arXiv subjects

Ayan Bhattacharjee

Publications and source records attributed to Ayan Bhattacharjee.

13 recordsLinked to original sources

arXiv2025

Contemporaneous X-ray and Optical Polarization of EHSP Blazar H 1426+428

We present the first contemporaneous X-ray and optical polarimetric measurement of the extremely high synchrotron peaked (EHSP) blazar H 1426+428. The X-ray polarimetric observations were undertaken using the Imaging X-ray Polarimetry Explorer (\textit{IXPE}) on 2024 May 27, and 2024 July 5. The \textit{IXPE} pointings were accompanied by contemporaneous optical observations of the Observatorio de Sierra Nevada, Calar Alto Observatory and the Perkins Telescope Observatory. While we observed the X-ray degree of polarization to be $>20\%$, the polarization in the optical band was found to be only $1-3\%$. This trend has been observed in several HSP blazars with available optical and X-ray polarimetric data and is typically explained in terms of energy stratification downstream of a shock. However, we observed a significant difference between the optical and X-ray polarization angles, a feature that has been observed in certain HSP blazars, such as Mrk 421, but remains a relatively rare or underreported phenomenon. We discuss possible scenarios for these findings within the framework of a partially turbulent jet model.

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arXiv2024

A Simulation Study of Low-Power Relativistic Jets: Flow Dynamics and Radio Morphology of FR-I Jets

Radio galaxies are classified into two primary categories based on their morphology: center-brightened FR-I and edge-brightened FR-II. It is believed that the jet power and interactions with the ambient medium govern the deceleration and decollimation of the jet-spine flows, which, in turn, influence this dichotomy. Using high-resolution, three-dimensional relativistic hydrodynamic simulations, we follow the development of flow structures on sub-kpc to kpc scales in kinetically dominant low-power relativistic jets. We find that the bulk Lorentz factor of the jet spine and the advance speed of the jet head, which depend on the energy injection flux and the jet-to-background density contrast, primarily determine the dynamics and structures of the jet-induced flows. The entrainment of ambient gas and the background density and pressure gradient may also play significant roles. To emulate radio morphology, we produce the synthetic maps of the synchrotron surface brightness for the simulated jets, by employing simple models for magnetic field distribution and nonthermal electron population and considering relativistic beaming effects at different inclination angles. Both the flow structures and radio maps capture the longitudinal and transverse structures of the jet-spine and shear layer, consistent with observations. We also compare different background effects and argue that the loss of pressure confinement beyond the galactic core may be a key factor in the flaring and disruption of FR-I jets. Our results confirm that mildly relativistic jets could explain the one-sidedness or asymmetries with the boosted main jet and deboosted counterjet pairs.

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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.

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arXiv2021

Accretion Flow Properties of GRS 1915+105 During Its $θ$ Class Using AstroSat Data

The Galactic microquasar GRS 1915+105 shows rich variability which is categorized into different classes. In this paper, we report temporal and spectral analysis of GRS 1915+105 to study the properties of the accretion flow when the light curve is showing $θ$ class variability. For this purpose, we use the LAXPC data from the Target of Opportunity observations of India's first multi-wavelength astronomy satellite AstroSat. The $θ$ class is marked by the recurrent appearance of U-shaped regions in the light curve, where the photon count rate first decreases rapidly and then increases slowly. For our analysis, we use U-shaped regions of first two orbits (02345 and 02346) on 2016 March 04. In both of the cases, the dynamic Power Density Spectra (PDS) showed significant power at around $4-5$ Hz, suggesting the presence of a low-frequency Quasi-Periodic Oscillation (QPO) around that frequency interval. The QPO frequency is found to increase with time when the enhancement of the energy flux also takes place. From the evolution of the spectra, we determine the evolution of the accretion flow parameters in both of these observations. Fitting the spectra with the transonic flow solution based Two-Component Advective Flow (TCAF) model in $4-25$ keV energy band shows that the Keplerian disk accretion rate rises with the rise in the radiation intensity, while the location of the centrifugal pressure driven shock front decreases. In both these data, a gradual increment of power-law photon index with intensity is observed suggesting the progressive softening of the source.

