arXiv subjects

Sandip K. Chakrabarti

Publications and source records attributed to Sandip K. Chakrabarti.

At least 55 records · Page 3Linked to original sources

arXiv2019

Evidence of Outflow Induced Soft Lags of Galactic Black Holes

The nature of lag variation of Galactic black holes remains enigmatic mostly because of nonlinear and non-local physical mechanisms which contribute to the lag of the photons coming from the region close to the central black holes. One of the widely accepted major sources of the hard lag is the inverse Comptonization mechanism. However, the exact reason or reasons for soft lags is yet to be identified. In this paper, we report a possible correlation between radio intensities of several outbursting Galactic black hole candidates and amounts of soft lag. The correlation suggests that the presence of major outflows or jets also change the disk morphology along the line of sight of the observer which produces soft lags.

astro-ph.HE↗

arXiv2019

Timing Properties of Shocked Accretion Flows around Neutron Stars -- II. Viscous Disks and Boundary Layers

We use Smoothed Particle Hydrodynamics to study viscous accretion flows around a weakly magnetic neutron star. We show the formation of multiple ``boundary" layers in presence of both cooling and viscosity. We find that with the introduction of a small viscosity in a sub-Keplerian flow, much like the wind accretion in HMXBs such as Cir X-1, only a single Normal Boundary Layer (NBOL) forms to adjust the rotational velocity component. With the increase of viscosity, the region extends radially and beyond some critical value, a RAdiative KEplerian Disk/layer (RAKED) forms between the sub-Keplerian flow and the NBOL. When viscosity is increased further only NBOL and RAKED remain. In all such cases, the CENtrifugal pressure dominated BOundary Layer (CENBOL) is formed, away from the star, as in the case of black holes. This is the first self-consistent study where such a transition from sub-Keplerian flows has been reported for neutron stars. We also identify the connection between accretion and ejection of matter, following the Two-Component Advective Flow for black holes, for neutron stars. The results are crucial in the understanding of the formation of disks, boundary layers and outflows in wind dominated neutron star systems.

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arXiv2019

Chemical and radiative transfer modeling of Propylene Oxide

The recent identification of the first complex chiral molecule, propylene oxide (PrO) in space opens up a new window to further study the origin of homochirality on the Earth. There are some recent studies to explain the formation of PrO however additional studies on the formation of this species are needed for better understanding. We seek to prepare a complete reaction network to study the formation of propylene oxide in the astrophysically relevant conditions. Based on our results, a detailed radiative transfer modeling has been carried out to propose some more transitions which would potentially be targeted in the millimeter wave domain. Gas-grain chemical network was used to explain the observed abundance of PrO in a cold shell surrounding the high-mass star-forming region of Sgr B2. Quantum chemical calculations were employed to study various reaction parameters and to compute multiple vibrational frequencies of PrO. To model the formation of PrO in the observed region, we considered a dark cloud model. Additionally, we used a model to check the feasibility of forming PrO in the hot core region. Some potential transitions in the millimeter wave domain are predicted which could be useful for the future astronomical detection. Radiative transfer modeling has been utilized to extract the physical condition which might be useful to know the properties of the source in detail. Moreover, vibrational transitions of PrO has been provided which could be very useful for the future detection of PrO by the upcoming James Webb Space Telescope (JWST).

astro-ph.SR↗

arXiv2019

Spectral properties of NGC 4151 and the Estimation of black hole mass using TCAF solution

We present X-ray spectral analysis of Seyfert 1.5 Active Galactic Nuclei (AGN) NGC~4151 using \textit{NuSTAR} observation during 2012. This is the first attempt to fit an AGN data using the physical Two Component Advective flow (TCAF) solution. We disentangle the continuum emission properties of the source in the energy range $3.0$ to $70.0$~keV using the spectrum obtained from TCAF model. This model was used as an additive local model directly in {\fontfamily{qcr}\selectfont XSPEC}. Additionally, we used a power law (PL) component, to take care of possible X-ray contribution from the jet, which is not incorporated in the present version of TCAF. Our primary aim is to obtain the flow properties and the mass of the central supermassive black hole from the available archival data. Our best estimate of the average mass obtained from spectral fits of three observations, is $M_{BH}=3.03^{+0.26}_{-0.26}\times 10^7 M_\odot$. This is consistent with earlier estimations in the literature such as reverberation mapping, gas kinematics and stellar dynamics around black holes. We also discuss the accretion dynamics and the flow geometry on the basis of model fitted physical parameters. Model fitted disk accretion rate is found to be lower than the low angular momentum halo accretion rate, indicating that the source was in a hard state during the observation.

