Indian Center for Space Physics

Ābu Road, India

Indian Center for Space Physics

Ābu Road, India
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Roy S.,NCRA TIFR | Pal S.,Indian Center for Space Physics | Pal S.,Ionospheric and Earthquake Research Center
Astrophysical Journal | Year: 2013

We report the discovery of a shell-like structure G354.4+0.0 of size 1.′6 that shows the morphology of a shell supernova remnant (SNR). Part of the structure shows polarized emission in a NRAO VLA sky survey map. Based on 330 MHz and 1.4 GHz Giant Metrewave Radio Telescope observations and existing observations at higher frequencies, we conclude that the partial shell structure showing synchrotron emission is embedded in an extended H II region of size ∼4′. The spectrum of the diffuse H II region turns over between 1.4 GHz and 330 MHz. The H I absorption spectrum shows this objected to be located more than 5 kpc from Sun. Based on its morphology, non-thermal polarized emission, and size, this object is one of the youngest SNRs discovered in the Galaxy with an estimated age of ∼100-500 yr. © 2013. The American Astronomical Society. All rights reserved.


Giri K.,Sn Bose National Center For Basic Science | Chakrabarti S.K.,Sn Bose National Center For Basic Science | Chakrabarti S.K.,Indian Center for Space Physics
Monthly Notices of the Royal Astronomical Society | Year: 2012

We study the time evolution of a rotating, axisymmetric, viscous accretion flow around black holes using a grid-based finite difference method. We use the Shakura-Sunyaev viscosity prescription. However, we compare with the results obtained when all the three independent components of the viscous stress are kept. We show that the centrifugal pressure supported shocks became weaker with the inclusion of viscosity. The shock is formed farther out when the viscosity is increased. When the viscosity is above a critical value, the shock disappears altogether and the flow becomes subsonic and Keplerian everywhere except in a region close to the horizon, where it remains supersonic. We also find that as the viscosity is increased, the amount of outflowing matter in the wind is decreased to less than a percentage of the inflow matter. Since the post-shock region could act as a reservoir of hot electrons or the so-called 'Compton cloud', the size of which changes with viscosity, the spectral properties are expected to depend on viscosity strongly: the harder states are dominated by low angular momentum and the low-viscosity flow with significant outflows while the softer states are dominated by the high-viscosity Keplerian flow having very few outflows. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Mondal S.,Indian Center for Space Physics | Chakrabarti S.K.,Indian Center for Space Physics | Chakrabarti S.K.,se National Center For Basic Science
Monthly Notices of the Royal Astronomical Society | Year: 2013

We study self-consistently the hydrodynamic and spectral properties of a general class of steady-state accretion discs where we couple both the hydrodynamics and the radiative transfer. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian disc are reprocessed by the electrons in the halo. We study the transonic properties of such a Comptonized flow. We use the Rankine-Hugoniot relation to obtain the shock locations in the disc and compute the radiated spectrum from this shocked disc. We identify the boundary of the parameter space spanned by the specific energy and angular momentum which allows the formation of the standing shocks. We show how the boundary changes in the presence of Compton cooling. Due to the radiative loss, some energy is removed from the accreting matter and the shock moves towards the black hole to maintain the pressure balance condition. We solve the two-temperature equations with Coulomb energy exchange between the protons and the electrons, and the radiative processes such as the bremsstrahlung and thermal Comptonization. We study the variation of the hydrodynamical and spectral properties as a function of the accretion rates of the Keplerian and sub-Keplerian components. Ours is the most accurate transonic solution of an inviscid flow around a black hole to date. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Giri K.,se National Center For Basic Science | Chakrabarti S.K.,se National Center For Basic Science | Chakrabarti S.K.,Indian Center for Space Physics
Monthly Notices of the Royal Astronomical Society | Year: 2013

We carry out a series of numerical simulations of viscous accretion flows having a reasonable spatial distribution of the viscosity parameter. We add the power-law cooling throughout the flow. We show that, in agreement with the theoretical solutions of viscous transonic flows, matter having the viscosity parameter above a critical value becomes a Keplerian disc while matter having lesser viscosity remains a low angular momentum, sub-Keplerian flow. The latter component produces centrifugal pressure supported shock waves. Thus, for instance, a flow having sufficiently high viscosity on the equatorial plane and low viscosity above and below would produce a two-component advective flow where a Keplerian discis surrounded by a rapidly infalling sub-Keplerian halo. We find that the post-shock region of the relatively cooler Keplerian disc is evaporated and the overall configuration is quite stable. This agrees with the theoretical model with twocomponents, which attempt to explain the spectral and timing properties of black hole candidates. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Garain S.K.,se National Center For Basic Science | Ghosh H.,se National Center For Basic Science | Chakrabarti S.K.,se National Center For Basic Science | Chakrabarti S.K.,Indian Center for Space Physics
Astrophysical Journal | Year: 2012

