arXiv subjects

Sandip K. Chakrabarti

Publications and source records attributed to Sandip K. Chakrabarti.

At least 19 recordsLinked to original sources

arXiv2026

An autonomous approach to model daytime behavior of sub-ionospheric VLF signals over short and medium propagation paths in India

We present an autonomous model to simulate the daytime variation of sub-ionospheric Very Low Frequency (VLF) signal amplitude, beginning with the computation of the D-region electron density by numerically solving the electron continuity equation (ECE). From the resulting altitude-dependent electron density profile (Ne), we extract Wait's ionospheric parameters (h' and beta) using a log-linear fitting method. The study focuses on two VLF propagation paths (one short and one medium in length) in India, originating from the VTX (18.2 kHz) transmitter. The model employs the Long Wave Propagation Capability (LWPC) framework to reproduce the daytime VLF signal amplitude profile. It accurately captures the daytime variations observed at the Bengaluru (BAN) station, where the ground-wave component is dominant, as well as at Khukurdaha, WB (KHK). A quantitative comparison between the simulated (Asim) and observed (Aobs) amplitudes shows good agreement, supporting the physical consistency and potential predictive capability of the proposed approach.

physics.space-ph↗

arXiv2026

Accretion dynamics and coronal geometry in Mrk 530: Insights from 24 years of X-ray monitoring

We present a long-term broadband study of the Seyfert galaxy Mrk~530 spanning $\sim$24 yr (2001--2024). The source remains largely stable across epochs, except in 2018, when a possible quasi-periodic oscillation is observed simultaneously in the UV and X-ray bands, with characteristic timescales of $\sim$90 and $\sim$60 days, characterized by low coherence. Time-resolved spectral analysis shows that this epoch is characterized by comparable coronal cooling and compressional heating timescales, a condition conducive to oscillatory behavior in the inner accretion flow. Other epochs exhibit a clear mismatch between these timescales, and no such variability is observed. The X-ray spectral properties display significant long-term evolution. The photon index and luminosity vary systematically, while a soft excess is present only in early epochs (2001--2006) and weakens thereafter, consistent with an evolving warm corona. Physically motivated models indicate that changes in the accretion rate regulate both the spectral slope and coronal geometry, with higher disc accretion rates producing enhanced cooling, a more compact corona, and softer spectra, and lower rates yielding an expanded hot flow and harder emission. These results suggest that accretion-driven coupling between the disc and corona governs both the long-term spectral evolution and transient short-timescale variability in Mrk~530.

astro-ph.HE↗

arXiv2026

Numerical study of multiple solar flare induced modulation of Very Low Frequency (VLF) diurnal profile

Earth's ionosphere is a perpetual detector of ionizing radiation received from celestial objects, particularly the Sun. Solar ionizing radiation in the form of extreme ultraviolet (EUV) and X-rays during both quiet and active phase of the Sun, and charged particles associated with a solar wind imprint their ionization signatures on the ionosphere. Although due to the bipolar nature of the geomagnetic field, the events, such as the solar coronal mass ejections (CMEs) and associated solar wind enhancement, usually disturb the polar ionosphere only, the UV and X-rays from the solar flares produce sudden ionospheric disturbances (SIDs) in low-mid-latitude part of the earth's ionosphere. Such ionospheric disturbances are studied with the help of the influence they exert on radio waves propagating through earth-ionosphere waveguide. For the lower part of the ionosphere, called the D region, prominent modification in electron-ion density during solar flares can be observed via deviation in earth bound Very Low Frequency (VLF) wave signal from its ambient diurnal profile. In earlier work, successful model of the deviation in VLF amplitude due to different classes of solar flares was formulated. There, calculation of rate of ionization with Monte Carlo simulation and ion-chemistry evaluation of plasma density enhancement followed by a radio propagation simulation was used. Presently, we attempt to numerically reconstruct the modulation in VLF signal from its diurnal pattern produced by multiple solar flares occurring over a single day. Successful reconstruction of the VLF signal modulation for such a complex flaring scenario points to the accuracy of our understanding of the ionization effect due to solar activity on the lower ionosphere, and strengthen our claim to use earth's ionosphere as a high energy space transient event detector.

