arXiv subjects

Biplab Raychaudhuri

Publications and source records attributed to Biplab Raychaudhuri.

15 recordsLinked to original sources

arXiv2026

On the effect of gravitational repulsion on geodesic deviation in the Schwarzschild-de Sitter spacetime

Here we discuss the effect of gravitational repulsion on geodesic deviation in the Schwarzschild-de Sitter spacetime. In particular, the geodesic deviation shows different behaviour inside or outside a critical surface, which is the divider between attractive and repulsive regions of gravity. This in turn makes it possible to locate the critical surface and regions of gravitational repulsion and attraction from the behaviour of geodesic deviation.

gr-qc↗

arXiv2026

On the assessment of the disk truncation and detection of type-II bursts from the accreting millisecond X-ray Pulsar IGR J17062-6143

We present a spectral analysis of the NuSTAR and NICER observations of the accreting millisecond X-ray pulsar IGR J17062-6143, performed in 2022. The source remained in the hard spectral state during the observations, with a luminosity of about 0.2-1.3$\%$ of the Eddington luminosity. The continuum emission of the NuSTAR spectrum is entirely dominated by a power-law component or by Comptonized emission of disk photons by a plasma with a high electron temperature ($\gtrsim100$ keV). The NuSTAR spectrum also reveals clear evidence of disk reflection, a broad Fe K line around 6-8 keV, and a Compton hump peaking at 20 keV, irrespective of the choice of the continuum models. Our spectral studies suggest a disk extending close to the neutron star surface ($\sim$7-17 $R_{\rm g}$) at low inclination angles ($\sim$20$^\circ$-40$^\circ$), as revealed by a couple of self-consistent relativistic reflection models, relxill and relxillCP. In addition, we detected type-II bursts for the first time in the NICER observation of this source. Light curve profiles of type-II bursts exhibit different patterns, mostly associated with the so-called mode-0 and mode-1 type-II bursts. The energy spectra of the persistent (pre-burst) and burst emission are well described by an absorbed Comptonization component, scattering diskbb- and blackbody-distributed photons, respectively, by a corona with a temperature of 1-3 keV. Although the origin of the type-II burst is not very clear, it has been substantially linked to magnetospheric gating of the accretion flow.

astro-ph.HE↗

arXiv2026

Emergent gravity from nonlinear perturbation of spherical accretion with variable adiabatic index

The main aim of the present work is to demonstrate that the analogue gravity phenomena are not an artifact of linear perturbation, rather gravity-like effects emerge through the non linear higher order perturbation of transonic fluid as well. To establish that fact, a spherically accreting astrophysical system has been considered where the hydrodynamic accretion with a relativistic, multi-component equation of state with position dependent adiabatic index onto compact astrophysical objects has been considered. By extending the acoustic metric formalism beyond the linear regime, it has been shown that the aforementioned perturbations satisfy a covariant wave equation in an effective acoustic spacetime with non-linear corrections, making the analogue geometry dynamical. As a consequence, the acoustic horizon can shift (inward or outward), depending on the relative amplitudes of density, temperature, and mass accretion-rate fluctuations. This provides a more realistic framework to investigate the dynamics of the non-linear analogue spacetime in astrophysically relevant accretion flows.

gr-qc↗

arXiv2025

Exploring the spectral characteristics of the periodic burster 4U 1323-62: Type-I X-ray burst and persistent emission

We report on the results obtained by the analysis of persistent and type-I thermonuclear X-ray burst emission observed from the periodic burster 4U 1323-62. These analyses are based on the NuSTAR observation performed on 2024 August 7 for a total exposure of around 90 ks. The persistent emission is well described by an absorbed thermal Comptonization model. An absorption edge is also detected at an energy of approximately 7.42 keV, which indicates the presence of absorbing material in the vicinity of this system. Six bursts have been observed during this observation, wherein we find the burst recurrence time to be approximately 4.52 hr. All the bursts exhibit the characteristics of a sharp rise and exponential decay. We perform the time-resolved spectroscopy of the burst spectra described by a model consisting of thermal emission from the neutron star surface and a varying persistent emission component to study the evolution of burst parameters. The enhancement of the persistent emission during burst exposure is characterized by the scaling parameter f a, which reflects the increasing strength of the burst-disc interaction with burst intensity, likely driven by Poynting-Robertson drag. The spectral analysis of bursts estimate the average apparent blackbody emitting radius of the neutron star to lie within 1.5-3.5 km. The ignition depths computed from the burst parameters indicate short Type-I thermonuclear bursts from a mixed hydrogen-helium fuel layer.

