arXiv subjects

Bikash Chandra Paul

Publications and source records attributed to Bikash Chandra Paul.

At least 19 recordsLinked to original sources

arXiv2026

Cyclotron Line Variability and Accretion Dynamics in Vela X-1

We present a comprehensive analysis of Vela X-1 using two new NuSTAR observations, placed in the context of four earlier datasets obtained between 2012 and 2020. The energy-resolved pulse profiles demonstrate a significant transformation from an asymmetric structure at low energies to distinct double peaks above 12 keV, whereas the pulse fraction escalates with photon energy but decreases with flux. Broadband spectra validate the Fe K alpha emission line and disclose both fundamental and harmonic cyclotron resonant scattering characteristics (CRSF). We observe no substantial link between CRSF energies and luminosity, contrary to previous findings; rather, the photon index and folding energy demonstrate distinct anti-correlations with flux, aligning with sub-critical accretion and increased Comptonization in the accretion column. Our results provide the first clear evidence that the harmonic CRSF in Vela X-1 does not follow the long-term decay previously claimed. The fundamental line energy also displays an irregular evolution, without a clear monotonic trend. Notably, the harmonic-to-fundamental energy ratio departs from the canonical value of two, suggesting that the line-forming regions are located at different heights within the accretion column. These results provide new constraints on the accretion geometry and magnetic field topology of Vela X-1, highlighting the importance of continued monitoring with current and future X-ray observatories.

astro-ph.HE↗

arXiv2025

Cyclic universe and uniform rate inflation in loop quantum cosmology

We investigate uniform rate inflationary universe in the framework of loop quantum cosmology (LQC) and find that this seemingly simple inflationary model is interlinked with various concepts such as cyclic evolution, HNI inflation and polymer quantized scalar fields, when the background spacetime is loop quantized. The potential for an \textit{exactly} uniform rate inflation in a loop quantized FRW spacetime turns out to be a polymerized version of the corresponding potential in general relativity, which mimics the potentials for a polymerized scalar field and that for hybrid natural inflation (HNI). There is also a radical modification of the background spacetime, leading to a cyclic universe with identical epochs separated by quantum bounces which replace the classical singularity. The energy density and Hubble rate are bounded. The predictions for cosmological perturbations depend on the value of the field at the end of inflation. The parameter space is explored to compare the results for spectral index and tensor-to-scalar ratio with observational constraints.

gr-qc↗

arXiv2025

Spherical accretion onto higher-dimensional Reissner-Nordström Black Hole

We obtain relativistic solutions of spherically symmetric accretion by a dynamical analysis of a generalised Hamiltonian for higher-dimensional Reissner-Nordström (RN) Black Hole (BH). We consider two different fluids namely, an isotropic fluid and a non-linear polytropic fluid to analyse the critical points in a higher-dimensional RN BH. The flow dynamics of the fluids are studied in different spacetime dimensions in the framework of Hamiltonian formalism. The isotropic fluid is found to have both transonic and non-transonic flow behaviour, but in the case of polytropic fluid, the flow behaviour is found to exhibit only non-transonic flow, determined by a critical point that is related to the local sound speed. The critical radius is found to change with the spacetime dimensions. Starting from the usual four dimensions it is noted that as the dimension increases the critical radius decreases, attains a minimum at a specific dimension ($D>4$) and thereafter increases again. The mass accretion rate for isotropic fluid is determined using Hamiltonian formalism. The maximum mass accretion rate for RN BH with different equations of state parameters is studied in addition to spacetime dimensions. The flow behaviour and mass accretion rate for a change in BH charge is also studied analytically. It is noted that the maximum mass accretion rate in a higher-dimensional Schwarzschild BH is the lowest, which however, increases with the increase in charge parameter in a higher-dimensional RN BH.

gr-qc↗

arXiv2025

Spectral-Timing Evolution of a Black hole X-ray binary Swift J1727.8-1613: Linking Disk Reflection and Type-C QPO Frequency During the 2023 Outburst