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arXiv2019

Constraining Mass of Cygnus X-1 from Analysis of the Hard State Spectral Data using TCAF Solution

The galactic black hole candidate Cygnus X-1, one of the brightest sources in the sky, is the first ever black hole candidate to be discovered. Despite being a very well-studied object due to its persistent brightness in X-rays, there has been much difficulty in determining its mass since its discovery. In spite of continuous efforts since the early nineteen seventies, there is yet no concensus on its mass for nearly four decades. The uncertainties in the mass measurements are due to the high degree of error involved in the measurement of its distance. In 2011, Orosz et al. constrained the mass of this object to be M = 14.8 +/- 1.0 M_Sun using dynamical methods. In this paper, we obtained the mass of Cygnus X-1, using a completely independent method, namely, carrying out the spectral analysis using Two Component Advective Flow (TCAF) solution based fits file and the archival data of RXTE PCA instrument. Our result does not require the distance of the source or the information about the companion. Each spectral fit with the TCAF gives one best fitted mass. Averaging fitted masses of Cygnus X-1 over a span of five months of observation during its persistent hard phase, mass of the source comes out to be M_avg = 14.20 +/- 0.36 M_Sun, which is consistent with the dynamically estimated mass.

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arXiv2019

Periodic X-ray Modulation and its relation with orbital elements in Compact Binaries

Stellar companion of a black hole orbiting in an eccentric orbit will experience modulating tidal force with a periodicity same as that of the orbital period. This, in turn, would modulate accretion rates, and the seed photon flux which are inverse Comptonized to produce harder X-rays. By analyzing complete all sky monitor (ASM) data (1.5-12 keV) of RXTE and all sky survey data (15-50 keV) of Swift/BAT we discover this periodicity in several objects. We also estimate eccentricities from the RMS power of the peak around quasi-orbital periods (QOP). Our method provides an independent way to obtain time periods and eccentricities of such compact binaries.

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arXiv2019

Does Cyg X-1 have a small Standard Accretion Disc?

We analyze several outbursts of a few transient sources using Proportional Counter Array (PCA) data (2.5-25 keV) as well as All Sky Monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) satellite. We find a time delay between the arrival times of the Keplerian disc component and the halo of the Two-Component Advective Flow (TCAF) when the spectral data is fitted with TCAF solution. We compare this time delay from the spectral fits with the TCAF solution of the transient low mass X-ray binaries (LMXBs) e.g., GX 339-4, H 1743-322 and MAXI J1836-194 with that of the high mass X-ray Binary (HMXB), Cyg X-1. We find that several days of time delays are observed in LMXBs while for Cyg X-1 the delay is negligible. We interpret the large delay to be due to the viscous delay of a large Keplerian component to reach the inner region as compared to nearly free-fall time taken by the low angular momentum halo component. The delay is of the order of a few days for the low mass X-ray binaries (LMXBs) where the feeding is primarily through the Roche-lobe. However, it is negligible in a wind-fed system like Cyg X-1 since a very small Keplerian disc is created here by slowly redistributing the low angular momentum of the wind. As a consequence, sporadic soft or intermediate spectral states are observed.

astro-ph.HE↗

arXiv2019

Timing Properties of Shocked Accretion Flows around Neutron Stars in Presence of Cooling

We carry out the first robust numerical simulation of accretion flows on a weakly magnetized neutron star using Smoothed Particle Hydrodynamics (SPH). We follow the Two-Component Advective Flow (TCAF) paradigm for black holes, and focus only on the advective component for the case of a neutron star. This low viscosity sub-Keplerian flow will create a normal boundary layer (or, NBOL) right on the star surface in addition to the centrifugal pressure supported boundary layer (or, CENBOL) present in a black hole accretion. These density jumps could give rise to standing or oscillating shock fronts. During a hard spectral state, the incoming flow has a negligible viscosity causing more sub-Keplerian component as compared to the Keplerian disc component. We show that our simulation of flows with a cooling and a negligible viscosity produces precisely two shocks and strong supersonic winds from these boundary layers. We find that the specific angular momentum of matter dictates the locations and the nature of oscillations of these shocks. For low angular momentum flows, the radial oscillation appears to be preferred. For flows with higher angular momentum, the vertical oscillation appears to become dominant. In all the cases, asymmetries w.r.t. the Z=0 plane are seen and instabilities set in due to the interaction of inflow and outgoing strong winds. Our results capture both the low and high-frequency quasi-periodic oscillations without invoking magnetic fields or any precession mechanism. Most importantly, these solutions directly corroborate observed features of wind dominated high-mass X-ray binaries, such as Cir X-1.

