arXiv subjects

Biswajit Paul

Publications and source records attributed to Biswajit Paul.

At least 37 records · Page 2Linked to original sources

arXiv2025

Flux enhanced localization and reentrant delocalization in the quench dynamics of two interacting bosons on a Bose-Hubbard ladder

We study the quench dynamics of two bosons possessing onsite repulsive interaction on a two-leg ladder and show that the presence of uniform flux piercing through the plaquettes of the ladder favors the localization of the bound states in the dynamics. We find that when the two bosons are symmetrically initialized on the edge rung of the ladder, they tend to edge-localize in their quantum walk - a phenomenon which is not possible in the absence of flux. On the other hand, when the bosons are initialized on the bulk rung they never localize and exhibit linear spreading in their quantum walk. Interestingly, however, we find that in the later case a finite flux favours localization of the bulk bound states in the presence of sufficiently weak quasiperiodic disorder which is otherwise insufficient to localize the particles in the absence of flux. In both the cases, we obtain that the localization in the dynamics strongly depends on the combined effect of the flux and interaction strengths, as a result which we obtain a signature of re-entrant delocalization as a function of flux (interaction) for fixed interaction (flux) strengths.

cond-mat.quant-gas↗

arXiv2025

Floquet realization of prethermal Meissner phase in a two-leg flux ladder

We show that a periodically driven two-leg flux ladder hosting interacting hardcore bosons exhibits a prethermal Meissner phase for large drive amplitudes and at special drive frequencies. Such a prethermal Meissner phase is characterized by a finite time-averaged chiral current. We find an analytic expression of these frequencies using Floquet perturbation theory. Our analysis reveals that the presence of the prethermal Meissner phase is tied to the emergence of strong Hilbert space fragmentation in these driven ladders. We support our analytical results by numerical study of finite-size flux ladders using exact diagonalization and discuss experiments using ultracold dipolar atom platforms that may test our theory.

cond-mat.quant-gas↗

arXiv2025

Nonlocality-Assisted Enhancement of Error-Free Communication in Noisy Classical Channels

The zero-error capacity of a noisy classical channel quantifies its ability to transmit information with absolute certainty, i.e., without any error. Unlike Shannon's standard channel capacity, which remains unaffected by pre-shared correlations, zero-error capacity can be enhanced through nonlocal correlations. In this work, we investigate zero-error communication utility of such correlations arising in the 2-2-m Bell scenario, where two parties have two inputs and m possible outcomes per input. For all m\geq2, we construct examples of noisy classical channels with zero zero-error capacity that, when assisted by extremal 2-2-m nonlocal correlations, can transmit one bit of information. While nonlocal correlations arising from quantum entangled states cannot achieve a positive zero-error capacity for these channels, they significantly enhance the probability of successfully transmitting a classical bit in a single use. Extending this analysis to the 2-m-2 Bell scenario, we identify channels with zero zero-error capacity that can nonetheless perfectly transmit log m bits of information when assisted by corresponding extremal nonlocal correlations. Our findings underscore the versatile utility of Bell nonlocal correlations in achieving zero-error communication.