astro-ph.SR↗

arXiv2024

Viscous Transonic Accretion Flows in Kerr Black Hole Geometry

We study viscous transonic accretion flows in vertical equilibrium in Kerr geometry. We employ the pseudo-Kerr formalism which accurately describes transonic flows around Kerr black holes and is applicable for modelling observational data. We study the effects of viscosity on the nature of sonic points and the parameter space that allows an accretion flow to possess multiple sonic points. We concentrate on the accretion solutions that can have centrifugal pressure supported shock waves and find that the shocks are weaker and are located farther from the black hole as the viscosity is enhanced. Moreover, if the viscosity is greater than a critical value, shocks do not form and the accretion flow can pass only through the inner sonic point close to the black hole and remains subsonic and Keplerian throughout the accretion disk. Since the resonance oscillation frequencies of the shock waves provide a measure of the observed Quasi Periodic Oscillation (QPO) frequencies, and since the location of shock waves depend on the spin of a black hole, it is clear that the QPO frequencies must depend on the spin of black hole as well. Our pseudo-Kerr approach makes it easier to compute spectra from an accretion flow with viscous dissipation and radiative cooling around a spinning black hole.

astro-ph.HE↗

arXiv2024

Effective potential approach to study hydrodynamics and particle dynamics in Kerr geometry

We derive the exact form of effective potential in Kerr geometry from the general relativistic radial momentum equation. The effective potential accurately mimics the general relativistic features, over the entire range of the spin parameter $-1<a<1$. We obtain the exact expression of the rate of dragging of inertial frames that can be used to study the relativistic precession of twisted accretion disks that are formed when the disk outskirts are tilted relative to the equatorial plane of the black hole. We then present an effective potential that provides a simplistic approach to study particle dynamics using physical concepts analogous to the Newtonian physics. We compare the equatorial as well as off-equatorial particle trajectories obtained using our potential with the general relativistic solutions. We find that our approach can capture the salient features of Kerr geometry and is applicable to studies of accretion processes around Kerr black holes.

gr-qc↗

arXiv2024

Accretion Flow Properties of EXO 1846-031 During its Multi-Peaked Outburst After Long Quiescence

We study the recent outburst of the black hole candidate EXO 1846-031 which went into an outburst in 2019 after almost 34 years in quiescence. We use archival data from Swift/XRT, MAXI/GSC, NICER/XTI and NuSTAR/FPM satellites/instruments to study the evolution of the spectral and temporal properties of the source during the outburst. Low energy (2-10 keV) X-ray flux of the outburst shows multiple peaks making it a multipeak outburst. Evolving type-C quasi-periodic oscillations (QPOs) are observed in the NICER data in the hard, hard intermediate and soft intermediate states. We use the physical Two Component Advective Flow (TCAF) model to analyze the combined spectra of multiple satellite instruments. According to the TCAF model, the accreting matter is divided into Keplerian and sub-Keplerian parts, and the variation in the observed spectra in different spectral states arises out of the variable contributions of these two types of accreting matter in the total accretion rate. Studying the evolution of the accretion rates and other properties of the accretion flow obtained from the spectral analysis, we show how the multiple peaks in the outburst flux arises out of variable supply of accreting matter from the pile-up radius. We determine the probable mass of the black hole to be $10.4^{+0.1}_{-0.2}~M_\odot$ from the spectral analysis with the TCAF model. We also estimate viscous time scale of the source in this outburst to be $\sim 8$ days from the peak difference of the Keplerian and sub-Keplerian mass accretion rates.

astro-ph.HE↗

arXiv2023

Survey of Bare Active Galactic Nuclei in the local universe (z < 0.2): I. On the origin of Soft-Excess