astro-ph.HE↗

arXiv2024

NuSTAR view of the X-ray transients Swift J174805.3-244637 and IGR J17511-3057

We report on the NuSTAR observations of the neutron star low-mass X-ray binary Swift J174805.3-244637 (hereafter Swift~J17480) and the accreting millisecond X-ray pulsar IGR~J17511-3057 performed on March 4, 2023, and April 8, 2015, respectively. We describe the continuum emission of Swift~J17480 with a combination of two soft thermal components and an additional hard X-ray emission described by a power-law. We suggest that the spectral properties of Swift~J17480 are consistent with a soft spectral state. The source IGR~J17511-3057 exhibits a hard spectrum characterized by a Comptonized emission from the corona. The X-ray spectrum of both sources shows evidence of disc reflection. For the first time, we employ the self-consistent reflection models ({\tt relxill} and {\tt relxillNS}) to fit the reflection features in the \nustar{} spectrum. From the best-fit spectral model, we find an inner disc radius ($R_{in}$) is precisely constrained to $(1.99-2.68)\:R_{ISCO}$ and inclination to $30\pm 1\degree$ for Swift~J17480. We determine an inner disc radius of $\lesssim 1.3\;R_{ISCO}$ and inclination of $44\pm 3\degree$ for IGR~J17511-3057. A low inclination angle of the system is required for both sources. For the source IGR~J17511-3057, spinning at $4.1$ ms, the value of co-rotation radius ($R_{co}$) is estimated to be $\sim 42$ km ($3.6\:R_{ISCO})$, consistent with the position of inner disc radius as $R_{in}\lesssim R_{co}$. We further place an upper limit on the magnetic field strength of the sources, considering the disc is truncated at the magnetospheric radius.

astro-ph.HE↗

arXiv2024

Relativistic X-ray reflection from the accreting millisecond X-ray pulsar IGR J17498-2921

The accreting millisecond X-ray pulsar IGR J17498-2921 went into X-ray outburst on April 13-15, 2023, for the first time since its discovery on August 11, 2011. Here, we report on the first follow-up \nustar{} observation of the source, performed on April 23, 2023, around ten days after the peak of the outburst. The \nustar{} spectrum of the persistent emission ($3-60$ \kev{} band) is well described by an absorbed blackbody with a temperature of $kT_{bb}=1.61\pm 0.04$\kev{}, most likely arising from the NS surface and a Comptonization component with power-law index $\Gamma=1.79\pm0.02$, arising from a hot corona at $kT_{e}=16\pm 2$ keV. The X-ray spectrum of the source shows robust reflection features which have not been observed before. We use a couple of self-consistent reflection models, {\tt relxill} and {\tt relxillCp}, to fit the reflection features. We find an upper limit to the inner disc radius of $ 6\: R_{ISCO}$ and $ 9\: R_{ISCO}$ from {\tt relxill} and {\tt relxillCp} model, respectively. The inclination of the system is estimated to be $\simeq 40\degr$ from both reflection models. Assuming magnetic truncation of the accretion disc, the upper limit of magnetic field strength at the pole of the NS is found to be $B\lesssim 1.8\times 10^{8}$ G. Furthermore, the \nustar{} observation revealed two type I X-ray bursts and the burst spectroscopy confirms the thermonuclear nature of the burst. The blackbody temperature reaches nearly $2.2$ keV at the peak of the burst.

astro-ph.HE↗

arXiv2022

A quest for the origin of the Sagnac effect

In the literature, there is no consensus on the origin of the relativistic Sagnac effect, particularly from the standpoint of the rotating observer. The experiments of Wang et al. \cite{wang2003modified,wang2004generalized} has, however, questioned the pivotal role of rotation of the platform in Sagnac effect. Recently, the relative motion between the reflectors which force light to propagate along a closed path and the observer has been ascribed as the cause of the Sagnac effect. Here, we propose a thought experiment on linear Sagnac effect and explore another one proposed earlier to demonstrate that the origin of the Sagnac effect is neither the rotation of frame affecting clock synchronization nor the relative motion between the source and the observer; Sagnac effect originates purely due to asymmetric position of the observer with respect to the light paths. Such a conclusion is validated by analysis of a gedanken Sagnac kind experiment involving rotation.

