arXiv subjects

Bikash Chandra Paul

Publications and source records attributed to Bikash Chandra Paul.

At least 37 records · Page 2Linked to original sources

arXiv2023

Study of recently discovered Be/X-ray pulsar MAXI J0655-013 using NuSTAR

We study the recently discovered Be/X-ray pulsar MAXI J0655--013 using the 2022 \emph{NuSTAR} observations. The paper is the first detailed study of the timing and spectral properties of the source. The pulse profiles of the pulsar vary with energy. The pulsed fraction is found to increase monotonically with energy. In between the two \emph{NuSTAR} observations, a large spin-up rate of $\sim$ -1.23 s d$^{-1}$ is observed, which can be due to large spin-up torque acting on the pulsar during an outburst. Such a large spin-up rate is observed for the first time in an X-ray pulsar during an outburst. The variation of the spin period with time can be employed to obtain the orbital parameters of the binary system, and we found the orbital period to be $\sim$ 27.9 d. The second \emph{NuSTAR} observation is done in a low luminosity state ($L_{X} \sim$3.9$\times$10$^{34}$ \unilum). We have detected the pulsation of the pulsar in such a low luminosity state. In such a low luminosity state, the pulsar MAXI J0655--013 might be accreting from the cold disk.

astro-ph.HE↗

arXiv2023

Super-critical accretion of BeXRB SXP 15.3

We have studied the temporal and spectral properties of SXP 15.3 observed by \textit{NuSTAR} in hard energy range 3-79 keV during late 2018. The timing analysis of \textit{NuSTAR} observation predicts coherent pulsation at $15.2388\;\pm\;0.0002\;s$. The pulse profiles in different energy bands demonstrates energy dependence. The shape of the pulse profile was generally suggestive of a fan-beamed dominated pattern, which when combined with the measured luminosity predicts that the source may be accreting in the super-critical regime. Non-monotonic increase of Pulse Fraction was observed with energy. The \textit{NuSTAR} observation finds that the pulse period of the source have spun-up at a rate of --0.0176 s $yr^{-1}$ when compared to the previous analysis by the same observatory more than 1 year ago. The source flux in the present \textit{NuSTAR} study in 3-79 keV energy range is $\sim\;1.36\;\times\;10^{-10}\;erg\;cm^{-2}\;s^{-1}$ which corresponds to a luminosity of $\sim\;6\;\times\;10^{37}\;erg\;s^{-1}$. The cyclotron line energy of the source is detected at $\sim$5 keV. Pulse phase resolved spectroscopy shows that the cyclotron line energy varies significantly with pulse phase and the photon index becomes softer with increasing flux. In addition, we have studied the evolution of luminosity with time using 2017 and 2018 \textit{Swift/XRT} observations. The analysis of \textit{Swift/XRT} data reveals that the photon index are positively correlated with the source luminosity which is a characteristic of super-critical accretion phenomena.

astro-ph.HE↗

arXiv2023

Nonsingular Black Holes in Higher dimensions

We present a class of new nonsingular black holes in higher dimensional theories of gravity. Assuming a specific form of the stress energy tensor exact analytic solutions of the field equation are generated in general theory of relativity (GR) and Rastall theory. The non-singular black hole solutions are obtained with a finite pressure at the centre in $D=4$ dimensions. For $D>4$ the transverse pressure is found finite at the centre for a set of model parameters. In the later case the transverse pressure is more than that in the usual four dimensions. The exact analytic solution of the field equations in higher dimensions for large $r$ coincides with the Schwarzschild black hole solution in the usual four and in higher dimensions which is singularity free. The different features of the generalized non-singular black hole in GR and modified GR are explored. A new vacuum nonsingular black hole is found in Rastall gravity. We also study the motion of massive and massless particles around the black holes.

