arXiv subjects

Sandip K. Chakrabarti

Publications and source records attributed to Sandip K. Chakrabarti.

At least 217 records · Page 12Linked to original sources

arXiv2000

Spectral Signature of Wind Generation From The Post-Shock Region in GRS1915+105 Accretion Disk

Accretion and outflows are common in systems which include black holes. Especially important is the case of the well known micro-quasar GRS1915+105 in our own galaxy, where super-luminal outflows are detected. We present a few observation which are suggestive of an outflow which is generated very close to the black hole, within a few tens of Schwarzschild radii. In the presence of mass loss (e.g., an outflowing wind), the electron density of matter within the centrifugal pressure supported region (which generates hard X-rays) goes down and it is easier to cool these electrons by soft photons coming from the Keplerian disk. If, on the other hand, the post-shock region gains mass from outside, the spectra would be harder. These properties of spectral `softening' of the low state and `hardening' of the high state have been detected in several days of RXTE data of GRS1915+105 which we present here.

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arXiv2000

Scattering of Dirac Waves off Kerr Black Holes

Chandrasekhar separated the Dirac equation for spinning and massive particles in Kerr geometry into radial and angular parts. Here we solve the complete wave equation and find out how the Dirac wave scatters off Kerr black holes. The eigenfunctions, eigenvalues and reflection and transmission co-efficients are computed. We compare the solutions with several parameters to show how a spinning black hole distinguishes mass and energy of incoming waves. Very close to the horizon the solutions become independent of the particle parameters indicating an universality of the behaviour.

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arXiv2000

Dirac equation in Kerr geometry and its solution

Chandrasekhar separated the Dirac equation for spinning and massive particles in Kerr geometry into radial and angular parts. In the present review, we present solutions of the complete wave equation and discuss how the Dirac wave scatters off Kerr black holes. The eigenfunctions, eigenvalues and reflection and transmission co-efficients are computed for different Kerr parameters. We compare the solutions with several parameters to show how a spinning black hole distinguishes mass and energy of incoming waves. Very close to the horizon, the solutions become independent of the particle parameters indicating an universality of the behaviour.

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arXiv2000

Solution of Dirac equation around a spinning Black Hole

Chandrasekhar separated the Dirac equation for spinning and massive particles in Kerr geometry in radial and angular parts. Chakrabarti solved the angular equation and found the corresponding eigenvalues for different Kerr parameters. The radial equations were solved asymptotically by Chandrasekhar. In the present paper, we use the WKB approximation to solve the spatially complete radial equation and calculate analytical expressions of radial wave functions for a set of Kerr and wave parameters. From these solutions we obtain local values of reflection and transmission coefficients.

gr-qc↗

arXiv2000

Adenine Abundance in a Collapsing Molecular Cloud

A vital ingredient of DNA molecule named adenine may be produced by successive addition of HCN during molecular cloud collapse and star formation. We compute its abundance in a collapsing cloud as a function of the reaction rate and show that in much of the circumstances the resulting amount may be sufficient to contaminate planets, comets and meteorites. We introduce a $f$-parameter which may be used to study the abundance where radiative association takes place.

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arXiv2000

Can DNA bases be produced during molecular cloud collapse?

Using a reasonably large chemical reaction network consisting of 421 species, we show that along with normal chemical evolution of molecular cloud during collapse and star formation, significant amount of adenine, a DNA base, may be produced after an evolution of 10^{6-7} years. This findings may shed light on whether life on earth had to begin from scratch or these molecules could have contaminated the earth from the beginning.

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arXiv2000

Correlation among QPO frequencies and Quiescence-state Duration in Black Hole Candidate GRS 1915+105

We discover a definite correlation between the frequency of the quasi-periodic oscillations (QPO) in quiescence states and the duration of the quiescence state of the transient X-ray source GRS 1915+105. We find that while the QPO frequency can be explained with the oscillation of shocks in accretion flows, the switching of burst to quiescence states (and vice versa) and their duration can be explained by assuming an outflow from the post-shock region. The duration of the quiescence state is inversely related to the QPO-frequency. We derive this relation. We also find the correlation between the observed low ($\sim 0.001-0.01$Hz) and the intermediate (1-10Hz) QPO frequencies. Our analytical solutions are verified by analyzing several days of public-domain data from RXTE.

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arXiv1999

Nucleosynthesis in accretion flows around Black Holes

Significant nucleosynthesis is possible in the centrifugal pressure-supported dense and hot region of the accretion flows which deviate from Keplerian disks around black holes. We compute composition changes and energy generations due to such nuclear processes. We use a network containing 255 species and follow the changes in composition. Highly viscous, high-accretion-rate flows deviate from a Keplerian disk very close to the black hole and the temperature of the flow is very small due to Compton cooling. No significant nucleosynthesis takes place in these cases. Low-viscosity and lower-accretion-rate hot flows deviate farther out and significant changes in composition are possible in these cases. We suggest that such changes in composition could be contributing to the metallicities of the galaxies. Moreover, the radial variation of the energy generation/absorption specifically due to proton capture and photo-dissociation reactions could cause instabilities in the inner regions of the accretion flows. For most of these cases sonic point oscillations may take place. We discuss the possibility of neutrino emissions.

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arXiv1999

Mass Outflow Rate From Accretion Discs around Compact Objects

We compute mass outflow rates from accretion disks around compact objects, such as neutron stars and black holes. These computations are done using combinations of exact transonic inflow and outflow solutions which may or may not form standing shock waves. Assuming that the bulk of the outflow is from the effective boundary layers of these objects, we find that the ratio of the outflow rate and inflow rate varies anywhere from a few percent to even close to a hundred percent (i.e., close to disk evacuation case) depending on the initial parameters of the disk, the degree of compression of matter near the centrifugal barrier, and the polytropic index of the flow. Our result, in general, matches with the outflow rates obtained through a fully time-dependent numerical simulation. In some region of the parameter space when the standing shock does not form, our results indicate that the disk may be evacuated and may produce quiescence states.