We analyse a sample of 21 `bare' Seyfert~1 Active Galactic Nuclei (AGNs), a sub-class of Seyfert~1s, with intrinsic absorption $\mathrm{N_{H}} \sim 10^{20}~ \mathrm{cm}^{-2}$, in the local universe (z $<$ 0.2) using {\it XMM-Newton} and {\it Swift}/XRT observations. The luminosities of the primary continuum, the X-ray emission in the 3 to 10 keV energy range and the soft-excess, the excess emission that appears above the low-energy extrapolation of the power-law fit of 3 to 10 keV X-ray spectra, are calculated. Our spectral analysis reveals that the long-term intrinsic luminosities of the soft-excess and the primary continuum are tightly correlated $(L_{PC}\propto L_{SE}^{1.1\pm0.04})$. We also found that the luminosities are correlated for each source. This result suggests that both the primary continuum and soft excess emissions exhibit a dependency on the accretion rate in a similar way.

astro-ph.GA↗

arXiv2023

Chemical modeling for predicting the abundances of certain aldimines and amines in hot cores

We consider six isomeric groups (CH3N, CH5N, C2H5N, C2H7N, C3H7N and C3H9N) to review the presence of amines and aldimines within the interstellar medium (ISM). Each of these groups contains at least one aldimine or amine. Methanimine (CH2NH) from CH3N and methylamine (CH3NH2) from CH5N isomeric group were detected a few decades ago. Recently, the presence of ethanimine (CH3CHNH) from C2H5N isomeric group has been discovered in the ISM. This prompted us to investigate the possibility of detecting any aldimine or amine from the very next three isomeric groups in this sequence: C2H7N, C3H7N and C3H9N. We employ high-level quantum chemical calculations to estimate accurate energies of all the species. According to enthalpies of formation, optimized energies, and expected intensity ratio, we found that ethylamine (precursor of glycine) from C2H7N isomeric group, (1Z)-1-propanimine from C3H7N isomeric group, and trimethylamine from C3H9N isomeric group are the most viable candidates for the future astronomical detection. Based on our quantum chemical calculations and from other approximations (from prevailing similar types of reactions), a complete set of reaction pathways to the synthesis of ethylamine and (1Z)-1-propanimine is prepared. Moreover, a large gas-grain chemical model is employed to study the presence of these species in the ISM. Our modeling results suggest that ethylamine and (1Z)-1-propanimine could efficiently be formed in hot-core regions and could be observed with present astronomical facilities. Radiative transfer modeling is also implemented to additionally aid their discovery in interstellar space.

astro-ph.GA↗

arXiv2022

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

We study the spectral properties and accretion flow behavior of an ultraluminous X-ray source M82\,X-1 using {\it 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 M82\,X-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 weighted average mass of the black hole is $273\pm43$ M$_\odot$, which accreted in nearly super-Eddington rate. The Compton cloud was compact with a size of $\sim13 r_g$ and the shock compression ratio had \textcolor{black}{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 estimated from the model fitted parameters can reproduce the observed QPOs. The robustness of the model parameters is verified by drawing the confidence contours among them.

astro-ph.HE↗

arXiv2022

Flux and spectral variability of Mrk 421 during its moderate activity state using NuSTAR: Possible accretion disc contribution?