We investigate the effects of cooling of the Compton cloud on the outflow formation rate in an accretion disk around a black hole. We carry out a time-dependent numerical simulation where both the hydrodynamics and the radiative transfer processes are coupled together. We consider a two-component accretion flow in which the Keplerian disk is immersed into an accreting low-angular momentum flow (halo) around a black hole. The soft photons which originate from the Keplerian disk are inverse-Comptonized by the electrons in the halo and the region between the centrifugal pressure supported shocks and the horizon. We run several cases by changing the rate of the Keplerian disk and see the effects on the shock location and properties of the outflow and the spectrum. We show that as a result of Comptonization of the Compton cloud, the cloud becomes cooler with the increase in the Keplerian disk rate. As the resultant thermal pressure is reduced, the post-shock region collapses and the outflow rate is also reduced. Since the hard radiation is produced from the post-shock region, and the spectral slope increases with the reduction of the electron temperature, the cooling produces softer spectrum. We thus find a direct correlation between the spectral states and the outflow rates of an accreting black hole. © 2012. The American Astronomical Society. All rights reserved.


Majumdar L.,Indian Center for Space Physics | Das A.,Indian Center for Space Physics | Chakrabarti S.K.,Indian Center for Space Physics | Chakrabarti S.K.,se National Center For Basic Science
Astrophysical Journal | Year: 2014

The main focus of this paper is to explore the possibility of finding two deuterated isotopomers of H2Cl+ (chloronium) in and around the interstellar medium. The presence of a chloronium ion has recently been confirmed by the Herschel Space Observatory's Heterodyne Instrument for the far-infrared. It observed para-chloronium toward six sources in the Galaxy. To date the existence of its deuterated isotopomers (HDCl+ and D2Cl +) have not been discussed in the literature. We find that these deuterated gas phase ions could be destroyed by various ion-molecular reactions, dissociative recombination (DR), and cosmic rays (CRs). We compute all of the ion-molecular (polar) reaction rates by using the parameterized trajectory theory and the ion-molecular (non-polar) reaction rates by using the Langevin theory. For DR- and CR-induced reactions, we adopt two well-behaved rate formulas. We also include these rate coefficients in our large gas-grain chemical network to study the chemical evolution of these species around the outer edge of the cold, dense cloud. In order to study spectral properties of the chloronium ion and its two deuterated isotopomers, we have carried out quantum chemical simulations. We calculated ground-state properties of these species by employing second-order Moller-Plesset perturbation theory (MP2) along with quadruple-zeta correlation consistent (aug-cc-pVQZ) basis set. Infrared and electronic absorption spectra of these species are calculated by using the same level of theory. The MP2/aug-cc-pVQZ level of theory is used to report the different spectroscopic constants of these gas phase species. These spectroscopic constants are essential to predict the rotational transitions of these species. Our predicted column densities of D2Cl+, HDCl +, along with spectral information may enable their future identification around outer edges of cold, dark clouds. © 2014. The American Astronomical Society. All rights reserved..


Singh C.B.,Indian Center for Space Physics | Chakrabarti S.K.,Indian Center for Space Physics | Chakrabarti S.K.,se National Center For Basic Science
Monthly Notices of the Royal Astronomical Society | Year: 2012

The spectral properties of black hole candidates and outflow rates depend crucially on the models of accretion flow, and thus they are interconnected. In a model of transonic flow, the centrifugal barrier forms a shock wave in the accretion disc at a few tens of Schwarzschild radii. The post-shock region (i.e. the region between the shock and the innermost critical point) can act as a Compton cloud as well a reservoir of outgoing jets/outflows. In order to compute the parameter space in which the outflow can form, we use a suitable Mach number relation that can be satisfied at the shock derived in our earlier work in the presence of dissipation and outflows. Assuming three models of accretion flow (i.e. models of vertical equilibrium, conical wedge shape and constant height), we examine the parameter space of the specific energy and the specific angular momentum for dissipative shock with mass loss. In the first two models, we find that the parameter space is reduced as the cooling rate is increased. However, in the case of the constant height model, the parameter space initially increases with the cooling process, but starts to decrease with a further increase in cooling. One common property in all three accretion flow models is that, above a critical amount of cooling, the parameter space disappears completely. This indicates that the spectrally soft states might not have a significant amount of outflows from the accretion disc. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Das A.,Indian Center for Space Physics | Chakrabarti S.K.,se National Center For Basic Science
Monthly Notices of the Royal Astronomical Society | Year: 2011