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arXiv2021

Two-Component Advective Flows around Neutron Stars

The fundamental difference between accretion around black holes and neutron stars is the inner boundary condition, which affects the behavior of matter very close to the compact objects. This leads to formation of additional shocks and boundary layers for neutron stars. Previous studies on the formation of such boundary layers focused on Keplerian flows that reached the surface of the star, either directly or through the formation of a transition layer. However, behavior of sub-Keplerian matter near the surface of a neutron star has not been studied in detail. Here, we study the effect of viscosity, in presence of cooling, on the sub-Keplerian flows around neutron stars, using Smoothed Particle Hydrodynamics. Our time-dependent study shows that multiple shocks, transition and boundary layers form in such type of accretion, when viscosity is significant, and one or more layers are absent when the viscosity is moderate. These flows are particularly of interest for the wind dominated systems such as Cir X-1. We also report the formation of a generalized flow configuration, Two-Component Advective Flow, for the first time.

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arXiv2020

A Unified Spectral Model for Accreting Neutron Stars

Recent studies show that accretion flows around weakly magnetic Neutron Stars undergo multiple shocks, before reaching the surface of the star, which contribute to the spectral and timing variabilities observed in the X-Rays. Here, we report, for the first time, the spectral properties of a unified model of shocked accretion flows around Neutron Stars, based on the Two-Component Advective Flow paradigm. We compare our theoretical results with the X-Ray spectral features of Z and Atoll sources, across different states. We also fit RXTE/PCA spectra of Sco X-1 and 4U 1705-44 to show the potential application of this new model.

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arXiv2020

Spectral Analysis of $χ$ Class Data of GRS 1915+105 Using TCAF Solution

The class variable source GRS 1915+105 exhibits a wide range of time variabilities in time scales of a few seconds to a few days. Depending on the count rates in different energy bands and the nature of the conventional color-color diagram, the variabilities were classified into sixteen classes, which were later sequenced in ascending order of Comptonization Efficiency (CE). The CE is the ratio of the phenomenological power-law and disk blackbody model fitted spectral photon fluxes. For a better understanding of the flow dynamics in one of the variability classes, namely $χ$, we fitted spectra with the physical two-component advective flow (TCAF) model. In $χ$ class, X-ray flux is steady with no significant variation, however, various $χ$ subclasses are observed at different X-ray flux and variations of count rates across different $χ$ subclasses must be linked to the variation of flow parameters such as the accretion rates, be it the Keplerian disc rate and/or the low angular momentum halo rate. We find that in the $χ_{2,4}$ classes, which are reportedly devoid of significant outflows, the spectra could be fitted well using TCAF solution alone. In the $χ_{1,3}$ class, which are always linked with outflows, a cutoff power-law model is needed in addition to the TCAF solution. At the same time, the normalization required by this model along with the variation of photon index and exponential roll-off factor provides us with the information on the relative dominance of the outflow in the latter two classes. TCAF fit also provides us with the size and location of the Compton cloud along with its optical depth. Thus by fitting with TCAF, a physical understanding of the flow geometry in different $χ$ classes of GRS 1915+105 has been obtained.

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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

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

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.

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arXiv2019

Generalized Flows around Neutron Stars

In this chapter, we present a brief and non-exhaustive review of the developments of theoretical models for accretion flows around neutron stars. A somewhat chronological summary of crucial observations and modelling of timing and spectral properties are given in sections 2 and 3. In section 4, we argue why and how the Two-Component Advective Flow (TCAF) solution can be applied to the cases of neutron stars when suitable modifications are made for the NSs. We showcase some of our findings from Monte Carlo and Smoothed Particle Hydrodynamic simulations which further strengthens the points raised in section 4. In summary, we remark on the possibility of future works using TCAF for both weakly magnetic and magnetic Neutron Stars.

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arXiv2019

The 2004 Outburst of BHC H1743-322: Analysis of spectral and timing properties using the TCAF Solution

The black hole transient H1743-322 exhibited several outbursts with temporal and spectral variability since its discovery in 1977. These outbursts occur at a quasi-regular recurrence period of around $0.5-2$ years, since its rediscovery in March 2003. We investigate accretion flow dynamics around the Low Mass X-ray Binary H1743-322 during its 2004 outburst using the RXTE/PCA archival data. We use Two Component Advective Flow (TCAF) solution to analyse the spectral data. From the fits with TCAF solution, we obtain day to day variation of physical accretion rates of Keplerian and sub-Keplerian components, size of the Compton cloud and its other properties. Analysis of the spectral properties of the 2004 outburst by keeping fitted normalization to be in a narrow range and its timing properties in terms of the presence and absence of QPOs, enable us to constrain the mass of the black hole in a range of $10.31 M_{\odot} - 14.07 M_{\odot}$ which is consistent with other estimates reported in the literature.

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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.

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