arXiv subjects

Saptarshi Sarkar

Publications and source records attributed to Saptarshi Sarkar.

6 recordsLinked to original sources

arXiv2026

Detection of giant pulses from the transitional millisecond pulsar J1227$-$4853

We report the discovery of giant pulse (GP) emission from the transitional millisecond pulsar (tMSP) PSR J1227$-$4853, using 174 hours of single-pulse data from the upgraded Giant Metrewave Radio Telescope (uGMRT). This marks the first detection of GPs from a transitional MSP and adds to the small number of millisecond pulsars known to exhibit such extreme variability. A total of 235 GPs were detected across observations at 550-750 MHz, with widths as narrow as 1.28 $μ$s and flux densities up to $\sim 10^4$ times the pulsar's mean flux density. The GPs are strongly localized in pulse phase, originating predominantly from the second and third main-pulse components, and are absent in the inter-pulse region. Their cumulative fluence distribution follows a power law above the completeness threshold, consistent with a defining characteristic of GP emission. The arrival times of the GPs deviate significantly from Poisson statistics, with the waiting-time distribution well described by a Weibull model having a shape parameter of $k = 0.30$, indicative of strong temporal clustering. During an epoch of enhanced activity, the GP rate increased by nearly two orders of magnitude to 124 $\mathrm{hr}^{-1}$, with a corresponding shape parameter of $k = 0.47$. This value is similar to that reported for a burst storm from the repeating fast radio burst FRB 20200120E, suggesting possible phenomenological parallels between GPs from compact binary systems and repeating FRBs.

astro-ph.HE↗

arXiv2026

Reconciling large-scale Lyman-$α$ correlations with the SCRIPT Semi-numerical Model

Recent analyses of high-redshift Lyman-$α$ forest observations have revealed strong correlations on scales exceeding 200 cMpc at redshift z = 6. Reproducing these large-scale correlations has proven challenging for current large-volume reionization simulations. In this work, we investigate these large-scale correlations using mock spectra generated from the extended SCRIPT semi-numerical reionization model. We find that while the fiducial model ensemble systematically predicts smaller correlation lengths than those inferred from the 67 sightlines in the extended XQR-30 sample, a small fraction of individual mock realizations can naturally reproduce the observed signal. Using a delete-2 jackknife analysis, we demonstrate that the observed large-scale correlation length is disproportionately driven by a rare pair of highly transmissive sightlines associated with high-redshift transmission spikes. By inserting two such highly transmissive sightlines into our mock realizations, the fraction of models consistent with the observed redshift evolution and correlation length increases significantly from 17.5% to 74.1%. Furthermore, we show that spatial fluctuations in the ionizing mean free path remain an essential physical ingredient for reproducing the observed correlation structure. Our results suggest that the unexpectedly large Lyman-$α$ correlations can be reconciled with existing reionization models when accounting for cosmic variance and the outsized statistical impact of rare, highly transmissive sightlines.

astro-ph.CO↗

arXiv2026

Accelerating reionization constraints: An ANN-emulator framework for the SCRIPT Semi-numerical Model

Constraining the Epoch of Reionization (EoR) with physically motivated simulations is hampered by the high cost of conventional parameter inference. We present an efficient emulator-based framework that dramatically reduces this bottleneck for the photon-conserving semi-numerical code SCRIPT. Our approach combines (i) a reliable coarse-resolution MCMC to locate the high-likelihood region (exploiting the large-scale convergence of SCRIPT) with (ii) an adaptive, targeted sampling strategy to build a compact high-resolution training set for an artificial neural network based emulator of the model likelihood. With only $\approx 10^3$ high-resolution simulations, the trained emulators achieve excellent predictive accuracy ($R^2 \approx 0.97-0.99$) and, when embedded within an MCMC framework, reproduce posterior distributions from full high-resolution runs. Compared to conventional MCMC, our pipeline reduces the number of expensive simulations by a factor of $\sim 100$ and lowers total CPU cost by up to a factor of $\sim 70$, while retaining statistical fidelity. This computational speedup makes inference in much higher-dimensional models tractable (e.g., those needed to incorporate JWST and upcoming 21 cm datasets) and provides a general strategy for building efficient emulators for next generation of EoR constraints.

astro-ph.IM↗

arXiv2022

Asynchronous wreath product and cascade decompositions for concurrent behaviours

We develop new algebraic tools to reason about concurrent behaviours modelled as languages of Mazurkiewicz traces and asynchronous automata. These tools reflect the distributed nature of traces and the underlying causality and concurrency between events, and can be said to support true concurrency. They generalize the tools that have been so efficient in understanding, classifying and reasoning about word languages. In particular, we introduce an asynchronous version of the wreath product operation and we describe the trace languages recognized by such products (the so-called asynchronous wreath product principle). We then propose a decomposition result for recognizable trace languages, analogous to the Krohn-Rhodes theorem, and we prove this decomposition result in the special case of acyclic architectures. Finally, we introduce and analyze two distributed automata-theoretic operations. One, the local cascade product, is a direct implementation of the asynchronous wreath product operation. The other, global cascade sequences, although conceptually and operationally similar to the local cascade product, translates to a more complex asynchronous implementation which uses the gossip automaton of Mukund and Sohoni. This leads to interesting applications to the characterization of trace languages definable in first-order logic: they are accepted by a restricted local cascade product of the gossip automaton and 2-state asynchronous reset automata, and also by a global cascade sequence of 2-state asynchronous reset automata. Over distributed alphabets for which the asynchronous Krohn-Rhodes theorem holds, a local cascade product of such automata is sufficient and this, in turn, leads to the identification of a simple temporal logic which is expressively complete for such alphabets.

cs.FL↗

arXiv2021

First-Order logic and its Infinitary Quantifier Extensions over Countable Words

We contribute to the refined understanding of the language-logic-algebra interplay in the context of first-order properties of countable words. We establish decidable algebraic characterizations of one variable fragment of FO as well as boolean closure of existential fragment of FO via a strengthening of Simon's theorem about piecewise testable languages. We propose a new extension of FO which admits infinitary quantifiers to reason about the inherent infinitary properties of countable words. We provide a very natural and hierarchical block-product based characterization of the new extension. We also explicate its role in view of other natural and classical logical systems such as WMSO and FO[cut] - an extension of FO where quantification over Dedekind-cuts is allowed. We also rule out the possibility of a finite basis for a block-product based characterization of these logical systems. Finally, we report simple but novel algebraic characterizations of one variable fragments of the hierarchies of the new proposed extension of FO.

cs.LO↗

arXiv2020

Wreath/cascade products and related decomposition results for the concurrent setting of Mazurkiewicz traces (extended version)

We develop a new algebraic framework to reason about languages of Mazurkiewicz traces. This framework supports true concurrency and provides a non-trivial generalization of the wreath product operation to the trace setting. A novel local wreath product principle has been established. The new framework is crucially used to propose a decomposition result for recognizable trace languages, which is an analogue of the Krohn-Rhodes theorem. We prove this decomposition result in the special case of acyclic architectures and apply it to extend Kamp's theorem to this setting. We also introduce and analyze distributed automata-theoretic operations called local and global cascade products. Finally, we show that aperiodic trace languages can be characterized using global cascade products of localized and distributed two-state reset automata.

cs.FL↗