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Soma Mandal

Publications and source records attributed to Soma Mandal.

18 recordsLinked to original sources
arXiv2025

Spectral-timing analysis of the kilohertz quasi-periodic oscillations and constraints on the mass of the neutron star in 4U 1636-536 using AstroSat observations

Kilohertz quasi-periodic oscillations (kHz QPOs) are believed to originate from the orbital timescales of the inner accretion flow, reflecting the dynamics of the innermost disk regions under strong gravitational forces. Despite numerous radiative and geometric models proposed so far, a comprehensive explanation of the observed properties of these variability components remains elusive. This study systematically examines kHz QPOs, their variability, and their connection to spectral properties in $4U 1636-536$ using AstroSat data. Our analysis tracks the source transition from hard to soft states in the hardness-intensity diagram. Broad spectral analysis (0.7-25 keV) using SXT and LAXPC data indicates a spectrum shaped by reflection from a thermal corona, with contributions from boundary layer emission and a soft disk component. We find significant changes in optical depth, blackbody temperature, and inner disk temperature that likely drive state transitions. Power density spectra reveal three variability types: a low frequency QPO (LFQPOs) (~30 Hz), and two simultaneous kHz QPOs. The LFQPOs and the upper kHz QPOs appear more prominently in soft spectral states. The presence of LFQPOs and twin kHz QPOs in soft spectral states enable us to estimate the neutron star mass at (2.37 $\pm$ 0.02) $M_\odot$ using the relativistic precession model (RPM). Additionally, time-lag and root mean square (rms) analysis provide insights into the size of the corona and the radiative origin of these variability components.

astro-ph.HE↗
arXiv2025

QPO signatures of disk restoration after type-I X-ray bursts from 4U~1636$-$536

Type--I thermonuclear bursts (TNBs) from neutron star low-mass X-ray binaries (NS LMXBs) originate on the neutron star's surface from the unstable burning of the accreted material. On the other hand, kHz quasi-periodic oscillations (QPOs) are thought to originate in the innermost regions of the in-spiralling accretion disk. Type-I TNBs are expected to impact the inner accretion flow, and consequently the kHz QPOs, due to the intense radiation pressure. In this work, we systematically study the evolution of the upper and the lower kHz QPOs immediately before and after a Type--I TNB on 4U 1636-536 using AstroSat observations in the 3--20,keV band. The analysis of the power-density-spectra show the presence of kHz QPOs within 200,seconds before the onset of the Type--I burst. However, we have not detected any prominent signature of the same within 100--200,sec after the burst. The kHz QPOs then re-emerges after $\approx$\,200\,sec. The fractional rms variation in the 3--20\,keV band drops by $\approx$\,5--6\,\%, supporting the non-existence of kHz QPOs in the 200\,sec post-Burst Zone. The time scale of 200\,sec coincides with the viscous time scale, highlighting a scenario where the inner disk is temporarily disrupted by the intense radiation from the Type--I TNB. The kHz QPO then re-establishes as the inner disk is restored.

astro-ph.HE↗
arXiv2024

Spectro-temporal evolution of 4U 1702-429 using AstroSat-NICER

We present the broadband spectral and timing properties of the atoll source 4U 1702-429 using two observations of AstroSat with the second one having simultaneous NICER data. For both observations, the spectra can be represented by a Comptonizing medium with a black body seed photon source which can be identified with the surface of the neutron star. A disk emission along with a distant reflection is also required for both spectra. For the first observation, the coronal temperature ($\sim 7$ keV) is smaller than the second ($\sim 13$ keV), and the disk is truncated at a larger radius, $\sim 150$ km, compared to the second, $\sim 25$ km, for an assumed distance of 7 kpc. A kHz QPO at $\sim 800$ Hz is detected in the first and is absent in the second observation. Modeling the energy-dependent r.m.s and time lag of the kHz QPO reveals a corona size of $\leq$ 30 km. A similar model can explain the energy dependence of the broadband noise at $\sim 10$ Hz for the second observation. The results suggest that kHz QPOs are associated with a compact corona surrounding the neutron star and may occur when the disk is truncated at large distances. We emphasize the need for more wide-band observations of the source to confirm these results.

astro-ph.HE↗
arXiv2024

Spectral and Timing evolution of GX 340+0 along its Z-track

We present the results from spectral and timing study of the Z source GX 340+0 using AstroSat's SXT and LAXPC data. During the observation the source traced out the complete Z-track, allowing for the spectral evolution study of the Horizontal, Normal and Flaring branches (HB, NB and FB) as well as the hard and soft apexes (HA and SA). The spectra are better and more physically described by a blackbody component and a hot Comptonizing corona with a varying covering fraction, rather than one having a disc component. Along the track, the Comptonized flux (as well as the covering fraction) monotonically decreases. It is the blackbody component (both the temperature and radius) which varies non-monotonically and hence gives rise to the Z-track behaviour. Rapid timing study reveals a prominent Quasi-periodic Oscillation (QPO) at ~ 50 Hz at the HB, HA and upper NB, while a QPO at ~ 6 Hz is seen for the other branches. The fractional r.m.s of the QPOs increase with energy and exhibit soft lags in all branches except SA and FB.

