arXiv subjects

Aritra Kundu

Publications and source records attributed to Aritra Kundu.

At least 19 recordsLinked to original sources

arXiv2026

Möbius functions for pseudo-Levi subgroups in finite general linear and symplectic groups

In this paper, we compute the Möbius function on the set of pseudo-Levi subgroups containing a fixed maximal torus of two families of finite groups, $GL_n(q)$ and $Sp_{2n}(q)$, for some natural number $n$ and a prime power $q$. The Möbius function for the set of pseudo-Levi subgroups of a finite group $G$ is important for an explicit evaluation of the formula expressing the decomposition of tensor products of characters, as proved in Theorem 2 of the paper ''Multiplicity of characters of finite reductive groups and Drinfeld doubles'' [arXiv:2512.01432].

math.RT↗

arXiv2026

The Dynamics of Old Inner Galaxy Stars in Milky Way-mass Galaxies Using FIRE-2 Simulations

Understanding how galaxies like the Milky Way assembled over cosmic time remains a central question in astrophysics. Understanding the processes that shaped their formation and evolution is greatly enhanced by the joint use of observational data and high-resolution cosmological simulations. Old stars in the inner regions of galaxies serve as powerful tracers of early dynamical events, having formed during the initial stages of galaxy assembly and retaining the kinematic imprints of those formative periods. We investigate the kinematic properties of old (age $>10$ Gyr) inner galaxy ($r_\mathrm{GC}<5$ kpc) stars in thirteen Milky Way-mass galaxies from the FIRE-2 cosmological zoom-in simulations, focusing on their origin, orbital structure, and kinematic alignment with the disk. Our analysis reveals that old stars in the inner galaxy are more likely to have been formed in their host galaxy, although accretion is seen most prominently during the earliest stages of galaxy formation. Many of these accreted stars tend to occupy kinematically hot orbits compared to their counterparts formed in the host galaxy, although some stars formed in-galaxy also retain kinematically hot orbits. Disk-like dynamics are present throughout all age bins, and are most prominent as age decreases. Although some old stellar populations retain disk-like structure, the prominence of this rotational component varies significantly across galaxies and between star populations. These results emphasize the diversity of early galaxy assembly histories and suggest that coherent angular momentum in accreted material can leave detectable kinematic signatures in present-day stellar halos.

astro-ph.GA↗

arXiv2026

Probing Atomic Dark Matter with Stellar Streams in Milky Way-Mass Galaxies

We present the first detailed analysis of the effects of dissipative dark matter on stellar streams. As a concrete example, we generate a cosmological hydrodynamic zoom-in simulation of a Milky Way-mass galaxy, assuming that the dark matter consists of Cold Dark Matter (CDM) with a sub-component ($\sim6\%$) of Atomic Dark Matter (ADM). The ADM subcomponent behaves as collisional, efficiently dissipative gas and allows for the formation of dense compact objects that enhance the central density of satellite galaxies, making them more resistant to tidal disruption. We show that stellar streams with stellar mass $M_{\rm{tot}, \star} \gtrsim 10^{5.5} \ \text{M}_\odot$ form later and exhibit prolonged star formation throughout their evolution, as compared to their CDM counterparts. Changes to star formation history are reflected on the chemical tracks of the stellar stream stars, where the youngest have enhanced [Fe/H] and [Mg/Fe] in the presence of ADM. Furthermore, a population of low-mass satellites with high ADM mass fractions is identified at low pericenter distances, which may affect the population of streams at $M_{\rm{tot}, \star} \lesssim 10^{5.5} \ \text{M}_\odot$. The results of this study should generalize to other dark matter models that lead to inner-density enhancements in satellites, such as elastic self-interacting dark matter in the gravothermal collapse regime.

astro-ph.GA↗

arXiv2026

Rise of the forsaken relics: connecting present-day stellar streams and phase-mixed galaxies to the Epoch of Reionization

The `near-far' approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint-end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ($z \gtrsim 6$), beyond the current JWST detection limits ($M_{\mathrm{UV}} \gtrsim -15$). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to $\sim50\%$ of the total stellar mass budget of the proto-MW/LG at $z =6-9$. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of $\sim2-3$ and reduces the halo-to-halo variation in the slope by $\sim20-40\%$. This enables robust constraints down to at least the resolution limit of the simulations, near $M_\star$ $\sim$ $10^{5}$ $M_\odot$ or $M_{\mathrm{UV}} \sim -10$ at $z \gtrsim 6$. We also show that `fossil record' reconstructions - which assume each present-day system descends from a single reionization-era progenitor - are sensitive to the stellar mass/UV magnitude thresholds, which introduces bias in the inferred low-mass/faint-end slopes. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with $M_{\mathrm{UV}} \lesssim -15$ to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction ($\sim50\%$) of streams with $M_\star$ $\gtrsim$ $10^{6}$ $M_\odot$ at $z=0$ should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.