The X-ray emission in BL Lac objects is believed to be dominated by synchrotron emission from their relativistic jets. However, when the jet emission is not strong, one could expect signatures of X-ray emission from inverse Compton scattering of accretion disc photons by the corona. Moreover, the observed X-ray variability can also originate in the disc, and gets propagated and amplified by the jet. Here, we present results on the BL Lac object Mrk 421 using the NuSTAR data acquired during 2017 when the source was in a moderate X-ray brightness state. For comparison with high jet activity, we also considered one epoch data in April 2013 during a very high X-ray brightness state. Our aim is to explore the possibility of the signature of accretion disc emission in the overall X-ray emission from Mrk 421. The spectral fitting of the data using the two component advective flow model shows gives (a) the size of the dynamic corona at the base of the jet from ~28 to 10 r$_s$, (b) the disc mass accretion rate from 0.021 to 0.051 $\dot M_{\rm Edd}$, (c) the halo mass accretion rate from 0.22 to 0.35 $\dot M_{\rm Edd}$, and (d) the viscosity parameter of the Keplerian accretion disc from 0.18 $-$ 0.25. In the assumed model, the total flux, disc and jet flux correlate with the radio flux observed during these epochs. We conclude that the spectra of all the epochs of Mrk 421 in 2017 are well described by the accretion disc based two component advective flow model. The estimated disc and jet flux relations with radio flux show that accretion disc can contribute to the observed X-ray emission, when X-ray data (that covers a small portion of the broad band spectral energy distribution of Mrk 421) is considered in isolation. However, the present disc based models are disfavoured with respect to the relativistic jet models when considering the X-ray data in conjunction with data at other wavelengths.

astro-ph.HE↗

arXiv2022

Similarities and Differences in Accretion Flow Properties between GRS 1915+105 and IGR J17091-3624: a Case Study

We perform a comparative spectro-temporal analysis on the variability classes of GRS 1915+105 and IGR J17091-3624 to draw inferences regarding the underlying accretion flow mechanism. The $ν$, as well as C2 class Rossi X-Ray Timing Explorer observation, have been considered for analysis. We investigate the intensity variation of the source in different energy domains that correspond to different components of the accretion flow and infer the relative dominance of these flow components during the dip/flare events. We correlate the dependence of the dynamic photon index ($Θ$) with intensities in different energy bands and comment on the transition of the source to hard/soft phases during soft dips/flares. We also report the presence of sharp QPOs at \sim7.1 Hz corresponding to both softer and harder domain in the case of $ν$ variability class of GRS 1915+105 and discuss the possible accretion flow configuration it suggests. Sharp QPO around \sim20 mHz is observed in $ν$ and C2 classes of IGR J17091-3624 in low and mid energy band (2.0-6.0 keV and 6.0-15.0 keV), but remains undetected in high energy (15.0-60.0 keV). The 2.5-25.0 keV background-subtracted spectra have also been fitted with TCAF along with a Compton reflection component. A plausible accretion flow mechanism in order to explain the observed variability has been proposed.

astro-ph.HE↗

arXiv2022

Transonic accretion and winds around Pseudo-Kerr black holes and comparison with general relativistic solutions

Spectral and timing properties of accretion flows on a black hole depend on their density and temperature distributions, which in turn come from the underlying dynamics. Thus, an accurate description of the flow which includes hydrodynamics and radiative transfer is a must to interpret the observational results. In the case of non-rotating black holes, Pseudo-Newtonian description of surrounding space-time enables one to make a significant progress in predicting spectral and timing properties. This formalism is lacking for spinning black holes. In this paper, we show that there exists an exact form of 'natural' potential derivable from the general relativistic (GR) radial momentum equation. Use of this potential in an otherwise Newtonian set of equations allows to describe transonic flows very accurately as is evidenced by comparing with solutions obtained from the full GR framework. We study the properties of the critical points and the centrifugal pressure supported shocks in the parameter space spanned by the specific energy and the angular momentum, and compare with the results of GR hydrodynamics. We show that this potential can safely be used for the entire range of Kerr parameter $-1<a<1$ for modeling of observational results around spinning black holes. We assume the flow to be inviscid. Thus, it is non-dissipative with constant energy and angular momentum. These assumptions are valid very close to the black hole as the infall timescale is much shorter as compared to the viscous timescale.

astro-ph.HE↗

arXiv2021

Simulation of Cosmic Rays in the Earth's Atmosphere and Interpretation of Observed Counts in an X-ray Detector at Balloon Altitude Near Tropical Region