gr-qc↗

arXiv2022

Spectral properties of the Be/X-ray pulsar 2S 1553-542 during Type II outbursts

We present an extended study of the Be/X-ray pulsar 2S 1553-542 during its Type II outbursts. We have incorporated \emph{NICER, Swift-XRT, RXTE-PCA, \emph{NuSTAR} and FERMI} observations to carry out the detailed phase and time resolved spectral analysis of the source. We have summarized the evidence of variability of the cyclotron feature observed in the X-ray continuum of the source with respect to the pulse phases of the pulsar by using the recent \emph{NuSTAR} observation of 2021 outburst of the source. The time resolved spectral analysis has been performed by considering \emph{RXTE} observations of the 2008 outburst of the pulsar. The hardness intensity diagram (HID) has been obtained using 2008 observations in which the intensity follows distinct branches with respect to the hardness ratio. Diagonal branch is observed in the high intensity state whereas the horizontal branch corresponds to the low intensity state. The transition from the diagonal to horizontal branch occurs at the luminosity of $(4.88\pm0.24)\;\times\;10^{37}\;erg\;s^{-1}$. The photon-index exhibits a weak positive correlation with flux along the diagonal branch and negative correlation along the horizontal branch. The existence of two different diagonal and horizontal branches further reflects the possibility of two different accretion states separated by the critical luminosity. The spin-up rate during the outburst phase is found to depend on the flux and is found to increase with an increase in flux.

astro-ph.HE↗

arXiv2022

Spectral & Timing analysis of Be/X-ray binary EXO 2030+375 during its giant 2021 outburst

We report the X-ray spectral and timing analysis of the high mass X-ray binary EXO 2030+375 during the 2021 type II outburst. We have incorporated NuSTAR, NICER, \textit{Swift}/BAT \& \textit{Fermi}/GBM observations to carry out a comprehensive analysis of the source. Pulse profiles in different energy ranges and time intervals have been generated and analyzed. We have performed a brief comparison of the observations amidst the peak outburst condition and also during the decaying state of the outburst. Pulse profiles are found to evolve with time and energy. An iron emission line at (6-7) keV is observed in the X-ray continuum. Distinct absorption features were observed in the spectra corresponding to the peak outburst state while such features were not detected during the later decaying phase of the outburst. We have estimated the characteristic spin-up time scale to be $\backsim$ 60 years. The continuum flux of the system and the varying luminosities covering the entire outburst period have been used to interpret the characteristics of the source. We have summarized the variability of various parameters along with their underlying physical implications.

astro-ph.HE↗

arXiv2022

A high-mass X-ray binary pulsar 4U 1907+09 with multiple absorption-line features in the spectrum

We report X-ray observations of the High Mass X-ray Binary (HMXB) pulsar 4U 1907+09. Spectral and Timing analysis of the source has been performed using NuSTAR observation. Timing analysis of the photon events revealed the coherent X-ray pulsation of the source with a pulse period of $442.92\;\pm\;0.03$ s. It is observed that the source is spinning down at a rate of $0.1971(4) s yr^{-1}$. The pulse profile is characterized by a decaying amplitude of the secondary peak and relative growth in the amplitude of the primary peak with an increase in energy. The broad-band spectral coverage of NuSTAR has been used to observe multiple absorption features in the X-ray continuum of the source. We confirm the presence of two prominent cyclotron absorption features at $\sim 17$ keV and $\sim 38$ keV respectively. In addition, we have detected an absorption-line feature at $\sim 8$ keV, with an equivalent width of $\sim 1.3$ keV. The variation of the spectral parameters with pulse phase has been observed using phase-resolved spectroscopy and the relevant variabilities of the parameters have been discussed with the underlying physical implications. The continuum evolution and variations in spectral parameters have also been studied by time-resolved spectroscopy.

astro-ph.HE↗

arXiv2022

NuSTAR observation of X-ray pulsar 1E 1145.1-6141

In this paper, we report on the hard X-ray observation of the X-ray pulsar 1E 1145.1-6141 performed with the Nuclear Spectroscopic Telescope Array mission (NuSTAR). The coherent pulsation of the source with a period of $\sim296.653\;\pm\;0.021\;s $ is detected. The source may be in the equilibrium phase, according to the most recent measurements of its pulse period. The pulse profile reveals a mild energy dependence and generally hints at a pencil-beam pattern. The pulse profile have evolved with time. The Pulse fraction is found to depend on energy with a fall in the value at $\sim 32\; keV$. The NuSTAR spectra can be approximated by a composite model with two continuum components, a blackbody emission, cut-off powerlaw, and a discrete component in the form of gaussian to account for the emission line of iron. The estimated absorbed flux of the source is $\sim6\times10^{-10}\;erg\;cm^{-2}\;s^{-1}$ which corresponds to a luminosity of $\sim5\times 10^{36}\;erg\;s^{-1}$. Pulse phase-resolved spectroscopy were performed to understand the evolution of spectral parameters with pulse phase. The estimated blackbody radius is found to be consistent with the size of the theoretical prediction.

