arXiv subjects

Kishan Deka

Publications and source records attributed to Kishan Deka.

5 recordsLinked to original sources

arXiv2026

WALOP-South: a four camera one shot imaging polarimeter for the PASIPHAE survey. Paper III -- PSF modelling

The two WALOP instruments, built for the PASIPHAE survey, will measure the linear polarization of large numbers of stars in the Galactic polar regions in the SDSS-$r$ band. They are designed with a wide field-of-view, enabling measurement of the Stokes parameters $I$, $q$, and $u$ for multiple stars simultaneously within a $35^\prime \times 35^\prime$ region of sky for WALOP-South and $30^\prime \times 30^\prime$ region for WALOP-North. In this paper, we present a polar shapelet-based PSF photometry framework for well-sampled stellar point sources applicable to WALOP-type wide-field polarimeters. Polar shapelets are a set of orthogonal basis functions, constructed from Gauss-Hermite or Gauss-Laguerre polynomials, that are well-suited to modelling localized PSF in a compact and efficient way. We developed an efficient PSF modelling method that uses polar shapelets as basis functions to reconstruct the spatial variation of the PSF shape across the CCD using Zemax-simulated images of one of the WALOP instruments, and show that a limited number of shapelet coefficients are sufficient to capture this variation consistently across different CCD locations. To simulate realistic star images, we introduce random sub-pixel shifts in the star centroids in the Zemax-simulated images, and account for this using a two-step iterative method that alternately estimates the PSF model and the sub-pixel centroid shift. Applying PSF photometry to the target faint stars, we demonstrate that the photometric accuracy of approximately 0.15% is achievable, and that the reconstructed PSF model can be incorporated into the photometry across different seeing conditions, meeting the polarimetric science requirements of the PASIPHAE survey.

astro-ph.IM↗

arXiv2025

Galactic foreground residue biases in cosmic-microwave-background lensing-convergence reconstruction and delensing of B-mode maps

Diffuse contamination from Galactic foreground emission is one of the main concerns for reconstruction of the cosmic microwave background (CMB) lensing potential for next-generation CMB polarisation experiments. Using realistic simulations, we investigated the impact of Galactic foreground residuals from multi-frequency foreground-cleaning methods on CMB lensing reconstruction and the de-lensing of B-mode maps. We also assessed how these residuals affect constraints on the tensor-to-scalar ratio for a CMB-S4--like experiment. We paid special attention to the errors coming from the small scale non-Gaussianity of the foreground residuals. We show that component separation is essential for the lensing reconstruction that reduces Galactic emission contribution to the lensing reconstruction errors by one order of magnitude. The residual foreground contribution is dominated by terms coming from Gaussian components of the residual maps. Errors coming from non-Gaussian components are around three orders of magnitude smaller than the Gaussian one, even for recent and the most complex models of the Galactic emission considered in this work. Although the bias in the reconstruction errors due to the Gaussian component of the residuals being small, it is comparable to the cosmic variance limit for the lensing power spectrum. For this reason, we corrected for this bias in the de-lensing of B-mode maps and constraining the tensor-to-scalar ratio. We also show that for the de-lensed B-mode maps with a simple quadratic estimator, that is, residuals of the Galactic emission after component separation, errors are two orders of magnitude smaller than uncertainties from leftover of the lensing signal. However, for high-sensitivity CMB experiments and more efficient de-lensing algorithms that remove up to 90% of the lensing signal, the foreground residuals will become one of the main sources of errors.

astro-ph.CO↗

arXiv2022

WALOP-South: A Four-Camera One-Shot Imaging Polarimeter for PASIPHAE Survey. Paper II -- Polarimetric Modelling and Calibration

The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument is an upcoming wide-field and high-accuracy optical polarimeter to be used as a survey instrument for carrying out the Polar-Areas Stellar Imaging in Polarization High Accuracy Experiment (PASIPHAE) program. Designed to operate as a one-shot four-channel and four-camera imaging polarimeter, it will have a field of view of $35\times 35$ arcminutes and will measure the Stokes parameters $I$, $q$, and $u$ in a single exposure in the SDSS-r broadband filter. The design goal for the instrument is to achieve an overall polarimetric measurement accuracy of 0.1 % over the entire field of view. We present here the complete polarimetric modeling of the instrument, characterizing the amount and sources of instrumental polarization. To accurately retrieve the real Stokes parameters of a source from the measured values, we have developed a calibration method for the instrument. Using this calibration method and simulated data, we demonstrate how to correct instrumental polarization and obtain 0.1 % accuracy in the degree of polarization, $p$. Additionally, we tested and validated the calibration method by implementing it on a table-top WALOP-like test-bed polarimeter in the laboratory.

astro-ph.IM↗

arXiv2021

Dynamics of QCD Matter -- current status

In this article, there are 18 sections discussing various current topics in the field of relativistic heavy-ion collisions and related phenomena, which will serve as a snapshot of the current state of the art. Section 1 reviews experimental results of some recent light-flavored particle production data from ALICE collaboration. Other sections are mostly theoretical in nature. Very strong but transient magnetic field created in relativistic heavy-ion collisions could have important observational consequences. This has generated a lot of theoretical activity in the last decade. Sections 2, 7, 9, 10 and 11 deal with the effects of the magnetic field on the properties of the QCD matter. There are several unanswered questions about the QCD phase diagram. Sections 3, 11 and 18 discuss various aspects of the QCD phase diagram and phase transitions. Recent years have witnessed interesting developments in foundational aspects of hydrodynamics and their application to heavy-ion collisions. Sections 12, 15, 16 and 17 of this article probe some aspects of this exciting field. Transport coefficients together with their temperature- and density-dependence, are essential inputs in hydrodynamical calculations. Sections 5, 8 and 14 deal with calculation/estimation of various transport coefficients (shear and bulk viscosity, thermal conductivity, relaxation times, etc.) of quark matter and hadronic matter. Sections 4, 6 and 13 deals with interesting new developments in the field. Section 4 discusses color dipole gluon distribution function at small transverse momentum in the form of a series of Bells polynomials. Section 6 discusses the properties of Higgs boson in the quark gluon plasma using Higgs-quark interaction. Section 13 discusses modification of coalescence model to incorporate viscous corrections and application of this model.

hep-ph↗

arXiv2020

Viscosity, non-conformal equation of state and sound velocity in Landau hydrodynamics

We find an analytical solution to relativistic viscous hydrodynamics for a 1+1 dimensional Landau flow profile. We consider relativistic Navier-Stokes form of the dissipative hydrodynamic equation, for a non-conformal system with a constant speed of sound, and employ the obtained solution to fit rapidity spectrum of observed pions in $\sqrt{s_{NN}}=$ 200, 17.3, 12.3, 8.76, 7.62, 6.27, 4.29, 3.83, 3.28 and 2.63 GeV collision energies. We find that at the freeze-out hypersurface with improved Landau's freeze-out prescription, the viscous corrections do not affect the rapidity spectra. We demonstrate that the solution of the non-conformal Landau flow lead to a better agreement with the experimental data compared to the conformal ideal solution. We also extract speed of sound from fit to the rapidity spectra for various collision energies and find a monotonous decrease with decreasing collision energies. Appealing to the fact that viscosity has negligible effect on rapidity spectra for Landau's freeze-out scenario, we argue that our calculations provides a framework for extracting the average value of speed of sound in relativistic heavy-ion collisions.

hep-ph↗