The study of secondary particles produced by the cosmic-ray interaction in the Earth's atmosphere is very crucial as these particles mainly constitute the background counts produced in the high-energy detectors at balloon and satellite altitudes. In the present work, we calculate the abundance of cosmic-ray generated secondary particles at various heights of the atmosphere by means of a Monte Carlo simulation and use this result to understand the background counts in our X-ray observations using balloon-borne instruments operating near the tropical latitude (geomagnetic latitude: $\sim 14.50^{\circ}$ N). For this purpose, we consider a 3D description of the atmospheric and geomagnetic field configurations surrounding the Earth, as well as the electromagnetic and nuclear interaction processes using Geant4 simulation toolkit. Subsequently, we use a realistic mass model description of the detector under consideration, to simulate the counts produced in the detector due to secondary cosmic-ray particles.

astro-ph.EP↗

arXiv2021

Background model of Phoswich X-ray detector on board small balloon

We performed a detailed modelling of the background counts observed in a phoswich scintillator X-ray detector at balloon altitude, used for astronomical observations, on board small scientific balloon. We used Monte Carlo simulation technique in Geant4 simulation environment, to estimate the detector background from various plausible sources. High energy particles and radiation generated from the interaction of Galactic Cosmic Rays with the atmospheric nuclei is a major source of background counts (under normal solar condition) for such detectors. However, cosmogenic or induced radioactivity in the detector materials due to the interaction of high energy particles and natural radioactive contamination present in the detector can also contribute substantially to the detector background. We considered detailed 3D modelling of the earth's atmosphere and magnetosphere to calculate the radiation environment at the balloon altitude and deployed a proper mass model of the detector to calculate the background counts in it. The calculation satisfactorily explains the observed background in the detector at 30 km altitude (atmospheric depth: 11.5 $g/cm^{2}$) during the balloon flight experiment from a location near 14.5$^{\circ}$N geomagnetic latitude.

astro-ph.IM↗

arXiv2021

Identification of pre-biotic molecules containing Peptide-like bond in a hot molecular core, G10.47+0.03

After hydrogen, oxygen, and carbon, nitrogen is one of the most chemically active species in the interstellar medium (ISM). Nitrogen bearing molecules have great importance as they are actively involved in the formation of biomolecules. Therefore, it is essential to look for nitrogen-bearing species in various astrophysical sources, specifically around high-mass star-forming regions where the evolutionary history is comparatively poorly understood. In this paper, we report the observation of three potential pre-biotic molecules, namely, isocyanic acid (HNCO), formamide (NH2CHO), and methyl isocyanate (CH3NCO), which contain peptide-like bonds (-NH-C(=O)-) in a hot molecular core, G10.47+0.03 (hereafter, G10). Along with the identification of these three complex nitrogen-bearing species, we speculate their spatial distribution in the source and discuss their possible formation pathways under such conditions. The rotational diagram method under the LTE condition has been employed to estimate the excitation temperature and the column density of the observed species. Markov Chain Monte Carlo method was used to obtain the best suited physical parameters of G10 as well as line properties of some species. We also determined the hydrogen column density and the optical depth for different continuum observed in various frequency ranges. Finally, based on these observational results, we have constructed a chemical model to explain the observational findings. We found that HNCO, NH2CHO, and CH3NCO are chemically linked with each other.