astro-ph.GA↗

arXiv2026

Run-and-Tumble Dynamics and Zeno--Anti-Zeno Transition in Biased Quantum Trajectories

We identify the transition from the oscillatory Rabi regime to the localized Zeno/Anti-Zeno regime in continuous measurement and feedback of a qubit as a quantum analogue of Motility-Induced Phase Separation (MIPS). A mapping between a biased monitored qubit and a classical ``Run-and-Tumble" active particle is studied. We demonstrate that the competition between coherent Rabi driving (analogous to active motility) and measurement-induced feedback bias (analogous to persistence) mimics the behavior of biological swimmers. This framework provides a picture of measurement-induced phase transitions using the language of active matter and offering a novel pathway for designing dissipative noisy quantum systems.

cond-mat.stat-mech↗

arXiv2025

Intramuscular microelectrode arrays enable highly-accurate neural decoding of hand movements

Decoding the activity of the nervous system is a critical challenge in neuroscience and neural interfacing. In this study, we present a neuromuscular recording system that enables large-scale sampling of muscle activity using microelectrode arrays with over 100 channels embedded in forearm muscles. These arrays captured intramuscular high-density signals that were decoded into patterns of activation of spinal motoneurons. In two healthy participants, we recorded high-density intramuscular activity during single- and multi-digit contractions, revealing distinct motoneuron recruitment patterns specific to each task. Based on these patterns, we achieved perfect classification accuracy (100%) for 12 single- and multi-digit tasks and over 96% accuracy for up to 16 tasks, significantly outperforming state-of-the-art EMG classification methods. This intramuscular high-density system and classification method represent an advancement in neural interfacing, with the potential to improve human-computer interaction and the control of assistive technologies, particularly for replacing or restoring impaired motor function.

q-bio.NC↗

arXiv2025

Quantum Dynamics with Stochastic Non-Hermitian Hamiltonians

We study the quantum dynamics generated by a non-Hermitian Hamiltonian subject to stochastic perturbations in its anti-Hermitian part, describing fluctuating gains and losses. The dynamics averaged over the noise is described by an `anti-dephasing' master equation. We characterize the resulting state evolution and analyze its purity. The properties of such dynamics are illustrated in a stochastic dissipative qubit. Our analytical results show that adding noise allows for a rich control of the dynamics, stabilizing the lossy state and making state purification possible to a greater variety of steady states.

quant-ph↗

arXiv2025

Spikes in Poissonian quantum trajectories

We consider the dynamics of a continuously monitored qubit in the limit of strong measurement rate where the quantum trajectory is described by a stochastic master equation with Poisson noise. Such limits are expected to give rise to quantum jumps between the pointer states associated with the non-demolition measurement. A surprising discovery in earlier work [Tilloy et al., Phys. Rev. A 92, 052111 (2015)] on quantum trajectories with Brownian noise was the phenomena of spikes observed in between the quantum jumps. Here, we show that spikes are observed also for Poisson noise. We consider three cases where the non-demolition is broken by adding, to the basic strong measurement dynamics, either unitary evolution or thermal noise or additional measurements. We present a complete analysis of the spike and jump statistics for all three cases using the fact that the dynamics effectively corresponds to that of stochastic resetting. We provide numerical results to support our analytic results.

quant-ph↗

arXiv2025

Cosmological predictions for minor axis stellar density profiles in the inner regions of Milky Way-mass galaxies

$Λ$CDM cosmology predicts the hierarchical formation of galaxies which build up mass by merger events and accreting smaller systems. The stellar halo of the Milky Way has proven to be useful a tool for tracing this accretion history. However, most of this work has focused on the outer halo where dynamical times are large and the dynamical properties of accreted systems are preserved. In this work, we investigate the inner galaxy regime, where dynamical times are relatively small and systems are generally completely phase-mixed. Using the FIRE-2 and Auriga cosmological zoom-in simulation suites of Milky Way-mass galaxies, we find the stellar density profiles along the minor axis (perpendicular to the galactic disk) within the NFW scale radii (R$\approx$15 kpc) are best described as an exponential disk with scale height <0.3 kpc and a power law component with slope $α\approx$-4. The stellar density amplitude and slope for the power law component is not significantly correlated with metrics of the galaxy's accretion history. Instead, we find the stellar profiles strongly correlate with the dark matter profile. Across simulation suites, the galaxies studied in this work have a stellar to dark matter mass ratio that decreases as $1/r^2$ along the minor axis.