astro-ph.HE↗

arXiv2019

Delayed Outburst of H 1743--322 in 2003 and relation with its other outbursts

The Galactic transient black hole candidate H 1743--322 exhibited a long duration outburst in 2003 after more than two and a half decades of inactivity. The 2003 event was extensively studied in multi-wavelength bands by many groups. The striking feature is that the total energy released is extremely high as compared to that in tens of outbursts which followed. In this paper, we look at this event and study both the spectral and temporal properties of the source using two component advective flow (TCAF) paradigm. We extract accretion flow parameters for each observation from spectral properties of the decay phase and determine the mass of the black hole. We computed the energy released during all the known outbursts since 2003 and showed that on an average, the energy release in an outburst is proportional to the duration of the quiescent state just prior to it, with the exception of the 2004 outburst. A constant rate of supply of matter from the companion cannot explain the energy release in 2004 outburst. However, if the energy release of 2003 is incomplete and the leftover is released in 2004, then the companion's rate of matter supply can be constant since 1977 till date. We believe that erratic behaviour of viscosity at the accumulation radius $X_p$ of matter as well as location the $X_p$ itself, rather than the random variation of mass transfer rate from the companion, could be responsible for non-uniformity in outburst pattern. We discuss several factors on which the waiting time and duration of the next outburst could depend.

astro-ph.HE↗

arXiv2019

Deuterium Enrichment of the Interstellar Medium

Despite low elemental abundance of atomic deuterium in interstellar medium (ISM), observational evidences suggest that several species in gas-phase and in ices could be heavily fractionated. We explore various aspects of deuterium enrichment by constructing a chemical evolution model in gas and grain phases. Depending on various physical parameters, gas and grains are allowed to interact with each other through exchange of their chemical species. It is known that HCO+ and N2H+ are two abundant gas phase ions in ISM and their deuterium fractionation are generally used to predict degree of ionization in various regions of a molecular cloud. To have a more realistic estimation, we consider a density profile of a collapsing cloud. We present radial distributions of important interstellar molecules along with their deuterated isotopomers. We carry out quantum chemical simulation to study effects of isotopic substitution on spectral properties of these important interstellar species. We calculate vibrational (harmonic) frequency of the most important deuterated species (neutral & ions). Rotational and distortional constants of these molecules are also computed to predict rotational transitions of these species. We compare vibrational (harmonic) and rotational transitions as computed by us with existing observational, experimental and theoretical results. We hope that our results would assist observers in their quest of several hitherto unobserved deuterated species.

astro-ph.GA↗

arXiv2019

Properties of the Black Hole Candidate XTE J1118+480 with the TCAF solution during its Jet Activity Induced 2000 Outburst

Galactic black hole candidate (BHC) XTE~J1118+480 during its 2000 outburst has been studied in a broad energy range using the archival data of PCA and HEXTE payloads of {\it Rossi X-ray Timing Explorer}. Detailed spectral and temporal properties of the source are studied. Low and very low frequency quasi-periodic oscillations (QPOs), with a general trend of increasing frequency are observed during the outburst. Spectral analysis is done using the combined data of PCA and HEXTE instruments with two types of models: the well known phenomenological power-law model and the current version of the {\it fits} file of two-component advective flow (TCAF) solution as an additive table model in XSPEC. During the entire period of the outburst, a non-thermal power-law component and the TCAF model fitted sub-Keplerian halo rate were found to be highly dominant. We suggest that this so-called outburst is due to enhanced jet activity. Indeed, the `outburst' subsides when this activity disappears. We estimated X-ray fluxes coming from the base of the jet and found that the radio flux is correlated with this X-ray flux. Though the object was in the hard state in the entire episode, the spectrum becomes slightly softer with the rise in Keplerian disk rate in the late declining phase. We also estimated the probable mass of the source from our spectral analysis with the TCAF solution. Our estimated mass of XTE~J1118+480 is $6.99^{+0.50}_{-0.74}~M_\odot$ i.e., in the range of $6.25-7.49~M_\odot$.