quant-ph↗

arXiv2025

Unveiling the intensity-dependent wake structure of Vela X-1 using MAXI/GSC

Context. Vela X-1 is among the earliest discovered high-mass X-ray binary (HMXB) pulsars. In such systems, the companion's stellar wind is strongly affected by ionisation from X-rays emitted by the compact object. A smooth, isotropic stellar wind model cannot account for the observed orbital variation in absorption column density. A stream-like photoionisation wake trailing the neutron star has been proposed to explain this variation. Aims. We investigated the variability of the circumbinary environment at different intensity levels of the Vela X-1 and used a model similar to the above-mentioned stream-like photoionisation wake to explain the asymmetric absorption column density present in the source. Methods. The 2.0-20.0 keV MAXI/GSC spectrum was well modelled with a Comptonised continuum absorbed by local and interstellar material. Nearly 13 years of MAXI/GSC data was used to constrain the variations in absorption column density in Vela X-1 from orbital-phase and intensity-and-orbital-phase resolved spectroscopy. Results. The long-term light curve of Vela X-1 shows orbit-to-orbit intensity level variations without any apparent super-orbital periodicity. The orbital-phase resolved spectroscopy in multiple intensity levels reveals asymmetric variation in absorption column density changes across the intensity levels. Conclusions. We confirm that the orbital variation in absorption column density in Vela X-1 cannot be explained by a smooth stellar wind alone using long-term MAXI/GSC data. An additional component, such as a photoionisation or accretion wake, is required. The wake structure is present across intensity levels, with geometry varying by intensity. The long-term MAXI/GSC data enabled us to vary wake parameters and derive best-fit stellar wind parameters for the time-averaged intensity, reproducing observed absorption column density across intensity levels. (Abbreviated.)

astro-ph.HE↗

arXiv2025

Sidebands to mHz QPOs in 4U 1626$-$67 in the second spin-down state

We report results from an $AstroSat$ Target-of-Opportunity (ToO) observation of 4U 1626$-$67, performed on 2023 May 18, soon after the discovery of torque reversal to spin-down in the source. The X-ray emission exhibited significant dependence on both energy and torque state. This work highlights the comparison of timing features of 4U 1626$-$67 with a previous $AstroSat$ observation from 2018, when the neutron star was in the spin-up state. The power density spectrum (PDS) of the 2023 observation comprised a sharp peak corresponding to $ν_{\rm NS}\sim$130 mHz X-ray pulsations along with a prominent quasi-periodic oscillation (QPO) feature at $ν_{\rm QPO}\sim$46 mHz with $\sim$20\% rms amplitude, which was positively correlated with energy. We also report the detection of sidebands to QPO occurring at a beat frequency ($ν_{\rm NS}-ν_{\rm QPO}$) of $\sim$83 mHz with $\sim$8\% rms amplitude, having $>3σ$ detection significance. Additionally, we utilized $Nuclear ~Spectroscopic ~Telescope ~ARray$ ($NuSTAR$) observations from the same torque state (2023 May-July) to analogize the presence and energy dependence of sidebands. The source retains timing properties in this spin-down torque state, similar to those seen in the previous spin-down phase. In sharp contrast, PDS from the 2018 observation was dominated by red noise, an absence of QPOs and a broadening in the wings of the pulse frequency peak, indicating a coupling between periodic and low-frequency aperiodic variability. Furthermore, we detected the known cyclotron resonance scattering feature (CRSF) at 37 keV in the Large Area X-ray Proportional Counter (LAXPC) spectrum. We explore various mechanisms that could possibly explain the presence of QPOs exclusively during the spin-down state.

astro-ph.HE↗

arXiv2025

Flux induced re-entrant dynamics in the quantum walk of interacting bosons

We study the QW of two interacting bosons on a two-leg ladder lattice in the presence of an artificial magnetic field. By considering an uniform flux piercing through the ladder, we show that in the limit of strong onsite repulsion and dominant rung-hopping, an initially slow dynamics becomes fast, then slow and fast again with increase in the flux strength indicating a re-entrant dynamics. This unusual behaviour is found to be associated with the formation, breaking and reformation of a bound pair state along the rung of the ladder. In addition to this we also find a re-entrant behaviour in the chiral dynamics where the chirality in the system first increases and then decreases with increase in interaction. We establish this unusual re-entrant behaviour in the dynamics by analysing the radial velocity, spreading of correlation, center-of-mass shearing and energy band diagrams.

cond-mat.quant-gas↗

arXiv2025

Discovery of a new phase-transient cyclotron line in A0535+26: Constraints on the accretion geometry