astro-ph.GA↗

arXiv2024

Statistical prethermalization in randomly kicked many-body classical rotor system

We explore the phenomena of prethermalization in a many-body classical system of rotors under aperiodic drives characterised by waiting time distribution (WTD), where the waiting time is defined as the time between two consecutive kicks. We consider here two types of aperiodic drives: random and quasi-periodic. We observe a short-lived pseudo-thermal regime with algebraic suppression of heating for the random drive where WTD has an infinite tail, as observed for Poisson and binomial kick sequences. On the other hand, quasi-periodic drive characterised by a WTD with a sharp cut-off, observed for Thue-Morse sequence of kick, leads to prethermal region where heating is exponentially suppressed. The kinetic energy growth is analyzed using an average surprise associated with WTD quantifying the randomness of drive. In all of the aperiodic drives we obtain the chaotic heating regime for late time, however, the diffusion constant gets renormalized by the average surprise of WTD in comparison to the periodic case.

cond-mat.stat-mech↗

arXiv2024

Design, Fabrication and Evaluation of a Stretchable High-Density Electromyography Array

The adoption of high-density electrode systems for human-machine interfaces in real-life applications has been impeded by practical and technical challenges, including noise interference, motion artifacts and the lack of compact electrode interfaces. To overcome some of these challenges, we introduce a wearable and stretchable electromyography (EMG) array, and present its design, fabrication methodology, characterisation, and comprehensive evaluation. Our proposed solution comprises dry-electrodes on flexible printed circuit board (PCB) substrates, eliminating the need for time-consuming skin preparation. The proposed fabrication method allows the manufacturing of stretchable sleeves, with consistent and standardised coverage across subjects. We thoroughly tested our developed prototype, evaluating its potential for application in both research and real-world environments. The results of our study showed that the developed stretchable array matches or outperforms traditional EMG grids and holds promise in furthering the real-world translation of high-density EMG for human-machine interfaces.

cs.RO↗

arXiv2023

Stochastic Operator Variance: an observable to diagnose noise and scrambling

Noise is ubiquitous in nature, so it is essential to characterize its effects. Considering a fluctuating Hamiltonian, we introduce an observable, the stochastic operator variance (SOV), which measures the spread of different stochastic trajectories in the space of operators. The SOV obeys an uncertainty relation and allows finding the initial state that minimizes the spread of these trajectories. We show that the dynamics of the SOV is intimately linked to that of out-of-time-order correlators (OTOCs), which define the quantum Lyapunov exponent $λ$. Our findings are illustrated analytically and numerically in a stochastic Lipkin-Meshkov-Glick (sLMG) Hamiltonian undergoing energy dephasing.

quant-ph↗

arXiv2022

Integrable hydrodynamics of Toda chain: case of small systems

Passing from a microscopic discrete lattice system with many degrees of freedom to a mesoscopic continuum system described by a few coarse-grained equations is challenging. The common folklore is to take the thermodynamic limit so that the physics of the discrete lattice describes the continuum results. The analytical procedure to do so relies on defining a small length scale (typically the lattice spacing) to coarse-grain the microscopic evolution equations. Moving from the microscopic scale to the mesoscopic scale then requires careful approximations. In this work, we numerically test the coarsening in a Toda chain, which is an interacting integrable system, i.e. a system with a macroscopic number of conserved charges. Specifically, we study the spreading of fluctuations by computing the spatiotemporal thermal correlations with three different methods: (a) Using microscopic molecular dynamics simulation with a large number of particles; (b) solving the generalized hydrodynamics equation; (c) solving the linear Euler scale equations for each conserved quantities. Surprisingly, the results for the small systems (c) match the thermodynamic results in (a) and (b) for macroscopic systems. This reiterates the importance and validity of integrable hydrodynamics in describing experiments in the laboratory, where we typically have microscopic systems.

cond-mat.stat-mech↗

arXiv2021

RMS-Flux Relation and Disc-Jet Connection in Blazars in the Context of the Internal Shocks Model

Recent analysis of blazar variability has revealed a proportionality between the mean flux and the root mean squared (rms) fluctuations about the mean flux. Although such rms-flux relation has been previously observed in the accretion disc/corona variability of X-ray binaries and Seyfert galaxies, and has been extensively modelled, its emergence in the jet light curves of blazars calls for a revised theoretical understanding of this feature. In this work, we analyse the time variability properties of realistic multi-wavelength jet light curves, simulated in the context of a simplified version of the internal shocks model, particularly focusing on the rms-flux relation. These shocks accelerate the jet electrons to relativistic energies, which then cool radiatively via synchrotron and inverse-Compton processes. We find that the rms-flux relation may be consistently recovered in the cases, in which the shocks have different amplitudes based on the speed of the colliding blobs generating them as opposed to all shocks having the same amplitude. We observe that the slope of the rms-flux relation depends on the wavelength at which the variability is observed and the energy distribution of the electron population. We find that the accretion disc and the jet variability are anti-correlated, with the latter lagging that of the disc. Our results provide crucial constraints on the physical properties of the jet, and the mode of connection through which the accretion disc and jet may be related.