physics.class-ph↗

arXiv2021

Synchronization gauge field, standing waves and one-way-speed of light

The absolute nature of many fundamental predictions of the theory of special relativity, including the relativity of simultaneity, has been questioned in the literature owing to the choice of distant clock synchronization process in the theory. Here we discuss the consequences of Anderson-Vetharaniam-Stedman (AVS) conventionality synchronization gauge, which reflects the choice of synchronization convention, on the standing wave observable. We found that although the position of the node(s) is gauge invariant and remain the same as in the standard case of the stationary wave formation following the Einstein synchronization, the anti-node(s) becomes a gauge dependent (conventional) element and the resulting wave travels between two nodes, contrary to the experimental observation. The experimental detection of standing wave substantiates that the one-way velocity is equal to the round-trip velocity implying the uniqueness of the Einstein synchronization convention. The present analysis thus eliminates the (unphysical) synchronization gauge freedom of special relativity.

physics.class-ph↗

arXiv2021

A new thought experiment on relativistic length contraction

Relativistic length contraction is revisited and a simple but new thought experiment is proposed in which an apparent asymmetric situation is developed between two different inertial frames regarding detection of light that comes from a chamber to an adjacent chamber through a movable slit. The proposed experiment does not involve gravity, rigidity or any other dynamical aspect apart from the kinematics of relative motion; neither does it involve any kind of non-uniformity in motion. The resolution of the seemingly paradoxical situation has finally been discussed.

physics.class-ph↗

arXiv2018

Broadband spectral study of the jet-disc emission in the radio-loud narrow-line Seyfert 1 galaxy 1H 0323+342

We present a broadband spectral study of the radio-loud narrow-line Seyfert 1 galaxy 1H~0323+342 based on multi-epoch observations performed with NuSTAR on 2014 March 15, and two simultaneous observations performed with Suzaku and Swift on 2009 July 26 and 2013 March 1. We found the presence of a strong soft X-ray excess emission, a broad but weak Fe line and hard X-ray excess emission. We used the blurred reflection (relxill) and the intrinsic disc Comptonization (optxagnf), two physically motivated models, to describe the broadband spectra and to disentangle the disk/corona and jet emission. The relxill model is mainly constrained by the strong soft X-ray excess although the model failed to predict this excess when fitted above 3keV and extrapolated to lower energies. The joint spectral analysis of the three datasets above 3keV with this model resulted in a high black hole spin ($a>0.9$) and moderate reflection fraction $R\sim 0.5$. The optxagnf model fitted to the two simultaneous datasets resulted in an excess emission in the UV band. The simultaneous UV-to-hard X-ray spectra of 1H~0323+342 are best described by a model consisting of a primary X-ray power-law continuum with $\Gamma \sim 1.8$, a blurred reflection component with $R\sim 0.5$, Comptonised disk emission as the soft X-ray excess, optical/UV emission from a standard accretion disk around a black hole of mass $\sim 10^7{\rm M_{\odot}}$ and a steep power law ($\Gamma \sim 3-3.5$) component, most likely the jet emission in the UV band. The fractional RMS variability spectra suggest that both the soft excess and the powerlaw component are variable in nature.

astro-ph.GA↗

arXiv2017

NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus~X--2

We report on the \nustar{} observation of the Z-type neutron star low-mass X-ray binary Cygnus X--2 performed on 7 January 2015. During this observation, the source exhibited sudden decrease in count rate (dips) and stronger variability in $3-79\kev$ X-ray lightcurve. The hardness-intensity diagram shows that the source remained in the so-called \quotes{normal branch} of the Z-track, although an extended \quotes{flaring branch} is observed during the dips. The source was in a soft spectral state with the $3-45\kev$ luminosity of $L\simeq(0.5-1.1)\times 10^{38}$ erg s$^{-1}$, assuming a distance of 8 kpc. Both the non-dip and dip X-ray spectra are well represented by models in which the soft band is dominated by the emission from the disc, while the hard X-ray band is dominated by the Comptonized emission from the boundary layer/corona and its reflected emission from the disc. The X-ray spectrum also revealed a broad Fe K$\alpha$ emission line which is nearly symmetric at the higher flux and asymmetric when the flux is reduced by a factor of $\sim 2$. The relativistic reflection model predicts the inner radius of the accretion disc as $R_{in}\simeq2.5-6.0\:R_{ISCO}\:(\simeq30-73$ Km) for the non-dip state and $R_{in}\simeq2.0-2.6\:R_{ISCO}\:(\simeq24-32$ Km) for the dip state. If the inner disc is truncated due to the pressure arising from a magnetic field, this implies an upper limit of the magnetic field strength of $\leq7.6\times10^{9}$ G at the magnetic poles which is consistent with other estimates.