astro-ph.HE↗
arXiv2023

Hidden Non n-locality In Linear Networks

We study hidden nonlocality in a linear network with independent sources. In the usual paradigm of Bell nonlocality, there are certain states which exhibit nonlocality only after the application of suitable local filtering operations, which, in turn, are some special stochastic local operations assisted with classical communication (SLOCC). In the present work, we introduce the notion of hidden non n-locality. The notion is detailed using a bilocal network. We provide instances of hidden nonbilocality and nontrilocality, where we notice quite intriguingly that nonbilocality is observed even when one of the sources distributes a mixed two-qubit separable state. Furthermore, a characterization of hidden nonbilocality is also provided in terms of the Bloch-Fano decomposition, wherein we conjecture that, to witness hidden nonbilocality, one of the two states (used by the sources) must have nonnull local Bloch vectors. Noise is inevitable in practical scenarios, which makes it imperative to study any possible method to enhance the possibility of detecting nonclassicality in the presence of noise in the network. We find that local filtering enhances the robustness to noise, which we demonstrate using bit-flip and amplitude-damping channels.

quant-ph↗
arXiv2023

Hidden Steering Nonlocality in Quantum Networks

By combining two objects with no quantum effect one can get an object with quantum effect. Such a phenomenon, often referred to as activation has been analyzed for the notion of steering nonlocality. Activation of steering nonlocality is observed for different classes of mixed entangled states in linear network scenarios. Characterization of arbitrary two qubit states, in ambit of steering activation in network scenarios has been provided in this context. Using the notion of reduced steering, instances of steerability activation are also observed in nonlinear network. Present analysis involves three measurement settings scenario(for both trusted and untrusted parties) where steering nonlocality is distinguishable from Bell nonlocality.

quant-ph↗
arXiv2018

The flux distribution of individual blazars as a key to understand the dynamics of particle acceleration

The observed log-normal flux distributions in the high energy emission from blazars have been interpreted as being due to variability stemming from non-linear multiplicative processes generated dynamically from the accretion disc. On the other hand, rapid minute scale variations in the flux point to a compact emitting region inside the jet, probably disconnected from the disc. In this work, we show that linear Gaussian variations of the intrinsic particle acceleration or escape time-scales can produce distinct non-Gaussian flux distributions, including log-normal ones. Moreover, the spectral index distributions can provide confirming evidence for the origin of the variability. Thus, modelling of the flux and index distributions can lead to quantitative identification of the micro-physical origin of the variability in these sources. As an example, we model the X-ray flux and index distribution of Mkn 421 obtained from over 9 years of MAXI observations and show that the variability in the X-ray emission is driven by Gaussian fluctuations of the particle acceleration process rather than that of the escape rate.

astro-ph.HE↗
arXiv2013

Alternating lags of QPO harmonics : A Generic model and its application to the 67 millihertz QPO of GRS 1915+105

A generic model for alternating lags in QPO harmonics is presented where variations in the photon spectrum are caused by oscillations in two parameters that characterize the spectrum. It is further assumed that variations in one of the parameters is linearly driven by variations in the other after a time delay $t_d$. It is shown that alternating lags will be observed for a range of $t_d$ values. A phenomenological model based on this generic one is developed which can explain the amplitude and phase lag variation with energy of the fundamental and the next three harmonics of the 67 mHz QPO observed in GRS 1915+105. The phenomenological model also predicts the variation of the Bicoherence phase with energy, which can be checked by further analysis of the observational data.

astro-ph.HE↗
arXiv2013

The kilo-second variability of X-ray sources in nearby galaxies

Chandra observations of 17 nearby galaxies were analysed and 166 bright sources with X-ray counts > 100, were chosen for temporal analysis. Fractional root mean square variability amplitudes were estimated for lightcurves binned at ~ 4 ksec and of length ~ 40 ksec. While there are nine ultra-luminous X-ray sources (ULXs) with unabsorbed luminosity (in 0.3-8.0 keV band) L > 10^39 erg/s in the sample for which the fractional r.m.s variability is constrained to be < 10%, only two of them show variability. One of the variable ULXs exhibits a secular transition and has a ultra-soft spectrum with temperature ~ 0.3 keV while the other is a rapidly varying source in NGC 0628, which has been previously compared to the Galactic micro-quasar GRS1915+105. These results seem to indicate that ULXs are typically not highly variable in ksec time-scales, except for some ultra-soft ones. Among the relatively low luminosity sources (L ~ 10^38 erg/s) we find five of them to be variable. Apart from an earlier known source in NGC 1569, we identify a source in NGC 2403, which exhibits persistent high amplitude fluctuations.The variability of the sources in general, do not seem to be correlated with hardness,which indicates that they may not be due to variations in any absorbing material, but instead could reflect inner accretion disk instabilities.

