arXiv subjects

Biswajit Paul

Publications and source records attributed to Biswajit Paul.

At least 19 recordsLinked to original sources

arXiv2026

Reprocessing of X-rays emission in Ultra-Luminous X-ray sources

Most of the Ultraluminous X-ray Sources (ULXs) harbor either a neutron star or a stellar-mass black hole as a compact object accreting at super-Eddington rates. In spite of having such a high accretion rate, the reprocessed emission in ULXs is quite meagre compared to that observed in Galactic X-ray binaries, except for some absorption lines in the winds. In this work, we use archival Chandra and \xmm\ observations to investigate the extent of reprocessed emission in ULXs through three diagnostics: (i) searches for Fe $\rm Kα$ lines in bright well-known ULXs and Ultraluminous X-ray Pulsars (ULXPs), (ii) evolution of hardness ratio around the eclipse transitions in the eclipsing ULXs, and (iii) the flux ratio between eclipse and out-of-eclipse (OOE) phases in eclipsing ULXs. We constrain the equivalent width (EW) of the Fe K$α$ line, with upper limits of $\sim$11-20 eV for a fixed line centroid (6.4 keV) and width (0.1 keV). Allowing the centroid and width to vary yields more conservative upper limits of up to $\sim$50 eV. Furthermore, we have not observed any significant changes in the hardness ratio in eclipsing ULXs during ingress or egress, whereas in Galactic eclipsing X-ray binaries the hardness ratio increases during the transitions. Finally, the reprocessing efficiency (eclipse-to-OOE flux ratio) is found to be larger in ULXs compared to Galactic eclipsing X-ray binaries. Based on these results, we discuss the possibility of a metal-poor or highly ionized environment surrounding the ULXs, which suppresses reprocessed emission features.

astro-ph.HE↗

arXiv2026

An SRG/eROSITA Study of Be X-ray binaries in Quiescence

Be X-ray binaries (BeXRBs) are mostly transient X-ray binaries with usually a neutron star as the compact object and a Be star as their optical companion. We performed a comprehensive analysis of BeXRBs in quiescence with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA), the soft X-ray instrument on board the Spectrum-Roentgen-Gamma satellite, using its first all-sky survey (eRASS1) data. The transient BeXRBs spend majority of time in their quiescent state, but are often studied during their outbursts. A quiescence study is useful to probe the emission mechanism of BeXRBs at low mass accretion rates. The spectra of BeXRBs detected in the eRASS1 main catalogue could be well fitted with a powerlaw as well as a blackbody model. In the absence of detection of a BeXRB, we report the luminosity upper limit for its position in the sky. The X-ray luminosity for the quiescent BeXRBs detected with eROSITA have $L_{X}\sim10^{33-36}$ erg s$^{-1}$ for both absorbed powerlaw and blackbody models in most cases. For the absorbed blackbody model, the emission seems to come from a fraction of the neutron star's surface. The quiescence luminosity of BeXRBs are used to distinguish between different accretion models at low mass accretion rates, like accretion in the propeller regime or from a cold accretion disc. The detection space of BeXRBs in quiescence for the eRASS1 data is compared against previously reported luminosities across multiple X-ray observatories.

astro-ph.HE↗

arXiv2026

Dynamical constraints on the companion mass from pulsar timing and cyclotron line confirmation in the SFXT IGR J16418-4532