We studied the chemical evolution of interstellar grain mantle by varying the physical parameters of the interstellar medium (ISM). To mimic the actual interstellar condition, gas-grain interactions via accretion from the gas phase and desorption (thermal evaporation and photoevaporation) from the grain surface were considered. We found that the chemical composition of the interstellar grain mantle is highly dependent on the physical parameters associated with molecular cloud. Interstellar photons have been found to play an important role in the growth and structure of the interstellar grain mantle. We considered the effects of interstellar photons (photodissociation and photoevaporation) in our simulation under various interstellar conditions. We noticed that the effects of interstellar photons dominate around the region of lower visual extinction. These photons contribute significantly to the formation of the grain mantle. The energy of the incoming photon is attenuated by the absorption and scattering by the interstellar dust. The topmost layers are assumed to be affected mainly by the incoming radiation. We have studied the effects of photodissociation by varying the number of layers which could be affected by it. Model calculations were carried out for the static (extinction parameter is changing with the density of the cloud) as well as the time-dependent case (i.e. both extinction parameter and number density of the cloud are changing with time) and the results are discussed in detail. Different routes to the formation of water molecules have been studied and it has been noticed that production of water molecules via O3 and H2O2 contributes significantly around the dense region. At the end, various observational evidences for the condensed phase species are summarized with their physical conditions and are compared with our simulation results. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Singh C.B.,Indian Center for Space Physics | Chakrabarti S.K.,Indian Center for Space Physics | Chakrabarti S.K.,se National Center For Basic Science
Monthly Notices of the Royal Astronomical Society | Year: 2011

We find a self-consistent solution for the outflow rate from an accretion disc around a black hole. The centrifugal pressure dominated shock in a transonic accretion flow can act as a Compton cloud by emitting radiation in the form of hard X-rays. It is also the base of an outflow where considerable matter is ejected. We modify the Rankine-Hugoniot relationship in the accretion flow when the post-shock region suffers energy as well mass-loss. After connecting the post-shock solution in the disc with the sonic surface properties of the outflow, we obtain the ratio of the outflow rate and inflow rate analytically. Our conclusions are (i) the outflow rate is at the most a few per cent of the inflow rate, (ii) the outflow is absent when the shock is relatively weak (more precisely, the compression ratio is less than about 2) and (iii) the outflow rate decreases with the increase of the energy loss at the post-shock region. Thus spectrally soft states will have lesser outflows. © 2010 The Authors. Journal compilation © 2010 RAS.


Debnath D.,Indian Center For Space Physics | Chakrabarti S.K.,Indian Center For Space Physics | Chakrabarti S.K.,se National Center For Basic Science | Mondal S.,Indian Center For Space Physics
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2014

Spectral and temporal properties of black hole candidates can be explained reasonably well using Chakrabarti-Titarchuk solution of two-component advective flow (TCAF). This model requires two accretion rates, namely the Keplerian disc accretion rate and the halo accretion rate, the latter being composed of a sub-Keplerian, low-angular-momentum flow which may or may not develop a shock. In this solution, the relevant parameter is the relative importance of the halo (which creates the Compton cloud region) rate with respect to the Keplerian disc rate (soft photon source). Though this model has been used earlier to manually fit data of several black hole candidates quite satisfactorily, for the first time, we made it user friendly by implementing it into XSPEC software of Goddard Space Flight Center (GSFC)/NASA. This enables any user to extract physical parameters of the accretion flows, such as two accretion rates, the shock location, the shock strength, etc., for any black hole candidate. We provide some examples of fitting a few cases using this model. Most importantly, unlike any other model, we show that TCAF is capable of predicting timing properties from the spectral fits, since in TCAF, a shock is responsible for deciding spectral slopes as well as quasi-periodic oscillation frequencies. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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