We present a comprehensive spectral-timing analysis of a BHXB Swift J1727.8$-$1613 during its 2023 outburst, using five pointed \textit{NuSTAR} observations sampling the luminous hard-intermediate state. Broadband 3-79 keV spectroscopy employs a physically motivated model combining a cool truncated disk (\texttt{diskbb}), relativistic reflection (\texttt{relxill} in reflection-only mode), and Comptonized continuum (\texttt{nthComp}) to probe the inner accretion geometry around a rapidly spinning black hole ($a_\ast\!=\!0.98$) at moderate inclination. Simultaneous timing analysis reveals type-C quasi-periodic oscillations (QPOs) with novel coherence evolution: the quality factor ($Q$) exhibits a striking non-monotonic dependence on both QPO frequency and luminosity, peaking near $ν_{\rm QPO}\!\sim\!1.2$~Hz and declining at both lower and higher frequencies. This turnover directly constrains Lense-Thirring precession geometry, implying optimal coherence at intermediate truncation radius. A tight photon-index-QPO-frequency correlation demonstrates that spectral softening and frequency rise are concurrent signatures of inward truncation-radius motion. The triadic luminosity evolution-rising disk and Compton, declining reflection-traces precession-driven geometry changes and corona beaming effects. Interpreting disk-normalization variability as apparent-area changes rather than physical radius swings provides new insight into disk-corona boundary layers. These quantitative results provide strong evidence for global Lense-Thirring precession regulation of both timing and spectral properties, establishing Swift J1727.8$-$1613 as a benchmark source for understanding accretion-geometry physics during black hole state transitions.

astro-ph.HE↗

arXiv2025

A study of the pulsar EXO 1745-248 in $f(Q)$ gravity with pseudo-spheroidal geometry

We present a singularity-free relativistic interior solution for constructing stable quark stellar models in the framework of a linear $f(Q)$ gravity ($f(Q) = αQ + ϕ$) satisfying the pseudo-spheroidal geometry. The physical features and the stability of the stellar model is explored with strange star (SS) candidate EXO 1745-248 ($M = 1.7\, M_{\odot}$ and $R = 9\, km$). The Durgapal-Banerjee transformation is employed to obtain the relativistic interior solution using the MIT Bag model equation of state (EoS): $P = \frac{1}{3}(ρ- 4 B_{g})$. For a linear form of $f(Q)$ gravity, we obtain the exterior vacuum solution, which reduces to the Schwarzschild-de Sitter (SdS) solution with the cosmological constant term, $Λ= \fracϕ{2α}$. The stellar model is analyzed for the different values of the spheroidicity parameter ($μ$). The value of $α$ is constrained using a viable physical limit on the Bag parameter ($B_{g} \in [57.55,95.11]\,MeV\,fm^{-3}$). The constraints on Mass-Radius relation indicates that physically acceptable SS models are permitted for $μ\geq 7$. The contribution of $μ$ to the energy density, pressure profiles, and other physical features is studied for the SS candidate EXO 1745-248. The stability of the stellar model obtained here is also analyzed through causality condition, adiabatic index and other stability criteria. We also investigate the stellar model for other SS candidates to test its viability. The relativistic interior solution obtained here can be used to construct viable and physically acceptable strange star models with very high compactness ratio in the framework of linear $f(Q)$ gravity.

gr-qc↗

arXiv2025

Wormholes in 4D Einstein-Gauss-Bonnet gravity with BEC Dark Matter density profile

The existence of Traversable Wormhole (TW) in the 4D Einstein-Gauss-Bonnet (4D-EGB) gravity is explored with phenomenological Bose-Einstein Condensates (BEC) dark matter density profile. In the framework of 4D EGB gravity, which one obtains by regularizing the higher-dimensional EGB gravity in the limit $D \to 4$ is considered to obtain a spherically symmetric TW. The Gauss-Bonnet coupling parameter ($α$) in this case is rescaled to $α\to \fracα{D-4}$. Considering the energy density profile of non-relativistic BEC matter, the shape function of the WH geometry and the Null energy condition (NEC) are determined with a constant redshift function. The applicability of realistic flaring-out condition and asymptotic flatness conditions are studied here and the domain of model parameters for realistic scenario is determined. We analyze the embedding diagram of the WH obtained here with proper radial distance, volume integral quantifier, and anisotropy. We obtained WH which is stable at the throat when $α= -0.0512471$ for a set of model parameters, which is estimated from sound speed measurement. The energy conditions are investigated, and it is noted that there is a range of Gauss-Bonnet coupling parameter $α\in [-4,-4.222]$, at which the energy conditions, including NEC, are obeyed at the throat. We explore NEC for other values of $α$ and found that it is not satisfied. The other energy conditions are also violated. A new result is thus obtained with 4D Einstein-Gauss-Bonnet gravity with BEC Dark Matter profile. We determine the parameter space for which the WH solution exists in the proposed modified gravity model.

