arXiv subjects

Sandip K. Chakrabarti

Publications and source records attributed to Sandip K. Chakrabarti.

At least 145 records · Page 8Linked to original sources

arXiv2010

Effects of Initial Condition and Cloud Density on the Composition of the Grain Mantle

Evolution of grain mantles in various interstellar environment is studied. We concentrate mainly on water, methanol, carbon di-oxide, which constitute nearly 90% of the grain mantle. We investigate how the production rates of these molecules depend on the relative gas phase abundances of oxygen and carbon monoxide and constrain the relevant parameter space which reproduces these molecules closed to the observed abundances. Allowing to accrete only H, O and CO on the grains and using the Monte-Carlo method we follow the chemical processes for a few million years. We allow formation of multi-layers on the grains and incorporate the freeze-out effects of accreting O and CO. We find that the formation of these molecules depends on the initial conditions as well as the average cloud density. Specifically, when the number density of accreting O is less than 3 times more than that of CO, methanol is always over-produced. Using available reaction pathways it appears to be difficult to match the exact observed abundances of all the three molecules simultaneously. Only in a narrow region of parameter space all these three molecules are produced within the observed limit. In addition to this, we found that the incorporation of the freeze-outs of O and CO leads to almost steady state on the grain surface. The mantle thickness grows anywhere between 60 to 500 layers in a period of two million years. In addition, we consider a case where the gas number density changes with time due to gradual collapse of the molecular cloud and present the evolution of composition of different species as a function of radius of the collapsing cloud.

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arXiv2010

Properties of the propagating shock wave in the accretion flow around GX 339-4 in the 2010 outburst

Context. The black hole candidate GX 339-4 exhibited an X-ray outburst in January 2010, which is still continuing. We here discuss the timing and the spectral properties of the outburst using RXTE data. Aims. Our goal is to study the timing and spectral properties of GX 339-4 using its recent outburst data and extract information about the nature of the accretion flow. Methods. We use RXTE archival data of the recent GX 339-4 outburst and analyze them with the NASA HEAsoft package, version 6.8. We then compare the observed quasi-periodic oscillation (QPO) frequencies with those from existing shock oscillation model and obtain the nature of evolution of the shock locations during the outburst. Results. We found that the QPO frequencies are monotonically increasing from 0.102 Hz to 5.69 Hz within a period of ~ 26 days. We explain this evolution with the propagating oscillatory shock (POS) solution and find the variation of the initial and final shock locations and strengths. The model fits also give the velocity of the propagating shock wave, which is responsible for the generation of QPOs and their evolutions, at ~ 10 m/s. We observe from the spectra that up to 2010 April 10, the object was in a hard state. After that, it went to the hard-intermediate state. On April 18, it had a state transition and went to the soft-intermediate state. On May 15, another state transition was observed and the source moved to the soft state. Conclusions. As in the previously fitted outburst sources, this source also showed the tendency of a rapidly increasing QPO frequency ($ν_{QPO}$) in a viscous time scale, which can be modeled quite accurately. In this case, the shock seems to have disappeared at about ~ 172 Schwarzschild radii, unlike in the 2005 outburst of GRO J1655-40, where the shock disappeared behind the horizon.

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arXiv2010

Monte-Carlo Simulations of Thermal Comptonization Process in a Two Component Accretion Flow Around a Black Hole in 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.

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arXiv2009

X-ray experiments for Space applications in intermediate energy range

X-ray experiments in the intermediate energy range (1-50 keV) are carried out at the Indian Centre for Space Physics (ICSP), Kolkata for space application. The purpose is to carry out developmental studies of space instruments to observe energetic phenomena from compact objects (black hole and compact stars) and active stars and their testing and evaluation. The testing/evaluation setup primarily consists of an X-ray generator, various X-ray imaging masks, an X-ray imager (CMOS) and an X-ray spectrometer (Si-PIN photo-diode). The X-ray generator (Mo target) operates in 1-50 kV anode voltage, and 1-30 mA beam current. A 45 feet long shielded collimator is used to collimate the beam which leads to the detector chamber having a 30 arc-sec angular diameter. Two types of imaging masks are used - conventional Coded Aperture Masks (CAM) and Tungsten Fresnel half-period zone-plates (ZPs) having angular resolutions of a few tens of arc-sec. The Moire fringe pattern produced by the composite shadows of two ZPs is inverse Fourier transformed to obtain the X-ray source distribution. CAMs are advantageous but the resolution obtained is limited by their smallest pixel size. Our setup has been extensively used in testing and evaluation of RT-2 payloads which have been launched recentlya (January 30, 2009). More experiments for improving imaging techniques are being designed and tested.

