• Sandip K. Chakrabarti

      Articles written in Journal of Astrophysics and Astronomy

    • Spacetime with self-gravitating thick disc

      Sandip K. Chakrabarti

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      We compute the metric coefficients and study some properties of the spacetime comprising of a Schwarzschild hole distorted by a self gravitating thick disc.

    • Vortex rings in the working surface of radio jets

      Sandip K. Chakrabarti

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      We study the effects of the presence of vortex rings surrounding a supersonic radio jet inside the cocoon of a radio lobe. We show that both the jet and the shocked ambient medium are pinched. Flow speed inside the cocoon is always close to the sonic value and it stays so by successively passing through several oblique shocks. We also discuss the possibility of the non-linear growth of the instabilities of the contact surface to explain the numerical results in the literature.

    • On the damping of the bending waves in Saturn’s ring

      Sandip K. Chakrabarti

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      We study numerically the motion of a single particle in the bending wave of finite thickness in Saturn’s ring. We include the forcing due to the planet, a moon, the coriolis force and the self gravity of the ring. In particular, we compute the variation of the velocity arising due to the variation of the amplitude and the phase of the epicyclic motion across the local vertical height of the ring. We suggest that the dissipation of energy due to the collision of ring particles in this shear layer damps out the bending wave of Saturn’s ring at the 5:3 vertical resonance of Mimas within a distance of 150 km from the site of its launching as is observed in Voyager data.

    • Multiple shocks in the rotating winds from self-gravitating discs

      Sandip K. Chakrabarti

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      We present the analytic theory of dissipative and non-dissi-pative shocks in the rotating outflows in both the pseudo-Newtonian and the Schwarzschild geometry. We include the effects of the self gravity of the surrounding massive disc and show that the flow may have as many as five critical points when the angular momentum and the disc mass are sufficiently high. This leads to the possibility of the multipleannular shocks within the flow. We derive the expressions correlating the pre-shock and the post-shock quantities for all the three principal types of discontinuities. From these relations it is shown that for given initial flow parameters such as the angular momentum and the energy there could be as many as eighteen formal shock locations out of which at most two are chosen in reality. Detailed classification of the parameter space in terms of the initial flow parameters will be discussed elsewhere

    • The Newtonian forces in the Kerr geometry

      Sandip K. Chakrabarti A. R. Prasanna

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      We study the properties of the ’Newtonian forces’ acting on a test particle in the field of the Kerr black hole geometry. We show that the centrifugal force and the Coriolis force reverse signs at several different locations. We point out the possible relevance of such reversals particularly in the study of the stability properties of the compact rotating stars and the accretion discs in hydrostatic equilibria

    • Standing shocks around black holes and estimation of outflow rates

      Santabrata Das Sandip K. Chakrabarti

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      We self-consistently obtain shock locations in an accretion flow by using an analytical method. One can obtain the spectral properties, quasi-periodic oscillation frequencies and the outflow rates when the inflow parameters are known. Since temperature of the CENBOL decides the spectral states of the black hole, and also the outflow rate, the outflow rate is directly related to the spectral states.

    • Radiatively driven winds from effective boundary layer around black holes

      Indranil Chattopadhyay Sandip K. Chakrabarti

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      Matter accreting onto black holes suffers a standing or oscillating shock wave in much of the parameter space. The post-shock region is hot, puffed up and reprocesses soft photons from a Keplerian disc to produce the characteristic hard tail of the spectrum of accretion discs. The post-shock torus is also the base of the bipolar jets. We study the interaction of these jets with the hard photons emitted from the disc. We show that radiative force can accelerate outflows but the drag can limit the terminal speed. We introduce an equilibrium speed υeq as a function of distance, above which the flow will experience radiative deceleration.

    • Interaction of accretion shocks with winds

      Kinsuk Acharya Sandip K. Chakrabarti D. Molteni

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      Accretion shocks are known to oscillate in presence of cooling processes in the disk. This oscillation may also cause quasi-periodic oscillations of black holes. In the presence of strong winds, these shocks have oscillations in vertical direction as well. We show examples of shock oscillations under the influence of both the effects. When the shocks are absent and the flow is cooler, the wind becomes weaker and the vertical oscillation becomes negligible.

    • Fast transition between high-soft and low-soft states in GRS 1915 + 105: Evidence for a critically viscous accretion flow

      S. Naik A. R. Rao Sandip K. Chakrabarti

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      We present the results of a detailed analysis of RXTE observations of classω (Klein-Woltet al. 2002) which show an unusual state transition between high-soft and low-soft states in the Galactic microquasar GRS 1915 + 105. Out of about 600 pointed RXTE observations, the source was found to exhibit such state transition only on 16 occasions. An examination of the RXTE/ASM data in conjunction with the pointed observations reveals that these events appeared as a series of quasi-regular dips in two stretches of long duration (about 20 days during each occasion) when hard X-ray and radio flux were very low. The X-ray light curve and colour-colour diagram of the source during these observations are found to be different from any reported so far. The duration of these dips is found to be of the order of a few tens of seconds with a repetition time of a few hundred seconds. The transition between these dips and non-dips which differ in intensity by a factor of ∼ 3.5, is observed to be very fast (∼ a few seconds). It is observed that the low-frequency narrow QPOs are absent in the power density spectrum (PDS) of the dip and non-dip regions of classω and the PDS is a power law in the 0.1–10 Hz frequency range. There is a remarkable similarity in the spectral and timing properties of the source during the dip and non-dip regions in this set of observations. These properties of the source are distinctly different from those seen in the observations of other classes. This indicates that the basic accretion disk structure during both dip and non-dip regions of classω is similar, but differ only in intensity. To explain these observations, we invoke a model in which the viscosity is very close to critical viscosity and the shock wave is weak or absent.

    • Study of accretion flows around an ultraluminous X-ray source M82 X-1 using NuSTAR data


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      We study the spectral properties and accretion flow behavior of an ultraluminous X-ray source M82X-1 using NuSTAR observations. We use the physical two-component advective flow (TCAF) model to fit the data and to derive the accretion flow properties of the source. From the model fitted parameters, we found that M82X-1 is harboring an intermediate mass black hole at its centre, where the mass varies from$156.04^{+13.51}_{-15.30}$ to $380.96^{+28.38}_{-29.76}$ $M_{\odot}$. The error weighed average mass of the black hole is $273 \pm 43$ $M_{\odot}$, whichaccreted in nearly super-Eddington rate. The Compton cloud was compact with a size of $\sim$13$r_g$ and the shock compression ratio had nearly intermediate values except for the epoch four. These indicate a possible significant mass outflow from the inner region of the disk. The quasi periodic oscillation (QPO) frequencies estimatedfrom the model fitted parameters can reproduce the observed QPOs. The robustness of the model parameters is verified by drawing the confidence contours among them.

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    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

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      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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