astro-ph.HE↗

arXiv2019

Inference on Disk-Jet Connection of MAXI J1836-194: Analysis with the TCAF Solution

Galactic transient black hole candidate (BHC) MAXI~J1836-194 was discovered on 2011 Aug 30, by MAXI/GSC and Swift/BAT. The source activity during this outburst was continued for ~3 months before entering into the quiescent state. It again became active in March 2012 and continued for another ~2 months. In this paper, 3-25 keV RXTE/PCA spectra of 2011 outburst and 0.5-10.0 keV Swift/XRT data during its 2012 outburst are analyzed with the two-component advective flow (TCAF) model based fits file in XSPEC. We calculate the X-ray contributions coming from jets/outflow using a newly developed method based on the deviation of the TCAF model normalization. We also studied the correlation between observed radio and estimated jet X-ray fluxes. The correlation indices ($b$) are found to be $1.79$, and $0.61$, when the 7.45 GHz VLA radio flux is correlated with the total X-ray and the jet X-ray fluxes in 3-25 keV range respectively. It has been found that the jet contributes in X-rays up to a maximum of 86\% during its 2011 outburst. This makes the BHC MAXI J1836-194 to be strongly jet dominated during the initial rising phase.

astro-ph.HE↗

arXiv2019

Evolution of spectral and temporal properties of MAXI J1836-194 during 2011 outburst

We study transient Galatic black hole candidate MAXI~J1836-194 during its 2011 outburst using RXTE/PCA archival data. 2.5-25~keV spectra are fitted with Two Component Advective Flow (TCAF) model fits file as an additive table local model in XSPEC. From TCAF model spectral fits, physical parameters such as Keplerian disk rate, sub-Keplerian halo rate, shock location and compression ratio are extracted directly for better understanding of accretion processes around the BHC during this outburst. Low frequency quasi-periodic oscillation (QPO) are observed sporadically during the entire epoch of the outburst, with a general trend of increasing frequency during rising and decreasing frequency during declining phases of the outburst, as in other transient BHCs. The nature of the variation of the accretion rate ratio (ratio of halo and disk rates) and QPOs (if observed), allows us to properly classify entire epoch of the outburst into following two spectral state, such as hard (HS), hard-intermediate (HIMS). These states are observed in the sequence of HS (Ris.) $\rightarrow$ HIMS (Ris.) $\rightarrow$ HIMS (Dec.) $\rightarrow$ HS (Dec.). This outburst of MAXI~J1836-194 could be termed as `failed' outburst, since no observation of soft (SS) and soft-intermediate (SIMS) spectral state are found during the entire outburst.

astro-ph.HE↗

arXiv2019

Monte Carlo Simulations of Thermal Comptonization Process in a Two Component Advective Flow around a Neutron Star

We explore spectral properties of a Two-Component Advective Flow (TCAF) around a neutron star. We compute the effects of thermal Comptonization of soft photons emitted from a Keplerian disc and the boundary layer of the neutron star by the post-shock region of a sub-Keplerian flow, formed due to the centrifugal barrier. The shock location $X_s$ is also the inner edge of the Keplerian disc. We compute a series of realistic spectra assuming a set of electron temperatures of the post-shock region $T_{CE}$, the temperature of the normal boundary layer (NBOL) $T_{NS}$ of the neutron star and the shock location $X_s$. These parameters depend on the disc and halo accretion rates ($\dot{m_d}$ and $\dot{m_h}$, respectively) which control the resultant spectra. We find that the spectrum becomes harder when $\dot{m}_h$ is increased. The spectrum is controlled strongly by $T_{NS}$ due to its proximity to the Comptonizing cloud since photons emitted from the NBOL cool down the post-shock region very effectively. We also show the evidence of spectral hardening as the inclination angle of the disc is increased.

astro-ph.HE↗

arXiv2018

General Relativistic Numerical Simulation of sub-Keplerian Transonic Accretion Flows onto Rotating Black Holes: Kerr Spacetime