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arXiv2009

Indian Payloads (RT-2 Experiment) Onboard CORONAS-PHOTON Mission

RT-2 Experiment (RT - Roentgen Telescope) is a low energy gamma-ray instrument which is designed and developed as a part of Indo-Russian collaborative project of CORONAS-PHOTON Mission to study the Solar flares in wide energy band of electromagnetic spectrum ranging from UV to high-energy gamma-rays (~2000 MeV). RT-2 instruments will cover the energy range of 15 keV to 150 keV extendable up to ~1 MeV. It consists of three detectors (two Phoswich detectors, namely, RT-2/S, RT-2/G and one solid-state imaging detector RT-2/CZT) and one processing electronic device (RT-2/E). Both Phoswich detectors will have time resolved spectrum, whereas the solid-state imaging detector will have high resolved image of the solar flares in hard X-rays. We have used Co-57 (122 keV) radio-active source for onboard calibration of all three detectors. In this paper, we briefly discuss the in-flight performance of RT-2 instruments and present initial flight data from the instruments. This mission was launched into polar LEO (Low Earth Orbit) (~550 km) on 30th January 2009 from Plesetsk Cosmodrome, Russia.

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arXiv2009

Fresnel Zone Plate Telescopes as high resolution imaging devices

Combination of Fresnel Zone Plates (FZP) can make excellent telescopes for imaging in X-rays. We present the results of our experiments with such telescopes with an X-ray source kept at a distance of 45 feet. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. In simulations, we allow the sources to be at finite distances (diverging beam) as well as at infinite distances (parallel beam) and show that the resolution is worsened when the source is nearby. We also present simulated results for the observation of the galactic center and show that the sources may be reconstructed with accuracy. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy. The Zone Plate based instrument has been sent for the first time in a recently launched KORONAS-FOTON satellite.

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arXiv2009

Hydrodynamic Simulations of Oscillating Shock Waves in a Sub-Keplerian Accretion Flow Around Black Holes

We study the accretion processes on a black hole by numerical simulation. We use a grid based finite difference code for this purpose. We scan the parameter space spanned by the specific energy and the angular momentum and compare the time-dependent solutions with those obtained from theoretical considerations. We found several important results (a) The time dependent flow behaves close to a constant height model flow in the pre-shock region and a flow with vertical equilibrium in the post-shock region. (c) The infall time scale in the post-shock region is several times higher than the free-fall time scale. (b) There are two discontinuities in the flow, one being just outside of the inner sonic point. Turbulence plays a major role in determining the locations of these discontinuities. (d) The two discontinuities oscillate with two different frequencies and behave as a coupled harmonic oscillator. A Fourier analysis of the variation of the outer shock location indicates higher power at the lower frequency and lower power at the higher frequency. The opposite is true when the analysis of the inner shock is made. These behaviours will have implications in the spectral and timing properties of black hole candidates.

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arXiv2009

Fresnel zone plate telescopes for X-ray imaging II: numerical simulations with parallel and diverging beams

We present the results of simulations of shadows cast by a zone plate telescope which may have one to four pairs of zone plates. From the shadows we reconstruct the images under various circumstances. We discuss physical basis of the resolution of the telescope and demonstrate this by our simulations. We allow the source to be at a finite distance (diverging beam) as well as at an infinite distance (parallel beam) and show that the resolution is worsened when the source is nearby. By reconstructing the zone plates in a way that both the zone plates subtend the same solid angles at the source, we obtain back high resolution even for sources at a finite distance. We present simulated results for the observation of the galactic center and show that the sources of varying intensities may be reconstructed with accuracy. Results of these simulations would be of immense use in interpreting the X-ray images from recently launched CORONAS-PHOTON satellite.

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arXiv2009

Fresnel zone plate telescopes for X-ray imaging I: experiments with a quasi-parallel beam

Combination of Fresnel Zone Plates (FZP) can make an excellent telescope for imaging in X-rays. We present here the results of our experiments with several pairs of tungsten made Fresnel Zone plates in presence of an X-ray source kept at a distance of about 45 feet. The quasi-parallel beam allowed us to study sources placed on the axis as well as off the axis of the telescope. We present theoretical study of the fringe patterns produced by the zone plates in presence of a quasi-parallel source. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. The images are also reconstructed by deconvolution from both the patterns. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy.

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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

Accretion flow behaviour during the evolution of the Quasi Periodic Oscillation Frequency of XTE J1550-564 in 1998 outburst

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550-564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 1998 September 7 to the 1998 September 19, when the QPO frequency increased monotonically from 81mHz to 13.1Hz. Immediately after that, QPO frequency started to decrease and on the 1998 September 26, the QPO frequency became 2.62Hz. After that, its value remained almost constant. This frequency drift can be modelled satisfactorily with a propagatory oscillating shock solution where the post-shock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655-40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called `outburst' may not be a full-fledged outburst.

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arXiv2009

Monte-Carlo Simulations of Thermal Comptonization Process in a Two Component Accretion Flow Around a Black Hole

We compute the effects of thermal Comptonization of soft photons emitted from a Kep- lerian disk around a black hole by the post-shock region of a sub-Keplerian flow, known as the CENtrifugal pressure dominated BOundary Layer (CENBOL). We show that the spectral state transitions of black hole candidates could be explained either by varying the outer boundary of the CENBOL, which also happens to be the inner edge of the Keplerian disk, or by changing the central density of the CENBOL which is governed by the rate of the sub-Keplerian flow. We confirm the conclusions of the previous the- oretical studies that the interplay between the intensity of the soft photons emitted by the Keplerian flow and the optical depth and electron temperature of the Comptonizing cloud is responsible for the state transitions in a black hole.