astro-ph.HE↗

arXiv2022

Spectral and Timing properties of the recently discovered Be/X-ray pulsar eRASSUJ 052914.9-662446

We have presented NuSTAR and Swift observations of the newly discovered Be/X-ray pulsar eRASSU J052914.9-662446. This is the first detailed study of the temporal and spectral properties of the pulsar using 2020 observations. A coherent pulsation of 1411.5$\pm$0.5 s was detected from the source. The pulse profile was found to resemble a simple single peaked feature which may be due to emission from the surface of the neutron star only. Pulse profiles are highly energy dependent. The variation of the pulse fraction of the pulse profiles are found to be non-monotonic with energy. The 0.5-20 keV Swift and NuSTAR simultaneous can be fitted well with power-law modified by high energy cutoff of $\sim$ 5.7 keV. The NuSTAR luminosity in the 0.5-79 keV energy range was $\sim$ 7.9x10 35 erg/s. The spectral flux in 3-79 keV shows modulation with the pulse phase.

astro-ph.HE↗

arXiv2022

Spectral and timing analysis of BeXRB eRASSU J050810.4-660653 recently discovered in the Large Magellanic Cloud (LMC)

We have studied the Be/X-ray binary (BeXRB) pulsar eRASSU J050810.4-660653 recently discovered in the Large Magellanic Cloud (LMC). Timing and spectral features of the source have been discussed in detail using NuSTAR \& XMM-Newton observations. Coherent pulsation of the source was detected at $\sim 40.578\;\pm\;0.001$ s using NuSTAR observation. We analyzed pulse profiles of the source in different energy bands using NuSTAR \& XMM-Newton data. The pulse-profile evolved with time but was generally suggestive of a pencil-beam dominated pattern, which combined with the measured luminosity, indicates that the source may be accreting in the sub-critical regime. The pulse fraction follows a linearly increasing trend with photon energy and is anti-correlated with luminosity. In the 1 year interval between the XMM and NuSTAR observations the pulse period shortened by 0.021 s which could be consistent with spin-up or orbital Doppler effect. The average flux of the source in (3-50) keV energy range is found to be $\sim 5.56\;\times\;10^{-11}\;erg\;cm^{-2}\;s^{-1}$ and the corresponding luminosity is $\sim 1.66\;\times\;10^{37}\;erg\;s^{-1}$. The variation of spectral parameters with pulse phase is studied using phase resolved spectroscopy which reveals that the observed photon index becomes harder with increasing flux.

astro-ph.HE↗

arXiv2022

Anisotropic strange stars in Einstein Gauss-Bonnet Gravity with Finch-Skea metric

We obtain a class of new anisotropic relativistic solution in Einstein Gauss-Bonnet (EGB) gravity with Finch-Skea metric in hydrostatic equilibrium. The relativistic solutions are employed to construct anisotropic stellar models for strange star with MIT Bag equation of state $ p_{r}= \frac{1}{3} \left( ρ- 4 B_{g}\right)$, where $B_{g}$ is the Bag constants. Considering the mass and radius of a known star PSR J0348+0432 we construct stellar models in the framework of higher dimensions. We also predict the mass and radius of stars for different model parameters. The Gauss-Bonnet coupling term ($α$) plays an important role in determining the density, pressure, anisotropy profiles and other features. The stability of the stellar models are probed analyzing the different energy conditions, variation of sound speed and adiabatic stability conditions inside the star. The central density and pressure of a star in EGB gravity are found to have higher values compared to that one obtains in Einstein gravity ($α=0$). We also explore the effect of extra dimensions for the physical features of a compact object. For this we consider $D=5$ and $D=6$ to obtain a realistic stellar model and found that in the formal case both positive and negative values of $α$ are allowed. But in the later case, only $α<0$ permits compact object in the Finch-Skea metric. We determine the best fit values of the model parameters for a number of observed stars for acceptable stellar models.