gr-qc↗

arXiv2025

Dynamical analysis of cosmological models in non-minimal scalar non-metricity gravity

We investigate the evolution of a spatially flat Friedmann-Robertson-Walker (FRW) universe in the framework of scalar non-metricity theory of gravity. In the model, we consider dark matter (DM) and dark energy (DE) described by the scalar field. In the paper, we examine the asymptotic behavior of the evolution of the observed universe. We probe the universe with and without an interaction between DM and the effective energy density including scalar field in the non-metricity theory that describes the DE. The critical points of the autonomous system and their stability are determined to understand the behaviour of the universe. The cosmological models accommodate the late accelerating phase of the universe for a given set of parameters, a matter-dominated saddle point is also found to exist which is flowed by a decelerating phase dominated by stiff fluid like DE. We obtain a new class of cosmological model accommodating early inflation, matter-dominated era, and a future decelerating phase followed by late acceleration of the universe.

gr-qc↗

arXiv2025

Dynamics of late time universe in $f(Q)$ gravity

We construct cosmological model in nonmetricity scalar functional gravitational Lagrangian $f(Q)$ which describes the dynamical evolution of the late accelerating universe. Cosmological models are constructed considering different functional of $f(Q)$ gravity where $Q$ in the gravitational action. We obtain cosmological model probing late universe with a constant jerk parameter. The observational constraints that are imposed on the model parameters for a realistic scenario estimated using the observational Hubble data and the Pantheon dataset. The evolution of the deceleration parameter, energy density and the equation of state (EoS) parameter are also explored. The transition of the universe from a deceleration to an accelerating phase is investigated in different framework of $f(Q)$ theories. We also analyzed the variation of the effective EoS parameter and found that the matter content in the universe favours quintessence type fluid in all the $f(Q)$-gravity. The energy conditions for a realistic scenario are examined and noted that the effective fluid violates the strong energy condition.

gr-qc↗

arXiv2025

Spectro-timing analysis of Be X-ray pulsar SMC X-2 during the 2022 outburst

We present broadband X-ray observations of the High Mass X-ray Binary (HMXB) pulsar SMC X-2, using concurrent NuSTAR and NICER observations during its 2022 outburst. The source is found to be spinning with a period of 2.37281(3) s. We confirm the existence of cyclotron resonant scattering feature (CRSF) at 31 keV in addition to the iron emission line in the X-ray continuum of the source. Spectral analysis performed with the physical bulk and thermal Comptonization model indicates that the bulk Comptonization dominates the thermal Comptonization. Using phase-resolved spectroscopy, we have investigated the variations of the spectral parameters relative to pulse phase that may be due to the complex structure of magnetic field of the pulsar or the impact of the emission geometry. It is observed that the spectral parameters exhibit significant variabilities relative to the pulsed phase. Time-resolved spectroscopy is employed to examine the evolution of the continuum and changes in the spectral characteristics. Measurements of luminosity along with variations in cyclotron line energy and photon index suggest that the source may be accreting in the super-critical regime.

astro-ph.HE↗

arXiv2024

NuSTAR detection of a broad absorption line in IGR J06074+2205

We present the broadband X-ray study of the BeXRB pulsar IGR J06074+2205 using NuSTAR observations. The temporal and spectral characteristics of the source are investigated. We detect coherent X-ray pulsations of the source and determine the spin period evolution. Using the current spin period data of the source, we show that the source is spinning down at 0.0202(2)\; s\; $day^{-1}$. The pulse profiles are found to evolve with energy and luminosity. Another interesting feature of the source is that the pulse profiles are dual peaked. The dual-peaked pulse profile is characterized by a decreasing secondary peak amplitude with increasing energy. We also observed a proportionate increase in the primary peak amplitude as the energy increases. The pulse fraction exhibits an overall increasing trend with the energy. The X-ray continuum of the source indicates the existence of a characteristic absorption feature with a centroid energy $\sim 55$ keV, which may be interpreted as a cyclotron line. We estimate the corresponding magnetic field strength to be $\sim4.74\times10^{12}$ G. A peculiar `10 keV' absorption feature is observed in the X-ray spectra of the second observation.The luminosity measurements predict that the source may be accreting in the sub-critical regime.