Supergiant Fast X-ray Transients (SFXTs) exhibit extreme X-ray variability on short timescales of minutes to hours, driven by accretion processes that are still poorly understood. IGR J16418â€“4532 is an intermediate SFXT in which X-ray pulsations have been detected and which has been proposed as a candidate cyclotron line source, making it a key system for probing neutron-star accretion physics and magnetic field properties in this class of sources. We aim to constrain the projected semi-major axis of the SFXT IGR J16418--4532, through timing analysis, and confirm the presence of a Cyclotron Resonant Scattering Feature (CRSF), thereby characterising the nature of the compact object in this system. We analysed a 148 ks long $\mathit{NuSTAR}$ observation and several archival $\mathit{RXTE}$ observations using epoch-folding techniques to search for pulsations while correcting the photon arrival times for a range of $a_x \sin(i)$ values. We performed phase-averaged spectroscopy using several different continuum models and assessed the significance of the cyclotron line feature. We detect coherent pulsations with $P_{\mathrm{spin}}$ = 1208.67$\pm$ 0.04 s and $P_{\mathrm{spin}}$ = 1209$.$4178 $\pm$ 0.0084 s in $\mathit{NuSTAR}$ and $\mathit{RXTE/PCA}$ light curves, respectively. The pulsations exhibit clear dependence on the assumed projected semi-major axis, and are detected with the highest significance for $a_x\sin i$ = 55.1 $\pm$ 3.5 lt-s $(\mathit{NuSTAR})$ and 51.3 $\pm$ 2.6 lt-s $(\mathit{RXTE/PCA})$. The light curves show several flares, and the pulse profiles are found to evolve over the course of the observation. Sliding window pulse profiles reveal the pronounced effect of the orbital delay on the pulse arrival times in uncorrected data, which is fully removed post-orbital correction. Using the inferred orbital parameters, we constrain the mass and radius of the companion star to be in the range of $\sim$12--18 $M_\odot$ and $\sim$13--16 $R_\odot$, respectively. We report the first successful constraint on the binary mass function of an SFXT obtained by considering the orbital motion of the neutron star. Spectral analysis reveals a robust cyclotron absorption line at $\sim$50 keV, independent of the choice of the continuum, suggesting a magnetic field strength of $\sim$5 $\times 10^{12}$ G. The cyclotron line confirmation and dynamical constraints on the companion's mass indicate that IGR J16418--4532 hosts a neutron star with a magnetic field of B$\approx$ 5 $\times 10^{12} G$ accreting from a $\sim$12--18 $M_\odot$ companion. We demonstrate that optimising the pulse detection by correcting the photon arrival times with trial $a_x \sin(i)$ values rather than modelling the pulse arrival time delays offers a route to obtain dynamical mass measurements in other SFXTs whose variable pulse profiles limit the scope of conventional timing techniques.

astro-ph.HE↗

arXiv2026

Statistics-controlled topological phases in a Su-Schrieffer-Heeger ladder

In strictly one-dimensional lattices, free fermions and hard-core bosons exhibit equivalent physical properties in many-particle systems. However, this statistical equivalence generally breaks down upon moving to higher dimensions. Our work investigates the significant effects of this difference on the topological phase transition in a minimal system lying away from one dimension: the two-leg Su-Schrieffer-Heeger (SSH) ladder. Starting in the limit of two decoupled SSH chains having opposite hopping dimerization patterns (staggered dimerization), we demonstrate that introducing inter-leg coupling drives a transition from a topological to a trivial phase for both spinless fermions and hard-core bosons. However, the topological to trivial phase transition boundary is strongly sensitive to the underlying particle statistics. We find that this intriguing effect favors a trivial phase sandwiched between the two topological phases with opposite characters for the case of hard-core bosons, resulting in a reentrant topological phase transition. In contrast, this reentrant phenomenon is absent for spinless fermions, with the topological phases being separated by a gapless line instead of an intervening trivial phase. This study demonstrates the interplay between particle statistics and topological features, revealing fundamentally different properties in an experimentally accessible set-up.