In November 2020, A0535+26 underwent one of its brightest outbursts, reaching nearly 12 Crab in X-ray flux. Observed by \textit{Insight-HXMT}, \textit{NuSTAR}, \textit{NICER}, and \textit{AstroSat}, this event provided valuable insights into Be/X-ray binaries. The pulse profiles evolved significantly with luminosity, transitioning from pencil-beam to fan-beam geometries. A0535+26, known for its fundamental cyclotron line at $\sim$44 keV, became only the second source to exhibit a negative correlation between cyclotron line energy and flux at high luminosities, with a plateau phase preceding the transition from positive to negative correlation. We report the discovery of a phase-transient low-energy cyclotron line, detected in a narrow phase range ($\sim$16\%) across all seven \textit{NuSTAR} observations during the rising, peak, and declining phases of the outburst. The new line exhibited dramatic variations with pulse phase and luminosity. We explain this behavior using an accretion geometry where the accretion column sweeps across the line of sight.

astro-ph.HE↗

arXiv2024

ICPR 2024 Competition on Multilingual Claim-Span Identification

A lot of claims are made in social media posts, which may contain misinformation or fake news. Hence, it is crucial to identify claims as a first step towards claim verification. Given the huge number of social media posts, the task of identifying claims needs to be automated. This competition deals with the task of 'Claim Span Identification' in which, given a text, parts / spans that correspond to claims are to be identified. This task is more challenging than the traditional binary classification of text into claim or not-claim, and requires state-of-the-art methods in Pattern Recognition, Natural Language Processing and Machine Learning. For this competition, we used a newly developed dataset called HECSI containing about 8K posts in English and about 8K posts in Hindi with claim-spans marked by human annotators. This paper gives an overview of the competition, and the solutions developed by the participating teams.

cs.CL↗

arXiv2024

Studying the X-ray absorption characteristics of Centaurus X-3 using nearly 14 years of MAXI/GSC data

Centaurus X-3 is a persistent high-mass X-ray binary with the long-term light curve from the source exhibiting orbit-to-orbit intensity variations with no apparent superorbital periodicity. We used $\sim$13.5 years of MAXI/GSC data to study the long-term behaviour of X-ray absorption caused by the stellar wind from the companion star and any absorbing structures present in the binary. We used orbital-phase-resolved spectroscopy to study the variation in the photoelectric absorption along the line of sight of the source for both the intensity-averaged data and intensity-resolved data after dividing all the data binned with orbital period into three intensity levels. We find an asymmetric variation in the photoelectric absorption along the line of sight across an orbit of the source. The orbital-phase-resolved spectra show a clear increase in photoelectric absorption after $ϕ_\text{orb}\sim$ 0.5, which deviates from a spherically symmetric stellar wind model. The flux of Cen X-3 shows significant variation between consecutive orbits. An intensity-resolved spectral analysis of the source was performed, followed by an intensity-resolved and orbital-phase-resolved spectral analysis, which showed that at the medium and high intensity levels, the orbital-phase-resolved photoelectric absorption is slightly asymmetric with respect to mid-phase ($ϕ_\text{orb}=$ 0.5). The asymmetry is very pronounced at the lowest intensity level and cannot be explained by a spherically symmetric wind from the companion star. The differences in the orbital phase-dependence of absorption for different intensity levels suggest that the presence of an accretion wake, photoionization wake, or tidal stream is more prominent at a lower intensity level for Centaurus X-3 than at a higher intensity level.

astro-ph.HE↗

arXiv2024

Network Nonlocality Without Entanglement Of All Sources

Entanglement and nonlocality are two important nonclassical features of quantum correlations. Recently the study of quantum correlations in networks has undergone remarkable progress owing to technological development towards scalable quantum networks. However, compared to standard Bell scenario, manifestation of the interplay between these two aspects has received less attention in network scenarios featuring independent sources. In this work we have analyzed the relation between entanglement content of the sources and detectable non n-locality in two distinct network topologies(linear and star). We have studied the extremal violations of n-local inequalities(compatible with linear and star network) for any fixed amount of entanglement(in terms of concurrence) of the independent sources. It is observed that each of the sources must be entangled for detecting non n-locality in linear network. However, the same is not true for star n-local network. Present analysis is revealing that entanglement of all the independent sources is not a necessity for generation of non n-local correlations in star topology. Characterization of sources in terms of minimum entanglement requirement for any fixed violation amount of the n-local inequalities is also provided. Interestingly, detection of non n-locality is ensured in the network if product of concurrence of all the sources involved exceeds 1/2. .