astro-ph.HE↗

arXiv2024

The ongoing spin-down episode of 4U 1626-67

We report the X-ray characteristics of the persistent X-ray pulsar 4U 1626-67 using simultaneous NuSTAR and NICER observations. The X-ray pulsar 4U 1626-67 has just encountered a torque reversal in 2023 and is presently in the spin-down state. We have examined the temporal and spectral characteristics of the source during its ongoing spin-down episode. The pulse profiles of the source are characterized by multiple substructures at lower energies and a wide asymmetric single-peaked structure at higher energies. The pulse fraction follows an overall increasing trend with energy. We confirm the existence of mHz quasi-periodic oscillation (QPO) exclusively during the current spin-down phase in all the observations. The source is spinning down at 0.00045(4)\; s\; $yr^{-1}$. The broadband spectrum during this phase is described by empirical NPEX model and a soft blackbody component with kT $\sim$ 0.25 keV. In addition to the iron emission line, we also confirm the presence of cyclotron line at $\sim$ 36 keV. The source flux continues to decrease during the current spin-down phase, and the corresponding luminosity $\sim$ (3.3-4.9)\;$\times 10^{36}\; ergs\; s^{-1}$ lies in the intermediate range of accreting X-ray pulsars that may be associated with a hybrid accretion geometry. The magnetic field strengths estimated using the cyclotron line measurements and QPO frequency are consistent. The evolution of the spectral parameters relative to the pulsed phase is examined using phase-resolved spectroscopy.

astro-ph.HE↗

arXiv2024

Barrow Holographic Dark Energy in Brane World Cosmology

Cosmological features of Barrow Holographic Dark Energy (BHDE), a recent generalization of original Holographic dark energy with a richer structure, are studied in the context of DGP brane, RS II brane-world, and the cyclic universe. It is found that a flat FRW scenario with pressure less dust and a dark energy component described as BHDE can accommodate late time acceleration with Hubble horizon considered as infrared cut off even in the absence of interaction between the dark sectors. Statefinder diagnostic reveals that these model resemble $ΛCDM$ cosmology in future. It is found that BHDE parameter $Δ$, despite its theoretically constrained range of values, is significant in describing the evolution of the universe, however, a classically stable cosmological model cannot be obtained in the RS-II and DGP brane. Viability of the models is also probed with observed Hubble data.

gr-qc↗

arXiv2024

Dynamical stability and phase space analysis of an Emergent Universe with non-interacting and interacting fluids

We investigate the evolution of a flat Emergent Universe obtained with a non-linear equation of state (nEoS) in Einstein's general theory of Relativity. The nEoS is equivalent to three different types of barotropic cosmic fluids, which are found from the nEoS parameter. The EU began expanding initially with no interaction among the cosmic fluids. Assuming an interaction that sets in at a time $t \geq t_i$ in the fluid components, we study the evolution of the EU that leads to the present observed universe. We adopt a dynamical system analysis method to obtain the critical points of the autonomous system for studying the evolution of an EU with or without interaction in fluid components. We also study the stability of critical points and draw the phase portraits. The density parameters and the corresponding cosmological parameters are obtained for both the non-interacting and interacting phases of the evolution dynamics.

gr-qc↗

arXiv2023

Luminosity dependent cyclotron line in Swift J1626.6-5156

We studied the timing and spectral properties of the Be/X-ray pulsar Swift J1626.6-5156 using the \emph{NICER} observations of its 2021 outburst. The most important observation is the positive correlation of the centroid energy of the fundamental cyclotron line with the luminosity. This observation agrees with the usual positive correlation of the centroid energy cyclotron line with luminosity in the sub-critical regime. The correlation between the two quantities is verified using two different continuum models. The photon index decreases with an increase in flux. Thus, the spectrum is softer when the flux is low, which may be due to a decrease in the optical depth of the accretion column with a decrease in the flux.