astro-ph.HE↗

arXiv2021

Dynamics of fluctuation correlation in periodically driven classical system

Having established the fact that interacting classical kicked rotor systems exhibit long-lived prethermal phase with quasi-conserved average Hamiltonian before entering into chaotic heating regime, we use spatio-temporal fluctuation correlation of kinetic energy to probe the above dynamic phases. We remarkably find the diffusive transport of fluctuation in the prethermal regime reminding us the underlying hydrodynamic picture in a generalized Gibbs ensemble with a definite temperature that depends on the driving parameter and initial conditions. On the other hand, the heating regime, characterized by a diffusive growth of kinetic energy, can sharply localize the correlation at the fluctuation center for all time. Consequently, we attribute non-diffusive and non-localize structure of correlation to the crossover regime, connecting the prethermal phase to the heating phase, where the kinetic energy displays complicated growth structure. We understand these numerical findings using the notion of relative phase matching where prethermal phase (heating regime) refers to an effectively coupled (isolated) nature of the rotors. We exploit the statistical uncorrelated nature of the angles of the rotors in the heating regime to find the analytical form of the correlator that mimics our numerical results in a convincing way.

cond-mat.stat-mech↗

arXiv2021

Revisiting the Mazur bound and the Suzuki equality

Among the few known rigorous results for time-dependent equilibrium correlations, important for understanding transport properties, are the Mazur bound and the Suzuki equality. The Mazur inequality gives a lower bound, on the long-time average of the time-dependent auto-correlation function of observables, in terms of equilibrium correlation functions involving conserved quantities. On the other hand, Suzuki proposes an exact equality for quantum systems. In this paper, we discuss the relation between the two results and in particular, look for the analogue of the Suzuki result for classical systems. This requires us to examine as to what constitutes a complete set of conserved quantities required to saturate the Mazur bound. We present analytic arguments as well as illustrative numerical results from a number of different systems. Our examples include systems with few degrees of freedom as well as many-particle integrable models, both free and interacting.

cond-mat.stat-mech↗

arXiv2020

Cosmic recombination history in light of EDGES measurements of the cosmic dawn 21-cm signal

The recent EDGES measurements of the global 21-cm signal from the cosmic dawn suggest that the kinetic temperature of the inter-galactic medium (IGM) might be significantly lower compared to its expected value. The colder IGM directly affects the hydrogen recombination of the universe during the cosmic dawn and dark ages by enhancing the rate of recombinations. Here, we study and quantify, the impact of the colder IGM scenario on the recombination history of the universe in the context of DM-baryonic interaction model which is widely used to explain the depth of the EDGES 21-cm signal. We find that, in general, the hydrogen ionisation fraction gets suppressed during the dark ages and cosmic dawn and the suppression gradually increases at lower redshifts until X-ray heating turns on. However, accurate estimation of the ionisation fraction requires knowledge of the entire thermal history of the IGM, from the epoch of thermal decoupling of hydrogen gas and the CMBR to the cosmic dawn. It is possible that two separate scenarios which predict very similar HI differential temperature during the cosmic dawn and are consistent with the EDGES 21-cm signal might have very different IGM temperature during the dark ages. The evolutions of the ionisation fraction in these two scenarios are quite different. This prohibits us to accurately calculate the ionisation fraction during the cosmic dawn using the EDGES 21-cm signal alone. We find that the changes in the ionisation fraction w.r.t the standard scenario at redshift $z \sim 17 $ could be anything between $\sim 0 \%$ to $\sim 36 \%$. This uncertainty may be reduced if measurements of HI 21-cm differential temperature at multiple redshifts are simultaneously used.

astro-ph.CO↗

arXiv2019

Anomalous heat transport in one dimensional systems: a description using non-local fractional-type diffusion equation

It has been observed in many numerical simulations, experiments and from various theoretical treatments that heat transport in one-dimensional systems of interacting particles cannot be described by the phenomenological Fourier's law. The picture that has emerged from studies over the last few years is that Fourier's law gets replaced by a spatially non-local linear equation wherein the current at a point gets contributions from the temperature gradients in other parts of the system. Correspondingly the usual heat diffusion equation gets replaced by a non-local fractional-type diffusion equation. In this review, we describe the various theoretical approaches which lead to this framework and also discuss recent progress on this problem.

cond-mat.stat-mech↗