quant-ph↗

arXiv2024

Probing the energy and luminosity-dependent spectro-timing properties of RX J0440.9+4431 with AstroSat

The Be/X-ray binary pulsar RX J0440.9+4431 went through a giant outburst in December 2022 with a peak flux of $\sim$2.3 Crab in 15--50 keV. We studied the broad-band timing and spectral properties of RX J0440.9+4431 using four $AstroSat$ observations, where the source transited between subcritical and supercritical accretion regimes. Pulsations were detected significantly above 100 keV. The pulse profiles were found to be highly luminosity- and energy-dependent. A significant evolution in the pulse profile shape near the peak of the outburst indicates a possible change in the accretion mode and beaming patterns of RX J0440.9+4431. The rms pulsed fraction was luminosity- and energy-dependent, with a concave-like feature around 20--30 keV. The depth of this feature varied with luminosity, indicating changes in the accretion column height and proportion of reflected photons. The broad-band continuum spectra were best fitted with a two-component Comptonization model with a blackbody component or a two-blackbody component model with a thermal Comptonization component. A quasi-periodic oscillation at 60 mHz was detected at a luminosity of $2.6 \times 10^{37}$ erg s$^{-1}$, which evolved into 42 mHz at $1.5 \times 10^{37}$ erg s$^{-1}$. The QPO rms were found to be energy dependent with an overall increasing trend with energy. For the first time, we found the QPO frequency varying with photon energy in an X-ray pulsar, which poses a challenge in explaining the QPO with current models such as the Keplarian and beat frequency model. Hence, more physically motivated models are required to understand the physical mechanism behind the mHz QPOs.

astro-ph.HE↗

arXiv2024

Dynamics of interacting particles on a rhombus chain: Aharonov-Bohm caging and inverse Anderson transition

The Aharonov-Bohm (AB) caging is the phenomenon of extreme localization of particles experiencing magnetic field in certain tight binding lattices. While the AB caging involves the localization of non-interacting particles, it often breaks down due to the effect of interaction resulting in delocalization. In this study, however, we show that interactions under proper conditions can restore the AB caging of particles. By analysing the dynamics of two bosons possessing both onsite and nearest neighbor interactions on a one dimensional diamond/rhombus lattice pierced by an artificial gauge field, we show that the AB caging is restored when both the interactions are of equal strengths. Furthermore, the AB caged bosons, with the onset of an antisymmetric correlated onsite disorder in the lattice, escape from the cages, demonstrating the phenomenon of inverse Anderson transition which is known to be exhibited by the non-interacting AB caged particles. We also obtain situation similar to the inverse Anderson transition when an external potential gradient is applied to the lattice. These findings offer route to realize the AB caging and inverse Anderson transition of interacting particles in experiments involving ultracold atoms in optical lattices or superconducting circuits.