gr-qc↗

arXiv2021

Timing and spectral properties of Be/Xray pulsar 4U 1901+03 during 2019 outburst

We have studied the timing and spectral properties of the BeXB 4U 1901+03 during the 2019 outburst using \textit{NuSTAR}, \textit{Swift}, and \textit{NICER} observations. Flares are in all observations and were of tens to hundreds of seconds duration. Pulse profiles were changing significantly with time and the luminosity of the source. An increase in the height of the peak of the pulse profiles was observed with energy. The pulse fraction increases with energy and at the end of the outburst. The variation of the pulse profile with time indicates the transition of the pulsar in different accretion regimes. The absorption like feature at 10 keV shows a positive correlation with the luminosity and along with other spectral parameters this feature was also pulse phase dependent. As the distance to the source is not precisely known hence we cannot confirm this feature to be CSRF and also cannot ignore other possible explanations of the feature. Another absorption like feature about 30 keV was observed in the spectra of the last two \textit{NuSTAR} observations and has line energy of about 30.37$\pm$0.55 and 30.23$\pm$0.62 keV respectively. We have also studied the variation of the line energy, width, and optical depth of this feature with pulse phase. The softening of the spectrum along with the increase in pulse fraction at the end of the outburst and absence of pulsation after 58665.09 MJD suggest that pulsar has entered propeller phase, also abrupt decrease in \textit{Swift}-XRT flux supports the fact.

astro-ph.HE↗

arXiv2021

R\' enyi Holographic Dark Energy in Higher Dimension Cosmology

Rényi holographic dark energy (RHDE) correspondence is discussed in higher dimensional cosmology, namely Kaluza-Klein (KK) cosmology. Both interacting and non-interacting cosmological scenario are considered here. It is found that the non-interacting model naturally leads to the late accelerated universe, unlike the standard Holographic Dark Energy models in 4$D$, which requires interaction to accommodate the late-time acceleration of the universe. The interacting model produces an accelerating universe at late time albeit failing to attain the estimated present value of the deceleration parameter. The evolution of different cosmologically relevant parameters have been estimated. We consider here two diagnostic tests namely, state-finder and $Om$ diagnostics to study the non-interacting model which is more favoured in the light of recent cosmological observations. Classical stability of the cosmological models are also discussed

gr-qc↗

arXiv2020

Gravitational collapse of anisotropic stars

We study the gravitational collapse of a spherically symmetric anisotropic relativistic star within Einstein theory of gravity making use of one of our recently developed collapsing stellar models [{\it Astrophys. Space Sci.} {\bf361} 99 (2016)]. The final state of continual gravitational collapse of a massive star under regular initial conditions is analyzed in terms of the formation of black holes. To study the evolution of an anisotropic star undergoing gravitational collapse, it is assumed that the dissipation process happens in the form of radial heat-flux. The interior space-time is described by static metric matched at the boundary with Vaidya metric that describes the exterior to the radiating star. The initial static configuration is described by the relativistic solution obtained by Paul and Deb [{\it Astrophys. Space Sci.} {\bf354} 421 (2014)]. The impact of anisotropy on the dynamical gravitational collapse of a massive star is studied. The relativistic causal heat transport equation of the Maxwell-Cattaneo equation is utilized to show the dependence of anisotropy on the temperature profile of the collapsing system.

gr-qc↗

arXiv2019

Inflation in anisotropic brane universe using tachyon field

Cosmological solution to the gravitational field equations in the generalized Randall-Sundrum model for an anisotropic brane with Bianchi I geometry and perfect fluid as matter sources has been considered. The matter on the brane is described by a tachyonic field. The solution admits inflationary era and at a later epoch the anisotropy of the universe washes out. We obtain two classes of cosmological scenario, in the first case universe evolves from singularity and in the second case universe expands without singularity.

