by the intergalactic magnetic fields that were produced during the course of the large scale structure formation of the universe. We adopt a novel model where the large scale extragalactic magnetic fields (EGMF) are estimated from local dynamic properties of the gas flows in hydrodynamic simulations of a concordance LambdaCDM universe. With the model magnetic fields, we calculate the deflection angle, time delay and energy spectrum of protons with E > 10^{19} eV that are injected at cosmological sources and then travel through the large scale structure of the universe, losing the energy due to interactions with the cosmic background radiation. Implications of this study on the origin of UHECRs are discussed.

astro-ph↗

arXiv2007

Parameter space study of magnetohydrodynamic flows around magnetized compact objects

We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around neutron stars and black holes and found all possible solution topologies for adiabatic accretion. We divide the parameter space spanned by the conserved energy and angular momentum of the flow in terms of the flow topologies. We also study the possibility of the formation of the MHD shock waves.

astro-ph↗

arXiv2007

Properties of accretion shock waves in viscous flows with cooling effects

We study the properties of the shock waves for a viscous accretion flow having low angular momentum in presence of synchrotron cooling. We present all possible accretion solutions in terms of flow parameters. We identify the region of the parameter space for steady and oscillating shocks and show the effect of various energy dissipation processes on it. We discuss the role of the shock waves while explaining the observations from black hole candidates.

astro-ph↗