cond-mat.quant-gas↗

arXiv2024

Luminosity dependence of the multiple cyclotron lines in 4U 0115$+$63

The Be X-ray binary 4U 0115$+$63 underwent a giant outburst in 2023 with the X-ray luminosity of the source reaching 10$^{38}$ erg s$^{-1}$. During the outburst, two target of opportunity observations were made with \textit{NuSTAR}. The main goal of this work is to model the multiple cyclotron scattering features (CRSFs) present in 4U 0115$+$63 and study their dependence on the luminosity of the source. The 3$-$79 keV broadband X-ray coverage of \textit{NuSTAR} allowed us to properly model the continuum and investigate the nature of the multiple cyclotron resonance scattering features present in the source spectrum. We used the epoch-folding technique to find the pulsation from the source and also studied the changes in the cyclotron line energy with an order of magnitude variation in the source luminosity. We detected five cyclotron lines during the 2023 outburst near 12, 16, 24, 34, and 47 keV. The $\sim$16 keV cyclotron line cannot be harmonically related to the other cyclotron lines at $\sim$12 keV and $\sim$24 keV. This indicates the presence of two fundamental lines in the spectrum of 4U 0115$+$63 at 12 keV and 16 keV. With the inclusion of the two latest \textit{NuSTAR} observations, we have expanded the data set of the CRSF line center to encompass a broad range of luminosity. This enables us to comprehensively investigate the relationship between the centroid energy of the cyclotron lines and luminosity. The CRSF line center shows no anticorrelation with luminosity, unlike previously reported. Instead, a weak positive correlation is found in four out of the five detected cyclotron lines of 4U 0115$+$63. The luminosity variation of the two fundamental CRSFs could be well explained with the prediction from the collisionless shock model.

astro-ph.HE↗

arXiv2024

$\mathcal{PT}$-symmetry of Particle mixing theories and the equation of motion matrix

A non-Hermitian complex scalar field model is considered from its $\mc{PT}$ symmetric aspect. A matrix constructed from the Euler-Lagrange equations of motion is utilized to analyze the states of the model. The model has two mass terms which determine the real or complex nature of the eigen values. A mismatch is found in the Lagrange equations of motion of the fields as the equations do not agree with the other after complex conjugation of the either. This is resolved by exploiting a preferred similarity transformation of the Lagrangian. The discrepancy even at the Hamiltonian level is found to have vanished once we consider the similarity transformed Hamiltonian.

hep-th↗

arXiv2024

Energy dependence of Quasi-periodic oscillations in accreting X-ray pulsars

We present the results from an investigation of the energy dependence of Quasi-Periodic Oscillations (QPOs) exhibited by accreting X-ray pulsars using data from archival \textit{XMM-Newton}, \textit{NuSTAR}, \textit{RXTE}, and \textit{NICER} observations. In a search for the presence of QPOs in 99 \textit{XMM-Newton} and \textit{NuSTAR} observations, we detected QPOs in eleven observations of five sources, viz., 4U 1626--67 (48 mHz), IGR J19294+1816 (30 mHz), V 0332+53 (2, 18 and 40 mHz), Cen X--3 (30 mHz), and XTE J1858+034 (180 mHz). A positive correlation of the QPO rms amplitude with energy is exhibited by 4U 1626--67, IGR J19294+1816, Cen X--3 and XTE J1858+034, while no energy dependence is observed in V 0332+53. We also analysed the energy spectrum to decouple thermal (soft-excess) from non-thermal emission and determine if the soft-excess has different QPO properties. We found no evidence for different QPO characteristics of the soft excess. The \textit{NuSTAR} observations of V 0332+53 during the Type-I outburst in 2016 show the presence of twin QPOs at 2.5 mHz and 18 mHz, while the \textit{XMM-Newton} and \textit{NuSTAR} observations during the Type-II outburst in 2015 show a QPO at 40 mHz. We review the observed QPO properties in the context of QPOs found in other types of accreting sources and the models usually used to explain the QPOs in accreting X-ray pulsars.

astro-ph.HE↗

arXiv2024

A comprehensive study of orbital evolution of LMC X-4: Existence of a second derivative of the orbital period