gr-qc↗

arXiv2018

A report on the type II X-ray burst from SMC X-1

We study RXTE PCA data for the high mass X-ray binary source SMC X-1 between 2003-10 and 2003-12 when the source was in high states. The source is found to be frequently bursting which can be seen as flares in lightcurves on an average of one in every 800 s, with an average of 4-5 X-ray burst per hour of type II. We note that typically burst was short lasting for few tens of seconds in addition few long bursts of more than hundred seconds were also observed. The flares apparently occupied 2.5$\%$ of the total observing time of 225.5 ks. We note a total of 272 flares with mean FWHM of the flare $\backsim$21 s. The rms variability and the aperiodic variability are independent of flares. As observed the pulse profiles of the lightcurves do not change its shape implying that there is no change in the geometry of accretion disk due to burst. The hardness ratio and the rms variability of lightcurves show no correlation with the flares. The flare-fraction shows a positive correlation with the peak-to-peak ratio of the primary and secondary peaks of the pulse profile. The observed hardening or the softening of the spectrum cannot be correlated with the flaring rate but may be due to the interstellar absorption of X-rays as evident from the change in the hydrogen column density ($n_{H}$). It is found that the luminosity of the source increases with the flaring rate. Considering the viscous timescale equal to mean recurrence time of flares we fixed the viscosity parameter $α$ $\backsim$ 0.16.

astro-ph.HE↗

arXiv2016

Relativistic Solutions of Anisotropic Compact Objects

We present a class of new relativistic solutions with anisotropic fluid for compact stars in hydrostatic equilibrium. The interior space-time geometry considered here for compact objects are described by parameters namely, $λ$, $k$, $A$, $R$ and $n$. The values of the geometrical parameters are determined here for obtaining a class of physically viable stellar models. The energy-density, radial pressure and tangential pressure are finite and positive inside the anisotropic stars. Considering some stars of known mass we present stellar models which describe compact astrophysical objects with nuclear density.

gr-qc↗

arXiv2015

Observational constraints on Modified Chaplygin Gas from Large Scale Structure

We study cosmological models with modified Chaplygin gas (in short, MCG) to determine observational constraints on its EoS parameters. The observational data of the background and the growth tests are employed. The background test data namely, H(z)-z data, CMB shift parameter, Baryonic acoustic oscillations (BAO) peak parameter, SN Ia data are considered to study the dynamical aspects of the universe. The growth test data we employ here consists of the linear growth function for the large scale structures of the universe, models are explored assuming MCG as a candidate for dark energy. Considering the observational growth data for a given range of redshift from the Wiggle-Z measurements and rms mass fluctuations from Ly-$α$ measurements, cosmological models are analyzed numerically to determine constraints on the MCG parameters. In this case, the Wang-Steinhardt ansatz for the growth index $γ$ and growth function $f$ (defined as $f=Ω_{m}^γ (a)$) are also taken into account for the numerical analysis. The best-fit values of the equation of state parameters obtained here are used to study the variation of the growth function ($f$), growth index ($γ$), equation of state parameter ($ω$), squared sound speed $c^2_{s}$ and deceleration parameter with redshift. The observational constraints on the MCG parameters obtained here are then compared with those of the GCG model for viable cosmology. It is noted that MCG models satisfactorily accommodate an accelerating phase followed by a matter dominated phase of the universe. The permitted range of values of the EoS parameters and the associated parameters ($f$, $γ$, $ω$, $Ω$, $c^2_{s}$, $q$) are compared with those obtained earlier using other observations.

astro-ph.CO↗

arXiv2014

Relativistic anisotropic star and its maximum mass in higher dimensions

We present a class of relativistic solutions of cold compact anisotropic stars in hydrostatic equilibrium in the framework of higher dimensions using spheroidal geometry. The solutions obtained with Vaidya-Tikekar metric are used to construct stellar models of compact objects and studied their physical features. The effects of anisotropy and extra dimensions on the global properties namely, compactness, mass, radius, equation of state are determined in higher dimensions in terms of the spheroidicity parameter ($λ$). It is noted that for a given configuration, compactness of a star is found smaller in higher dimensions compared to that in four space-time dimensions. It is also noted that the maximum mass of compact objects increase with the increase of space-time dimensions which however attains a maximum when $D=5$ for a large ($λ=100$), thereafter it decreases as one increases number of extra dimensions. The effect of extra dimensions on anisotropy is also studied.

gr-qc↗