cond-mat.quant-gas↗

arXiv2026

Detection of hard X-ray pulsations in non-ULX X-ray pulsar 3X J0042 in M31

We report the detection of hard X-ray pulsations from the M31 accreting non-ULX X-ray pulsar 3X J0042 (3XMM J004232.1+411314) using archival NuSTAR observations. The source has a spin period of ~3 s and an orbital period of ~4.15 hr, with coherent pulsations previously detected only in XMM-Newton observations. We reanalyse all five publicly available NuSTAR observations by correcting the photon arrival times for binary motion and optimizing the orbital epoch and projected semi-major axis. Coherent pulsations are detected in all five observations. The broad-band pulse profiles show similar morphology across the five observations spanning ~2.3 yr, with no significant long-term evolution. The pulse profile also shows energy dependence, with structured, non-sinusoidal profiles below ~20 keV and a less structured profile at higher energies. The pulse periods show an overall decrease over the ~2.3 yr NuSTAR baseline, continuing the long-term spin-up trend observed in the ~16 yr of XMM-Newton observations, but with a faster average spin-up rate of ~-1.1e-10 s/s compared to the earlier reported ~-6e-11 s/s. The short orbital period and inferred low-mass companion star shows it to be a low-mass X-ray binary containing a magnetized neutron star, with systems such as Her X-1, 3A 1822-371, and XMM J174457-2850.3 providing closer Galactic comparisons. This work represents the first detection of hard X-ray pulsations from a non-ULX extragalactic accreting pulsar.

astro-ph.HE↗

arXiv2026

Coupling induced emergent topology in a two-leg fermionic ladder

We investigate the ground state properties of spinless fermions on a two leg ladder, by allowing the nearest-neighbour hopping dimerization in one leg and uniform hopping in the other. In the non-interacting limit, we find that, at half-filling, the system exhibits robust topological behavior if the inter-leg hopping is allowed. Though depending on the dimerization pattern, the dimerized leg can be either topological or trivial in nature, here we show that by connecting such a leg to a uniform leg through inter-chain coupling, the overall system becomes topological irrespective of the dimerization pattern in the dimerized leg. As a result, a topological phase transition occurs as a function of the inter-leg hopping. When the inter-leg interaction is turned on, the topological phase survives, and we obtain an interaction induced topological phase transition. Finally, we reveal that when uniform interactions are included on all the bonds of the ladder, the topological phase transitions to a symmetry-broken charge-density wave (CDW) phase.

cond-mat.quant-gas↗

arXiv2026

Emergence of a molecular quantum liquid in one dimension

We investigate the fate of a one-dimensional lattice superfluid formed by hard-core bosons, aka `atoms' (alternatively, a free spinless Fermi sea) subjected to nearest-neighbor attractive Hubbard-like interactions only in subgroups of two sites. The system, as expected, stabilizes a fluid of dimerized molecules at large attractive interactions. However, the composite molecules have an effective meek hopping scale and dominant repulsive interactions solely due to virtual quantum fluctuations. Interestingly, at an intermediate attractive potential, the system realizes a phase-separated region where the system is in an absorbing state. We show that this phase-separated region is due to an emergent attractive interaction between the dimers which leads to a local charge-density wave puddle where particles effectively cluster with local half-filling. Moreover the molecular superfluid gets spontaneously charge-ordered in the addition of an unpaired atom, reflecting the extreme sensitivity of the system to the existence of lone atoms. Using density-matrix renormalization group studies and effective low-energy Hamiltonians, we isolate the quantum processes to uncover the physics behind molecule formation in a strongly interacting one-dimensional system.

cond-mat.quant-gas↗

arXiv2026

Floquet-induced suppression of thermalization in a quasiperiodic Ising chain

Many-body localized (MBL) systems are known to thermalize in periodically driven systems. In this work, we demonstrate that under proper driving protocol, this thermalization this thermalization can be resisted such that the MBL phase turns into a non-ergodic extended phase, known as the many-body critical (MBC) phase. Considering a kicked quasiperiodic Ising chain, we show that while at high-frequency driving the ergodic, MBL, and the MBC phases coexist, at moderate driving frequencies the MBL phase is completely suppressed and the MBC phase proliferates in the parameter space. Using quasienergy statistics, Floquet eigenstates, autocorrelation dynamics, and entanglement growth, we characterize the emergent phases and identify non-monotonic signatures revealing richness of the nonergodic phases. Our results establish Floquet driving as a powerful route to stabilizing nonergodic extended many-body phases beyond the conventional Floquet-MBL paradigm.