astro-ph.HE↗

arXiv2016

Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30

Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disk. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here we investigate the reality of the broad iron line detected earlier from the neutron star low mass X-ray binary 4U~1820--30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broadband spectral study of the atoll source using \suzaku{} and simultaneous \nustar{} \& \swift{} observations. We have used different continuum models involving accretion disk emission, thermal blackbody and thermal Comptonization of either disk or blackbody photons. The \suzaku{} data show positive and negative residuals in the region of iron K band. These features are well described by two absorption edges at $7.67\pm0.14\kev$ and $6.93\pm0.07\kev$ or partial covering photoionized absorption or by blurred reflection. Though, the simultaneous \swift{} and \nustar{} data do not clearly reveal the emission or absorption features, the data are consistent with the presence of either absorption or emission features. Thus, the absorption based models provide an alternative to the broad iron line or reflection model. The absorption features may arise in winds from the inner accretion disk. The broadband spectra appear to disfavour continuum models in which the blackbody emission from the neutron star surface provides the seed photons for thermal Comptonization. Our results suggest emission from a thin accretion disk ($kT_{disk} \sim 1\kev$), Comptonization of disk photons in a boundary layer most likely covering a large fraction of the neutron star surface and innermost parts of the accretion disk, and blackbody emission ($kT_{bb} \sim 2\kev$) from the polar regions.

astro-ph.HE↗

arXiv2016

NuSTAR and Swift joint view of neutron star X-ray binary 4U~1728--34 : Disc reflection in the island and lower banana states

We analyze two simultaneous \nustar{} and \swift{} data of the Atoll type neutron star (NS) X-ray binary 4U~1728--34 observed on 1 and 3 October, 2013. Based on power density spectra, hardness ratio and colour-colour diagram, we infer that the first and second observations belong to the island state and lower banana state, respectively. During island state, a low-frequency quasi-periodic oscillation (LFQPO) at $\sim4.53$ Hz is observed along with four type-I X-ray bursts ($\sim 10-15$ s duration each). The X-ray luminosity of the source during the island and lower banana states are found to be $L_{X}= 1.1$ and $1.6\times10^{37}$ erg s$^{-1}$, respectively which correspond to $\sim 6\%$ and $\sim 9\%$ of the Eddington luminosity. The combined burst spectrum is well represented by a blackbody with a characteristic temperature of $2.22\pm0.05\kev$ and the blackbody radius of $9.87\pm1.86$ km which is consistent with the typical radius of the neutron star. The persistent energy spectra from both observations in the energy band $1-79\kev$ are well described with thermal emission from the NS surface ($kT_{bb}\simeq1-2.5\kev$), Comptonized emission of thermal seed photons from the hot boundary layer/corona and the strong reflection component from the accretion disc. We detect a broad Iron line in the $5-8 \kev$ band and reflection hump in the $15-30 \kev$ band. These features are well modelled by the {\tt relxill} reflection model. From joint spectral fitting, we constrain the inclination angle of the binary system and inner disc radius to be $22\degree-40\degree$ and $(2.8-4.3)\times R_{ISCO}$, respectively. From inner disc radius, we estimate the magnetic field to be $(3.3-6.5)\times10^{8}$ Gauss.

astro-ph.HE↗

arXiv2015

XMM-Newton view of a hard X-ray transient IGR J17497-2821

We present spectral and energy dependent timing characteristics of the hard X-ray transient IGR J17497-2821 based on XMM-Newton observations performed five and nine days after its outburst on 2006 September 17. We find that the source spectra can be well described by a hard (Gamma ~ 1.50) powerlaw and a weak multicolour disk blackbody with inner disk temperature kT_{in} ~ 0.2 KeV. A broad iron K - alpha line with FWHM ~ 27000 Km/s, consistent with that arising from an accretion disk truncated at large radius, was also detected. The power density spectra of IGR J17497 - 2821, derived from the high resolution (30 micro second) timing mode XMM-Newton observations, are characterised by broadband noise components that are well modelled by three Lorentzians. The shallow power law slope, low disk luminosity and the shape of the broadband power density spectrum indicate that the source was in the hard state. The rms variability in the softer energy bands (0.3-2 KeV) found to be ~ 1.3 times that in 2-5 and 5-10 KeV energy bands. We also present the energy dependent timing analysis of the RXTE/PCA data, where we find that at higher energies, the rms variability increases with energy.

astro-ph.HE↗

arXiv2011

Scalar Perturbation of Black Holes in (2+1)-Dimensional Dilaton Gravity

Scalar perturbation represented by the Klein-Gordon field equation of the system of black holes in 2+1 dimensional Static charged dilaton gravity proposed by Chan and Mann is investigated analytically. The radial equation is analyzed near horizon and at the asymptotic region to show the absence of superradiance instability.

gr-qc↗