We study time evolution of sub-Keplerian transonic accretion flows onto black holes using a general relativistic numerical simulation code. We perform simulations around the black holes having non-zero rotation. We first compare one-dimensional simulation results with theoretical results and validate the performance of our code. Next, we present results of axisymmetric, two-dimensional simulation of advective flows. In the literature, there is no solution which describes steady shock solutions in two dimensions. However, our simulations produce these centrifugal force supported steady shock waves even in presence of strong dragging of inertial frames. Since the post-shock region could be hot and upscatter photons through Comptonization, these shock would put imprints on the spectra. Thus, our solutions, which represent truly new results, could be useful to measure spins through radiation spectrum of accreting Kerr black holes.

astro-ph.HE↗

arXiv2018

Signature of Two-Component Advective Flow in several Black Hole candidates obtained through time-of-arrival analysis of RXTE/ASM Data

We study several Galactic black hole candidates using long-time RXTE/ASM X-ray data to search for telltale signatures of differences in viscous timescales in the two components used in the Two-Component Advective Flow (TCAF) paradigm. In high-mass X-ray binaries (HMXBs) mainly winds are accreted. This nearly inviscid and dominant sub-Keplerian flow falls almost freely towards the black hole. A standard Keplerian disc can form out of this sub-Keplerian matter in presence of a significant viscosity and could be small in size. However, in low-mass X-ray binaries (LMXBs), highly viscous and larger Keplerian accretion disc is expected to form inside the sub-Keplerian disc due to the Roche-lobe overflow. Due to two viscous timescales in these two components, it is expected to have a larger lag between the times-of-arrival of these components in LMXBs than that in HMXBs. Direct cross-correlation between the photon fluxes will not reveal this lag/delay since they lack linear dependence; however, they are coupled through the viscous processes which bring in both matter. To quantify the aforesaid time lag, we introduce an index (Θ), which is a proxy of the usual photon index (Γ). Thus, when Θ, being dynamically responsive to both fluxes, is considered as a reference, the arrival time lag between the two fluxes in LMXBs is found to be much larger than that in HMXBs. Our result establishes the presence of two dynamical components in accretion and shows that the Keplerian disc size indeed is smaller in HMXBs as compared to that in LMXBs.

astro-ph.HE↗

arXiv2018

Anomalous Outbursts of H 1743-322

Using black body and power-law photon counts of All Sky Monitor (ASM) in Rossi X-ray Timing Explorer (RXTE) satellite it has been established recently by us that there is a significant time lag between the infall timescales of two components in the Two-Component Advective Flow (TCAF) paradigm, where a standard slow moving Keplerian disc is surrounded by a fast moving halo. The time lag is clearly due to the difference in viscosity in the flow components and the size of the Keplerian disc may be considered to be proportional to the arrival time lag. In this paper, using RXTE/ASM (1.5-12 keV) data, we examine eight successive outbursts of the low-mass X-ray binary H 1743-322 since 2003 from a new angle. A dynamic photon index, Θ indicates that the size of the Keplerian disc is biggest during the brightest outburst of 2003. The size diminishes thereafter during subsequent weaker outbursts. These results are corroborated when two energy fluxes corresponding to the two flows are cross-correlated with reference to Θ. Moreover, Θ decides spectral transitions of any outburst. We show from the behaviour of Θ alone that the outburst of October 2008 was an anomalous outburst. In fact, each normal outburst was either preceded or followed by an otherwise premature outburst. This makes H 1743-322 an enigmatic source and a subject of further study.

astro-ph.HE↗

arXiv2018

An Approach to Estimate the Binding energy of Interstellar Species

One of the major obstacles to accurately model the interstellar chemistry is an inadequate knowledge about the binding energy (BE) of interstellar species with dust grains. In denser region of molecular cloud, where very complex chemistry is active, interstellar dust is predominantly covered by H2O molecules and thus it is essential to know the interaction of gas phase species with water ice to trace realistic physical and chemical processes. To this effect, we consider water (cluster) ice to calculate the BE of several atoms, molecules, and radicals of astrochemical interest. Systematic studies have been carried out to come up with a relatively more accurate BE of astrophysically relevant species on water ice. We increase the size of the water cluster methodically to capture the realistic situation. Sequentially one, three, four, five and six water molecules are considered to represent water ice analogue in increasing order of complexity. We notice that for most of the species considered here, as we increase the cluster size, our calculated BE value starts to converge towards the experimentally obtained value. More specifically, our computed results with water c-pentamer (average deviation from experiment ~ 15.8%) and c-hexamer (chair) (average deviation from experiment ~ 16.7%) configuration are found to be more nearer as the experimentally obtained value than other water clusters considered.

astro-ph.GA↗