cond-mat.dis-nn↗

arXiv2026

Cyclic cosmology from Cuscuton-Gallileons subjected to Lie point transformations

Spacetime transformations in any physically viable theory should follow Lie Point symmetry. In this work, we explore the Cuscuton model extended to Galileons, as introduced by de Rham et al in \cite{Rham2017}. We find the true degrees of freedom by converting the model into an equivalent first order model. Despite being a higher derivative model, it possesses only \textit{two} degrees of freedom. We calculate the Noether symmetry parameters corresponding to Lie point transformations, which lead to the vanishing of the original Cuscuton term's coefficient and restrict the potential to an exponential form. Interestingly, the coefficient corresponding to the original Cuscuton term vanish. Additionally, we also use the Killing analysis to find out the charges corresponding to the Killing vectors and the Killing tensors. The cosmological implications are examined through dynamical analysis, revealing that under the condition where the coefficient $a_2$ vanishes, the equation of state parameters exhibit damped oscillatory behavior .

gr-qc↗

arXiv2026

Quantum Incompatibility in Parallel vs Antiparallel Spins

We explore the joint measurability of incompatible qubit observables on ensembles of parallel and antiparallel spin-1/2 pairs. In parallel configuration, both spins are prepared in the same state, whereas in antiparallel case, each spin is paired with its flipped counterpart. We show that the antiparallel configuration uniquely enables exact simultaneous prediction of three mutually orthogonal spin components -- an advantage not achievable with parallel states. Extending beyond three observables, we examine joint measurability for larger sets of spin measurements and further generalize our analysis to state configurations beyond the parallel and antiparallel cases. As we show, our results reveal a deep connection to the 'mean King retrodiction task' proposed by Vaidman, Aharonov, and Albert, and have implications for a cryptographic protocol introduced by Jeffrey Bub. We further demonstrate how the enhanced compatibility in the antiparallel configuration can facilitate efficient estimation of unknown measurement devices. Finally, we discuss prospects for experimentally realizing the enhanced measurement compatibility in antiparallel configuration by analyzing the effect on finite sub-ensembles of states.

quant-ph↗

arXiv2025

Probing accretion dynamics and spin evolution in the X-ray pulsar RX J0520.5-6932 during its 2024 Outburst

After nearly a decade of quiescence, the transient Be/X-ray binary pulsar RX J0520.5-6932 underwent an outburst in 2024. We performed X-ray monitoring of the source with NICER and AstroSat near the peak of the event. Our primary objective is to investigate the energy and luminosity dependence of the pulsed emission, characterize the spin evolution, and study the broadband X-ray spectral properties of RX J0520.5-6932 during the outburst. The AstroSat/LAXPC and NICER light curves reveal pronounced short-duration flaring activity lasting ~400-700 s, with enhancements by a factor of ~2. The pulse profile exhibits a strong dependence on both energy and intensity, evolving from a simple single-peaked structure at low energies to complex multi-peaked shapes at intermediate energies, and reverting to simpler morphologies at higher energies. Pulse profiles during the flares differ significantly from those in the persistent state, indicating changes in the pulsed beam pattern with a change in the intensity on a short timescale. Broadband spectral analysis reveals a soft excess and an emission feature at ~1 keV, likely arising from reprocessed emission in the accretion disc and fluorescence from Ne K and Fe L ions. Continuous NICER monitoring over nearly one orbital cycle enabled us to track spin evolution with accretion-driven spin-up and spectral variability in the soft X-ray band. Additionally, a declining spin-up rate is observed during the outburst, likely due to a gradual reduction in mass accretion rate. Our results provide a comprehensive view of the complex accretion dynamics in RX J0520.5-6932 during its 2024 outburst. The strong variability in pulse shape and spin behaviour highlights rapid changes in the accretion geometry and torque as a function of accretion rate. [Abridged]