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arXiv2009

Quasi Periodic Oscillations due to Axisymmetric and Non-Axisymmetric Shock Oscillations in Black Hole Accretion

Quasi-Periodic Oscillations (QPOs) are very puzzling since they remain totally unexplained by popular earlier models of accretion disks. The significant rms value in power density spectrum implies that the oscillation involves in the dynamical and non-linear variation of certain region of the accretion disk itself. The nature of the energy dependence implies that the region which produces Comptonized hard tail is also responsible for QPOs. Similarly, the occurrences of the QPOs are strongly related to the jet formation and the spectral states. These features are the natural consequences of the advective disk paradigm that we are advocating. In the mid 90s, some of the present authors first pointed out that the QPOs in all possible types of black holes may be simply due to the oscillations of the CENBOL, the CENtrifugal pressure supported BOundary Layer which is formed in the sub-Keplerian flows around a black hole. This CENBOL could be axi-symmetric as well as non-axisymteric in nature since its boundary, namely, the centrifugally driven shocks could be axi-symmetric or non-axisymmetric. In addition, we pointed out that the transition radius where the flow becomes Keplerian to sub-Keplerian, as well as the location of the inner sonic point can also oscillate and produce the QPOs. Since the shock locations are functions of the specific angular momentum ($λ$) and specific energy (${\cal E}$) of the flow, our model naturally predicts that the QPO frequency should vary with mass, spin, $λ$ and ${\cal E}$. The QPO frequencies with specific ratios, such as, 2:3 must be due to non-axisymmetric effects when the shock switches between the two-armed and the three-armed spirals. We also discuss the possible effects that the disk inclination might have with the line of sight.

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arXiv2009

Spectral and timing evolution of GRO J1655-40 during its outburst of 2005

In a recent outburst which lasted for 260 days, the black hole candidate GRO J1655-40 exhibited a behaviour similar to its last outburst observed almost eight years ago. We analyze a total of 150 observational spells in 122 days of data spreaded over the entire outburst phase of Feb. 2005 to Oct. 2005. From our study, a comprehensive understanding of the detailed behaviour of this black hole candidate has emerged. Based on the degree of importance of the black body and the power-law components we divide the entire episode in four spectral states, namely, hard, soft, very soft and intermediate. Quasi-Periodic oscillations (QPOs) were found in two out of these four states, namely, in the hard and the intermediate states. In the hard state, at the rising phase of the outburst, QPO frequency ranged from 0.034 - 17.78Hz and the spectra was fitted by a disk black body, power-law and iron emission line at 6.2 - 6.5 keV. In the intermediate state, QPOs vary from 13.17Hz to 19.04Hz and the QPO frequency modulation in this state was not significant. The spectra in this state are well fitted by the disk black body and the power-law components. In the hard state of the declining phase of the outburst, we found QPOs of decreasing frequency from 13.14 Hz to 0.034 Hz. The spectra of this state were fitted by a disk black body and power-law components, but in the initial few days a cooler Comptonized component was required for a better fit. In the soft and very soft states, the spectral states are mostly dominated by the strong disk black body component.

astro-ph.GA↗

arXiv2008

Spectrum of two component flows around a super-massive black hole: an application to M87

We calculate the spectra of two-component accretion flows around black holes of various masses, from quasars to nano-quasars. Specifically, we fit the observational data of M87 very satisfactorily using our model and find that the spectrum may be well fitted by a sub-Keplerian component alone, and there is little need of any Keplerian component. The non-thermal distribution of electrons produced by their acceleration across the standing shock in the sub-Keplerian component is enough to produce the observed flat spectrum through the synchrotron radiation.

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arXiv2008

Evolution of the quasi-periodic oscillation frequency in GRO J1655-40 -- Implications for accretion disk dynamics

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot blobs embedded in Keplerian disks. Any movement of these perturbations is expected systematically to change the QPO frequency. Our goal is to find systems where such a systematic drifts have been observed. We also try to find the real cause of such drifts and whether they shed some light on the accretion disk dynamics. Using archival data of the recent outburst of GRO J1655-40, we report the presence of such systematic drifts not only during the rising phase from the 25th of February 2005 to the 12th March 2005, when the QPO frequency monotonically increased from 82mHz to 17.78Hz but also in the decline phase from the 15th September 2005 to the 5th of October 2005, when the QPO frequency decreased from 13.14Hz to 34mHz. We fitted the frequency drifts with the propagatory oscillating shock solution. In the shock-oscillation solution, the frequency is inversely proportional to the infall time scale from the shock location. We obtained the shock location and strength through such a fit. The astonishing smoothness of the variation of the QPO frequency over a period of weeks directly supports the view that it may due to the drift of an oscillating shock rather than the movements of a blob inside a differentially rotating disk.

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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.

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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.

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