arXiv subjects

R. Samanta

Publications and source records attributed to R. Samanta.

5 recordsLinked to original sources

arXiv2025

Fastest or Significant: A Systematic Framework for Validating Global Minimum Variability Timescale Measurements of Gamma-ray Bursts

The minimum variability timescale (MVT) is a key observable used to probe the central engines of Gamma-Ray Bursts (GRBs) by constraining the emission region size and the outflow Lorentz factor. However, its interpretation is often ambiguous: statistical noise and analysis choices can bias measurements, making it difficult to distinguish genuine source variability from artifacts. Here we perform a comprehensive suite of simulations to establish a quantitative framework for validating Haar-based MVT measurements. We show that in multi--component light curves, the MVT returns the most statistically significant structure in the interval, which is not necessarily the fastest intrinsic timescale, and can therefore converge to intermediate values. Reliability is found to depend jointly on the MVT value and its signal-to-noise ratio ($\mathrm{SNR}_{\mathrm{MVT}}$), with shorter intrinsic timescales requiring proportionally higher $\mathrm{SNR}_{\mathrm{MVT}}$ to be resolved. We use this relation to define an empirical MVT Validation Curve, and provide a practical workflow to classify measurements as robust detections or upper limits. Applying this procedure to a sample of Fermi-GBM bursts shows that several published MVT values are better interpreted as upper limits. These results provide a path toward standardizing MVT analyses and highlight the caution required when inferring physical constraints from a single MVT measurement in complex events.

astro-ph.HE↗

arXiv2023

NANOGrav spectral index $\gamma=3$ from melting domain walls

We discuss cosmic domain walls described by a tension red-shifting with the expansion of the Universe. These melting domain walls emit gravitational waves with the low-frequency spectral shape $\Omega_{gw}\propto f^{2}$ corresponding to the spectral index $\gamma=3$ favoured by the recent NANOGrav 15 yrs data. We discuss a concrete high-energy physics scenario leading to such a melting domain wall network in the early Universe. This scenario involves a feebly coupled scalar field, which can serve as a promising dark matter candidate. We identify parameters of the model matching the gravitational wave characteristics observed in the NANOGrav data. The dark matter mass is pushed to the ultra-light range below $10^{-11}-10^{-12}\,\text{eV}$ which is accessible through planned observations thanks to the effects of superradiance of rotating black holes.

hep-ph↗

arXiv2021

Anomalous Hydrodynamics in One Dimensional Electronic Fluid

We construct multi-mode viscous hydrodynamics for one dimensional spinless electrons. Depending on the scale, the fluid has six (shortest lengths), four (intermediate, exponentially broad regime), or three (asymptotically long scales) hydrodynamic modes. Interaction between hydrodynamic modes leads to anomalous scaling of physical observables and waves propagating in the fluid. In a four-mode regime, all modes are ballistic and acquire KPZ-like broadening with asymmetric power-law tails. "Heads" and "tails" of the waves contribute equally to thermal conductivity, leading to $\omega^{-1/3}$ scaling of its real part. In a three-mode regime, the system is in the universality class of classical viscous fluid[9,24]. Self-interaction of the sound modes results in KPZ-like shape, while the interaction with the heat mode results in asymmetric tails. The heat mode is governed by Levy flight distribution, whose power-law tails give rise to $\omega^{-1/3}$ scaling of heat conductivity.

cond-mat.mes-hall↗

arXiv2019

Density matrix calculation of the dark matter abundance in the Higgs induced right-handed neutrino mixing model

We present new results on the calculation of the dark matter relic abundance within the Higgs induced right-handed (RH) neutrino mixing model, solving the associated density matrix equation. For a benchmark value of the dark matter mass $M_{\rm DM} = 220\,{\rm TeV}$, we show the evolution of the abundance and how this depends on reheat temperature, dark matter lifetime and source RH neutrino mass $M_{\rm S}$, with the assumption $M_{\rm S} < M_{\rm DM}$. We compare the results with those obtained within the Landau-Zener approximation, showing that the latter largely overestimates the final abundance giving some analytical insight. However, we also notice that since in the density matrix formalism the production is non-resonant, this allows source RH neutrino masses below the W boson mass, making dark matter more stable at large mass values. This opens an allowed region for initial vanishing source RH neutrino abundance. For example, for $M_{\rm S} \gtrsim 1\,{\rm GeV}$, we find $M_{\rm DM}\gtrsim 20\,{\rm PeV}$. Otherwise, for $M_{\rm S} > M_W\sim 100\,{\rm GeV}$, one has to assume a thermalisation of the source RH neutrinos prior to the freeze-in of the dark matter abundance. This results into a large allowed range for $M_{\rm DM}$, depending on $M_{\rm S}$. For example, imposing $M_{\rm S} \gtrsim 300\,{\rm GeV}$, allowing also successful leptogenesis, we find $0.5 \lesssim M_{\rm DM}/{\rm TeV} \lesssim 500$. We also discuss in detail leptogenesis with two quasi-degenerate RH neutrinos, showing a case when observed dark matter abundance and matter-antimatter asymmetry are simultaneously reproduced. Finally, we comment on how an initial thermal source RH neutrino abundance can be justified and on how our results suggest that also the interesting case where $M_{\rm DM} < M_{\rm S}$, embeddable in usual high scale two RH neutrino seesaw models, might be viable.

hep-ph↗

arXiv2019

Thermal Transport in One Dimensional Electronic Fluid

We study thermal conductivity for one-dimensional electronic fluid. The many-body Hilbert space is partitioned into bosonic and fermionic sectors that carry the thermal current in parallel. For times shorter than bosonic Umklapp time, the momentum of Bose and Fermi components are separately conserved, giving rise to the ballistic heat propagation and imaginary heat conductivity proportional to $T / i\omega$. The real part of thermal conductivity is controlled by decay processes of fermionic and bosonic excitations, leading to several regimes in frequency dependence. At lowest frequencies or longest length scales, the thermal transport is dominated by L{\'e}vy flights of low-momentum bosons that lead to a fractional scaling, $\omega^{-\frac{1}{3}}$ and $L^{1/3}$, of heat conductivity with the frequency $\omega$ and system size $L$ respectively.

cond-mat.str-el↗