astro-ph.HE↗

arXiv2025

Unveiling the Nature of Superorbital Modulation of SMC X-1 using NinjaSat

We report a long-term, high-cadence timing and spectral observation of the X-ray pulsar SMC X-1 using NinjaSat, a 6U CubeSat in low-Earth orbit, covering nearly a full superorbital cycle. SMC X-1 is a high-mass X-ray binary exhibiting a 0.7 s X-ray pulsar and a non-stationary superorbital modulation with periods ranging from approximately 40 to 65 days. Its peak luminosity of $1.3\times10^{39}$~\lumcgs\ makes it a local analogue of ultraluminous X-ray pulsars powered by supercritical accretion. We find that the spin-up rate during the high state remains consistent with the long-term average, with no significant correlation between spin-up rate and flux. This result indicates that the modulation is primarily geometric rather than accretion-driven. The hardness ratio and spectral shape are stable throughout the entire superorbital cycle, supporting obscuration by optically thick material or energy-independent scattering. In addition, the 2--20 keV pulse profile varies with superorbital phase, which may be explained either by variable covering fraction due to geometric obscuration, or by free precession of the neutron star. This represents the first complete measurement of spin-up rate and spectral evolution across a single superorbital cycle in SMC X-1, highlighting the scientific capability of CubeSat-based observatories.

astro-ph.HE↗

arXiv2025

Non-Hermitian comb effect in coupled clean and quasiperiodic chains

We study localization properties in a system of non-Hermitian quasiperiodic chain coupled to a uniform chain or clean chain by inter-chain hopping. We find that in the limit of weak inter-chain coupling, such a coupled system exhibits transitions from delocalized to intermediate phase with increase in the non-Hermiticity parameter. However, for stronger inter-chain coupling strengths, the delocalized phase undergoes a transition to localized phase and then to an intermediate phase. Interestingly, the intermediate phase in this case exhibits the non-Hermitian comb effect (NHCE), i.e., the coexistence of localized and extended states rather than being well separated from each other by any mobility edge which is conventional in any intermediate phase. We further show that such a NHCE originates from the isolated site limit of the quasiperiodic chain and provide an analytical explanation supporting the numerical signatures.

cond-mat.quant-gas↗

arXiv2025

XMM-Newton view of pulsating iron fluorescent emission from Centaurus X3

Cen X-3 is a bright, high-mass X-ray binary pulsar. We present a pulse phase-resolved X-ray spectral analysis of data from an archival XMM-Newton observation of Cen X-3 taken during the high state of the source. The observation was entirely in the out-of-eclipse part of the binary orbit. We study the pulse phase variability of the three K$α$ fluorescent emission lines from near-neutral, Helium-like and Hydrogen-like iron, along with the iron K$β$ emission line. All four lines show clear modulation with the pulse phase of the neutron star, and modulation is found to be higher for the lines from highly ionised iron compared to the neutral lines. Structures within the light travel distance corresponding to the pulse period of the neutron star are likely responsible for the pulse phase modulation of the emission lines. We have also investigated the orbital phase dependence of the pulse phase variability in the iron lines by dividing the data into four segments at different orbital phases of Cen X-3. The pulse phase modulation behaviour of the four lines is quite identical at different orbital phases of Cen X-3, indicating the pulsed iron emission region is persistent in nature and probably phase aligned with respect to the observer. The accretion stream intercepting the line of sight can probably produce the observed phase dependence of the iron fluorescence emission lines.

astro-ph.HE↗

arXiv2025

Thermonuclear X-ray bursts across the eclipse transitions in the LMXBs EXO 0748-676 and XTE J1710-281

The primary radiation from thermonuclear X-ray bursts observed in the neutron star low-mass X-ray binary (LMXB) systems can interact with various parts of the binary system. This interaction gives rise to secondary radiation in different wavelength ranges, known as reprocessed emission. In eclipsing LMXBs, the reprocessed emission from the bursts can be examined during eclipses, as the primary emission is blocked and only the reprocessed emission is visible. We searched for bursts during eclipses in the archival RXTE data of the eclipsing LMXBs and found them in EXO 0748-676 and XTE J1710-281. In EXO 0748-676, seven bursts were found to occur near eclipse egress, with their tails extending beyond the eclipse, and one such burst was found for XTE J1710-281. We estimate the reprocessing fraction at orbital phases near eclipse egress by modeling the peculiar eclipse bursts detected in both systems, which have tails extending beyond the eclipses. We observe an increasing trend in reprocessing fraction as these eclipse bursts occur closer to the eclipse egress. We discuss the possibilities of reprocessing in the ablated wind from the companion star, the accretion disc, and the disc wind in EXO 0748-676 and XTE J1710-281. Additionally, we observe two decay components in the bursts in EXO 0748-676, which could suggest a complex composition of the accreting fuel. From the burst rise timescales, we place an upper limit on the size of the reprocessing regions in both EXO 0748-676 and XTE J1710-281, finding it comparable to the size of the respective X-ray binaries.