astro-ph.HE↗
arXiv2006

On The Problem with Only Zeroth Landau Level Occupancy of Electrons / Protons in Strongly Magnetized Neutron Star Matter in $β$-equilibrium Condition- The Role of Anomalous Magnetic Moments

It is shown explicitly that if only the zeroth Landau level is occupied by electrons in strongly magnetized neutron star matter in $β$-equilibrium condition, then both the proton and neutron matter sectors become spin symmetric. Whereas, the study of Pauli para-magnetism of neutron matter sector shows that such a scenario is physically impossible. It is also shown that in dense stellar hadronic matter in $β$-equilibrium in presence of strong quantizing magnetic field, with $σ-ω-ρ$ exchange type mean field interaction and with the inclusion of magnetic dipole moments does not allow electrons to occupy only the zeroth Landau level.

astro-ph↗
arXiv2006

Neutron Star Equation of State and the Possibility of Complex Self-Energy in Landau Theory of Fermi Liquid in Presence of Strong Quantizing Magnetic Field

Using a relativistic version of Landau theory of Fermi liquid with $σ-ω$ and $ρ$ mesons exchange, we have obtained an equation of state for dense neutron star matter in presence of strong quantizing magnetic field. It is found that in this scenario the self energies of both neutron and proton components of dense neutron star matter become complex under certain physical conditions. To be more specific, it is observed that in the exchange diagrams of $σ$, $ω$ and neutral $ρ$ transfer processes and in the direct interaction diagram with $ρ_\pm$ transfer reactions, the nucleon self-energies become complex in nature.

astro-ph↗
arXiv2005

Chiral Properties of QCD Vacuum in Magnetars- A Nambu-Jona-Lasinio Model with Semi-Classical Approximation

The breaking of chiral symmetry of light quarks at zero temperature in presence of strong quantizing magnetic fiels is studied using Nambu-Jona-Lasinio (NJL) model with Thomas-Fermi type semi-classical formalism. It is found that the dynamically generated light quark mass can never become zero if the Landau levels are populated and the mass increases with the increase of magnetic field strength.

astro-ph↗
arXiv2004

Chiral Symmetry Breaking in Presence of Strong Quantizing Magnetic Fields- A Nambu-Jona-Lasino Model with Semi-Classical Approximation

The breaking of chiral symmetry of light quarks at zero temperature in presence of strong quantizing magnetic field is studied using Nambu-Jona-Lasinio (NJL) model with Thomas-Fermi type semi-classical formalism. It is found that the dynamically generated light quark mass can never become zero if the Landau levels are populated and increases with the increase of magnetic field strength.

astro-ph↗
arXiv2003

Impossibility of Spin Polarized States for Neutron Star / Proto-neutron Star Matter in $β$-Equilibrium Condition

It is shown explicitly that a ferromagnetic transition of neutron star (NS) / proto-neutron star (PNS) matter in the $β$-equilibrium condition with $σ-ω-ρ$ exchange type of mean field approximation can actually occur if and only if the neutrinos remain trapped within the system, and perhaps it is also necessary for the neutrinos / anti-neutrinos to carry some finite non-zero mass. It is further shown that the electrons also play a significant role in this transition process. It is therefore, very much unlikely that such a transition of spin polarization can really take place in old neutron stars of very low temperature, whereas, the possibility of spontaneous ferromagnetic transition cannot be ruled out in a newly born PNS.

astro-ph↗
arXiv2003

Collapse/Flattening of Nucleonic Bags in Ultra-Strong Magnetic Field

It is shown explicitly using MIT bag model that in presence of ultra-strong magnetic fields, a nucleon either flattens or collapses in the direction transverse to the external magnetic field in the classical or quantum mechanical picture respectively. Which gives rise to some kind of mechanical instability. Alternatively, it is argued that the bag model of confinement may not be applicable in this strange situation.

astro-ph↗
arXiv2002

Electrons as Quasi-Bosons in Strong Magnetic Fields and the Stability of Magnetars

Following the recent ideas of Dryzek, Kato, Muñoz and Singleton \cite{R1} (henceforth we call this paper as R1), that the composite system consisting of an electron along with its screened electric field and within the sphere of influence, the trapped magnetic field of white dwarf can behave like a boson, we have argued that such an exotic transition (bosonization) of electronic component in strongly magnetized neutron star matter in $β$-equilibrium can make the existence of magnetars physically viable.

astro-ph↗
arXiv2002

Pauli Paramagnetism of Neutron Star Matter and the Upper Limit for Neutron Star Magnetic Fields

A relativistic version of Pauli paramagnetism for $n-p-e$ system inside a strongly magnetized neutron star has been developed. An analytical expressions for the saturation value of magnetic field strength for each of these constituents at which they are completely polarized have been obtained. From the fully polarized configuration of electronic component, an upper limit for neutron star magnetic field is predicted. It has been concluded that indeed, magnetars, as stronly magnetized young neutron stars can not exist if the constituents are electron, proton and neutron in $β$-equilibrium. An alternative model has been proposed.

astro-ph↗