astro-ph.HE↗

arXiv2023

Long-term evolution of Cyclotron Line energy in an eclipsing pulsar 4U 1538-522

We present the timing and spectral analysis of the HMXB source 4U 1538-522 using NuSTAR observations. One of the observations partially covers the X-ray eclipse of the source along with eclipse ingress. The source is found to spin down at the rate of 0.163 $\pm$ 0.002 s $\text{yr}^{-1}$ between $\sim$ (54973-58603) MJD. It is evident that at time $\sim$ 58620 MJD, a torque reversal occurred, thereafter the source exhibited a spin-up trend at the rate - (0.305 $\pm$ 0.018) s $\text{yr}^{-1}$ until 59275 MJD. A recent NuSTAR observation finds the pulse period of the source: (526.2341 $\pm$ 0.0041) s. The pulse profile exhibits a transition from double-peaked to single-peaked nature above $\sim$ 30 keV. We analyzed the overall trend of the temporal evolution of fundamental Cyclotron Resonance Scattering Feature (CRSF), $\text{E}_\text{cyc}$, incorporating recent NuSTAR measurements. Initially, during the time span $\sim$ (50452.16-55270.8) MJD, the cyclotron line energy is found to increase at a rate of 0.11 $\pm$ 0.03 keV $\text{yr}^{-1}$ which is further followed by a decrease at a rate - 0.14 $\pm$ 0.01 keV $\text{yr}^{-1}$ between (55270.8-59267) MJD. The combined measurements in the time span (50452.16-59267) MJD reveal that the cyclotron line energy is increasing linearly at a rate of 0.08 $\pm$ 0.02 keV $\text{yr}^{-1}$.

astro-ph.HE↗

arXiv2023

Low luminosity observation of BeXRB source IGR J21347+4737

In this paper, we report the results of the detailed temporal and spectral studies of the BeXRB J21347+4737 based on the data from the NuSTAR and \textit{SWIFT/XRT} in a wide energy range of 0.5-50 keV. Coherent pulsation with a period of 322.738$\;\pm\;0.018$ s was found in the light curve, implying, the source pulsation has spun down by 0.341 s $yr^{-1}$ when compared with the coherent pulsation estimated from XMM Newton more than 7 years ago. The pulse profile of the source demonstrates energy dependence and has evolved with time. The pulse fraction of the source observed by NuSTAR initially decreases with energy upto $\sim$15 keV, followed by a non-monotonic increasing trend above 15 keV. The source spectrum can be well approximated by an absorbed power-law model with modification by an exponential cutoff at high energies. The absorbed flux of the source is $4\times10^{-11}\;erg\;cm^{-2}\;s^{-1}$ and its corresponding luminosity is $3.5\times10^{35}\;erg\;s^{-1}$. The study of pulse-phase resolved spectroscopy shows a strong variation of spectral parameters on the phase. No additional emission or absorption features in the form of Fe line or Cyclotron lines were observed both in the phase-averaged and phase-resolved spectra of IGR J21347+4737.

astro-ph.HE↗

arXiv2023

Spectral study of neutron star low mass X-ray binary source 1A 1744-361

We present X-ray observations of the recent outburst of 2022 from the neutron star low mass X-ray binary (LMXB) source 1A 1744-361. Spectral properties of the source have been analyzed using joint NuSTAR and NICER observations. During our observations, the source happens to be in the banana state (soft state) of the hardness intensity diagram (HID). In addition to a power-law with a high energy cutoff, the spectrum is found to exhibit broad iron $K_α$ emission along with distinct absorption features. A prominent absorption feature observed at 6.92 keV may be interpreted as $K_α$ absorption line from hydrogen-like iron. The absorption feature observed at 7.98 keV may be interpreted as a blend of Fe XXV and Ni XXVII transitions. We have summarized the evidence of variability of the spectral features observed in the X-ray continuum by time-resolved spectroscopy.

astro-ph.HE↗

arXiv2023

Observational constraints on the Emergent Universe with interacting non-linear fluids and its stability analysis

We investigate a flat Emergent Universe (EU) with a nonlinear equation of state which is equivalent to three different compositions of fluids. In the EU, initially, the evolution of the universe began with no interaction, but as time evolves, an interaction sets in among the three fluids leading to the observed universe. The characteristic of an EU is that it is a singularity-free universe that evolves with all the basic features of the early evolution. A given nonlinear equation of state parameter permits a universe with three different fluids. We get a universe with dark energy, cosmic string, and radiation domination to begin with, which at a later epoch transits into a universe with three different fluids with matter domination, dark matter, and dark energy for a given interaction strength among the cosmic fluids. Later the model parameters are constrained using the observed Hubble data and Type Ia Supernova (SnIa) data from the Pantheon data set. The classical stability analysis of the model is performed using the square speed of sound. It is found that a theoretically stable cosmological model can be obtained in this case, however, the model becomes classically unstable at the present epoch when the observational bounds on the model parameters are taken into account.

gr-qc↗