astro-ph.HE↗

arXiv2025

Measurement Incompatibility Based In-equivalence Between Bell and Network Nonlocality

It is a well-known fact that measurement incompatibility is a necessary resource to generate nonlocal correlations in usual Bell scenario that typically involves single quantum source. We can provide with some contrasting findings if we consider connected structure of multiple quantum sources. Precisely, we demonstrate that non n-locality can be detected in standard quantum network even when only a single party performs incompatible measurements. More interestingly, for any finite n greater than 2, non n-local correlations can be generated in any standard linear n-local network when all the parties perform compatible measurements. Such an observation is topology specific as one of the parties must perform incompatible measurement to exhibit non n-locality in any non-linear network endowed with star topology. However, we observe that in any non-standard network(all sources independent and nonlocal), to generate genuine non n-local correlations, all the parties must perform incompatible measurements. Such a finding is intuitive as more resource is required to generate stronger form of quantum non-classicality. We also demonstrate that merely providing resource of measurement incompatibility to all the parties is not sufficient for non n-locality detection in any quantum network

quant-ph↗

arXiv2025

Spectral analysis of ultraluminous X-ray pulsars with models of X-ray pulsars

A fraction of the Ultra Luminous X-ray (ULX) sources are known to be accreting neutron stars as they show coherent X-ray pulsations with pulse periods ranging from ~1-30 seconds. While initially thought to host intermediate-mass black holes, ULXs have since been recognized as a diverse class of objects, including ULX pulsars. These pulsars require models specifically tailored to account for their unique accretion physics, distinct from those used for Galactic black hole binaries. The X-ray spectra of all Galactic accreting X-ray pulsars (including sources in the Magellanic Clouds) are dominated by a high energy cut-off power-law and some of the sources show a soft excess, some emission lines, cyclotron absorption features, etc. In this work, we undertake a comprehensive analysis of the broadband X-ray spectra of five ULX pulsars using simultaneous XMM-Newton and NuSTAR observations and show that their X-ray spectra can be effectively described by spectral models, similar to those used for the local accretion-powered X-ray pulsars. A soft excess is detected in all the sources which is also consistent with the local X-ray pulsars that have low absorption column density. We have marginal detection or low upper limit on the presence of the iron K-alpha emission line from these sources, which is a key difference of the ULX pulsars with the local accreting X-ray pulsars. We discuss the implication of this on the nature of the binary companion and the accretion mechanism in the ULX pulsars.

astro-ph.HE↗

arXiv2025

Revealing Hidden Non n-Locality In n-Local Star Network

Keeping pace with technological advancement, in the past decade, use of scalable networks have extended the study of quantum non-classicality beyond the regime of Bell-CHSH nonlocality. Present work provide characterization of non n-locality that can be exploited by incorporating filtering operations in star-shaped n-local networks. This in turn provide a framework of sequential n-local networks capable of generating non n-local correlations by involving some suitable form of stochastic local operations assisted with classical communications(SLOCC). It is observed that for effectiveness of such sequential networks, Bell-CHSH nonlocality(upto SLOCC operations) of every individual two-qubit state, distributed in the network, is not mandatory. However, there does not exist any separable local filter, which when applied in n-local network involving only Bell local states(upto SLOCC operations), can reveal non n-locality. Interestingly, instances revealing advantage of nonseparable mutli-qubit local filters over separable mutli-qubit local filters(by central node) are obtained. Such an advantage is attributable to the specific topology of star-shaped n-local networks and thus can never be reflected in Bell scenario.

quant-ph↗