We report here results from pulse arrival time delay analysis of the eclipsing high mass X-ray binary pulsar LMC X-4 using observations made with the Rossi X-ray Timing Explorer, XMM-Newton, NuSTAR and AstroSat. Combining the orbital parameters determined from these observations with the historical measurements dating back to 1998, we have extended the $T_{π/2}$ epoch history of LMC X-4 by about 4600 binary orbits spanning about 18 years. We also report mid-eclipse time measurements ($T_{ecl}$) using data obtained from wide-field X-ray monitors of MAXI-GSC and Swift-BAT. Combining the new $T_{π/2}$ and $T_{ecl}$ estimates with all the previously reported values, we have significantly improved the orbital evolution measurement, which indicates that the orbital period is evolving at a time scale ($P_{\rm orb}/\dot{P}_{\rm orb}$ ) of about 0.8 Myr. For the first time in an accreting X-ray pulsar system, we confirm the existence of a second derivative of the orbital period, having an evolution time scale ($\dot{P}_{orb}/\ddot{P}_{orb}$) of about 55 yr. Detection of a second derivative of the orbital period in LMC X-4 makes its orbital evolution timescale more uncertain, which may also be true for other HMXBs. Independent solutions for the orbital evolution measurement using the mid-eclipse data and the pulse timing data are consistent with each other, and help us put an upper limit of 0.009 on the eccentricity of the binary system.

astro-ph.HE↗

arXiv2024

Flares during Eclipses of High Mass X-ray Binary Systems Vela X-1, 4U 1700-37, and LMC X-4

In eclipsing X-ray binary systems, the direct X-ray emission is blocked by the companion star during the eclipse. We observe only reprocessed emission that contains clues about the environment of the compact object and its chemical composition, ionization levels, etc. We have found flares in some X-ray binaries during their eclipses. The study of eclipse flares provides additional clues regarding the size of the reprocessing region and helps distinguish between different components of the X-ray spectrum observed during the eclipse. In the archival data, we searched for flares during eclipses of high-mass X-ray binaries and found flares in three sources: Vela X-1, LMC X-4, and 4U 1700-37. Comparing spectral properties of the eclipse flare and non-flare data, we found changes in the power-law photon index in all three sources and multiple emission lines in Vela X-1 and 4U 1700-37. The fluxes of prominent emission lines showed a similar increase as the overall X-ray flux during the eclipse flare, suggesting the lines originate in the binary environment and not in the interstellar medium. We also observed a soft excess in 4U 1700-37 that remains unchanged during both eclipse flare and non-flare states. Our analysis suggests that this emission originates from the extremely thin shell of the stellar wind surrounding the photosphere of its companion star. The detection of short (100-200 seconds) count-rate doubling timescale in 4U 1700-37 and LMC X-4 indicates that the eclipse reprocessing occurs in a region larger than, but comparable to the size of the companion star.

astro-ph.HE↗

arXiv2024

Detection of a cyclotron line in the Be X-ray pulsar IGR J06074$+$2205

IGR J0607.4+2205 is a transient Be X-ray binary discovered two decades ago. IGR J0607.4+2205 underwent an outburst in 2023 during which it was observed twice with \textit{NuSTAR}. The main goal of this work is to model the broadband X-ray spectrum of IGR J0607.4+2205 during the outburst and to study the variations of the spectral and timing features at different intensities. We extracted the light curve and spectrum of the source from the two \textit{NuSTAR} observations carried out during the recent outburst in the energy range of 3$-$78 keV. We used the epoch folding technique to find pulsation from the source and to study the changes in emission characteristics from the source with energy across an order of magnitude variation in source luminosity. IGR J0607.4+2205 shows pulsations with a period of $\sim$347.6 s during both the observations, with a pulse fraction of $\geq$50\%. The broadband spectrum of the source was modelled using a power-law continuum with a high-energy cutoff. During the first observation, a cyclotron absorption line at $\sim$51 keV was also present in the source with an optical depth of $\sim$1.3. However, no cyclotron line feature was detected in the second observation when the source was an order of magnitude fainter. Additionally, soft excess was detected in the second observation, which was modelled with a black body component emerging from close to the neutron star (NS). We report the first ever detection of a cyclotron line in the broadband spectrum of IGR J0607.4+2205 centred at 51$\pm$1 keV. The magnetic field strength of the NS is estimated to be $\sim$4$\times$$10^{12}$ G from the centroid energy of the absorption line. A significant change is observed in the pulse profile with luminosity during the decay of the outburst, indicating an associated change in the beaming pattern.

astro-ph.HE↗