astro-ph.SR↗

arXiv2021

Radiative transfer modeling of the observed line profiles in G31.41+0.31

An inverse P-Cygni profile of H13CO+ (1-0) in G31.41+0.31 was recently observed, which indicates the presence of an infalling gas envelope. Also, an outflow tracer, SiO, was observed. Here, exclusive radiative transfer modelings have been implemented to generate synthetic spectra of some key species (H13 CO+, HCN, SiO, NH3, CH3 CN, CH3OH, CH3SH, and CH3NCO) and extract the physical features to infer the excitation conditions of the surroundings where they observed. The gas envelope is assumed to be accreting in a spherically symmetric system towards the central hot core region. Our principal intention was to reproduce the observed line profiles toward G31.41+0.31 and extract various physical parameters. The LTE calculation with CASSIS and non-LTE analysis with the RATRAN radiative transfer codes are considered for the modeling purpose. The best-fitted line parameters are derived, which represents the prevailing physical condition of the gas envelope. Our results suggest that an infalling gas could explain the observed line profiles of all the species mentioned above except SiO. An additional outflow component is required to confer the SiO line profile. Additionally, an astrochemical model is implemented to explain the observed abundancests various species in this source.

astro-ph.GA↗

arXiv2021

Spectral signature of mass outflow in Two Component Advective Flow Paradigm

Outflows are common in many astrophysical systems. In the Two Component Advective Flow ({\fontfamily{qcr}\selectfont TCAF}) paradigm which is essentially a generalized Bondi flow including rotation, viscosity and cooling effects, the outflow is originated from the hot, puffed up, post-shock region at the inner edge of the accretion disk. We consider this region to be the base of the jet carrying away matter with high velocity. In this paper, we study the spectral properties of black holes using {\fontfamily{qcr}\selectfont TCAF} which includes also a jet ({\fontfamily{qcr}\selectfont JeTCAF}) in the vertical direction of the disk plane. Soft photons from the Keplerian disk are up-scattered by the post-shock region as well as by the base of the jet and are emitted as hard radiation. We also include the bulk motion Comptonization effect by the diverging flow of jet. Our self-consistent accretion-ejection solution shows how the spectrum from the base of the jet varies with accretion rates, geometry of the flow and the collimation factor of the jet. We apply the solution to a jetted candidate GS\,1354-64 to estimate its mass outflow rate and the geometric configuration of the flow during 2015 outburst using {\it NuSTAR} observation. The estimated mass outflow to mass inflow rate is $0.12^{+0.02}_{-0.03}$. From the model fitted accretion rates, shock compression ratio and the energy spectral index, we identify the presence of hard and intermediate spectral states of the outburst. Our model fitted jet collimation factor ($f_{\rm col}$) is found to be $0.47^{+0.09}_{-0.09}$.

astro-ph.HE↗

arXiv2021

Effect of binding energies on the encounter desorption

The abundance of interstellar ice constituents is usually expressed with respect to the water ice because, in denser regions, a significant portion of the interstellar grain surface would be covered by water ice. The binding energy (BE), or adsorption energy of the interstellar species regulates the chemical complexity of the interstellar grain mantle. Due to the high abundance of water ice, the BE of surface species with the water is usually provided and widely used in astrochemical modeling. However, the hydrogen molecules would cover some part of the grain mantle in the denser and colder part of the interstellar medium. Even at around ~ 10K, few atoms and simple molecules with lower adsorption energies can migrate through the surface. The BE of the surface species with H2 substrate would be very different from that of a water substrate. However, adequate information regarding these differences is lacking. Here, we employ the quantum chemical calculation to provide the BE of 95 interstellar species with H2 substrate. These are representative of the BEs of species to a H2 overlayer on a grain surface. On average, we notice that the BE with the H2 monomer substrate is almost ten times lower than the BE of these species reported earlier with the H2 O c-tetramer configuration. The encounter desorption of H and H2 was introduced (with ED (H, H2 ) =45 K and ED (H2 , H2 ) =23 K) to have a realistic estimation of the abundances of the surface species in the colder and denser region. Our quantum chemical calculations yield higher adsorption energy of H2 than that of H (ED (H, H2 ) = 23 - 25 K and ED (H2, H2 ) =67 - 79 K). We further implement an astrochemical model to study the effect of encounter desorption with the resent realistic estimation. The encounter desorption of the N atom (calculations yield ED (N, H2 ) =83 K) is introduced to study the differences with its inclusion.

astro-ph.GA↗