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arXiv1999

Estimation and Effects of the mass outflow rate from shock compressed flow around compact objects

Outflows are common in many astrophysical systems which contain black holes and neutron stars. Difference between stellar outflows and outflows from these systems is that the outflows in these systems have to form out of the inflowing material only. The inflowing material can form a hot and dense cloud surrounding the compact object, either because of a centrifugal barrier, or a denser barrier due to pair plasma or pre-heating effects. This barrier behaves like a stellar surface as far as the mass loss is concerned. We estimate the outflow rate from the regions of shock compressed flow. The outflow rate is directly related to the compression ratio of the gas at the shocks. These estimated rates roughly match the rates in real observations as well as those obtained from numerical experiments. In special geometries, where the solid angle of the outflow is higher, the disk evacuation takes place creating quiescence states. Outflows are shown to be important in deciding the spectral states and Quasi Periodic Oscillations (QPO)s of observed X-rays.

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arXiv1999

Semi-analytical Solution of Dirac equation in Schwarzschild Geometry

Separation of the Dirac equation in the spacetime around a Kerr black hole into radial and angular coordinates was done by Chandrasekhar in 1976. In the present paper, we solve the radial equations in a Schwarzschild geometry semi-analytically using Wentzel-Kramers-Brillouin approximation (in short WKB) method. Among other things, we present analytical expression of the instantaneous reflection and transmission coefficients and the radial wave functions of the Dirac particles. Complete physical parameter space was divided into two parts depending on the height of the potential well and energy of the incoming waves. We show the general solution for these two regions. We also solve the equations by a Quantum Mechanical approach, in which the potential is approximated by a series of steps and found that these two solutions agree. We compare solutions of different initial parameters and show how the properties of the scattered wave depend on these parameters.

gr-qc↗

arXiv1999

Neutron Disks Around Black Holes

We study nucleosynthesis in low accretion rate hot advective flows around black holes. We find that matter is generally photo-dissociated into protons and neutrons inside the disk. These neutrons stay around black holes for longer time because they are not coupled to magnetic fields while the protons accrete into the hole. We find the nature of the resulting neutron disks and estimate the rate at which these disks contaminate the surroundings.

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arXiv1998

Global Inflow and Outflow Solutions (GIOS) around a Black Hole

Twenty five years have passed by since models of accretions and jets have separately emerged. Today, it is understood that these two objects are related to each other in a fundamental way. In a binary system, matter from an accretion disk enters into a black hole. A part of it is bounced back because of the centrifugal barrier, radiation pressure or magnetohydrodynamic effects, to form jets and bipolar outflows which carry away excess angular momentum. In the case of AGNs containing black holes, accretion disks form out of stellar winds and similar processes as above form cosmic radio jets. We present a general review of the study of the accretion disks and outflows in a coherent manner, especially emphasizing global inflow-outflow solutions (GIOS). We also present a few observational consequences of wind production from the accretion disks on spectral properties of the accretion disks.

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arXiv1998

Mass Outflow Rate From Advective Accretion Disks around Compact Objects

We compute mass outflow rates from advective accretion disks around compact objects, such as neutron stars and black holes. These computations, for the first time, are done using combinations of exact transonic inflow and outflow solutions which may or may not form standing shock waves. Assuming that the bulk of the outflow is from the effective boundary layers of these objects, we find that the ratio of the outflow rate and inflow rate varies anywhere from a few percent to even close to a hundred percent (i.e., close to disk evacuation case) depending on the initial parameters of the disk, the degree of compression of matter near the centrifugal barrier, and the polytropic index of the flow. Our result, in general, matches with the outflow rates obtained through a fully time-dependent numerical simulation. In some region of the parameter space when the standing shock does not form, our results indicate that the disk may be evacuated and may produce quiescence states.

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arXiv1998

Spectral Softening due to Winds in Accretion Disks

Accretion flows may produce profuse winds when they have positive specific energy. Winds deplete matter from the inner region of the disk and makes the inner region thinner, optically. Since there are fewer electrons in this region, it becomes easier to Comptonize this part by the soft photons which are intercepted from the Keplerian disk farther out. We present a self-consistent picture of winds from an accretion disk and show how the spectra of the disk is softened due to the outflowing wind.

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arXiv1998

Estimation of the Mass Outflow Rate From Compact Objects

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arXiv1998

Accretion Disks Around Black Holes: Twenty Five Years Later

We study the progress of the theory of accretion disks around black holes in last twenty five years and explain why advective disks are the best bet in explaining varied stationary and non-stationary observations from black hole candidates. We show also that the recently proposed advection dominated flows are incorrect.

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arXiv1998

Identification of Astrophysical Black Holes

Black holes are by definition black, and therefore cannot be directly observed by using electromagnetic radiations. Convincing identification of black holes must necessarily depend on the identification of a very specially behaving matter and radiation which surround them. A major problem in this subject of black hole astrophysics is to quantify the behaviour of matter and radiation close to the horizon. In this review, the subject of black hole accretion and outflow is systematically developed. It is shown that both the stationary as well as the non-stationary properties of the observed spectra could be generally understood by these solutions. It is suggested that the solutions of radiative hydrodynamic equations may produce clear spectral signatures of black holes. Other circumstantial evidences of black holes, both in the galactic centers as well as in binary systems, are also presented.

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