arXiv subjects

Tuhin Ghosh

Publications and source records attributed to Tuhin Ghosh.

At least 37 records · Page 2Linked to original sources

arXiv2024

Systems design, assembly, integration and lab testing of WALOP-South Polarimeter

Wide-Area Linear Optical Polarimeter (WALOP)-South is the first wide-field and survey-capacity polarimeter in the optical wavelengths. On schedule for commissioning in 2024, it will be mounted on the 1 m SAAO telescope in Sutherland Observatory, South Africa to undertake the PASIPHAE sky survey. PASIPHAE program will create the first polarimetric sky map in the optical wavelengths, spanning more than 2000 square degrees of the southern Galactic region. The innovative design of WALOP-South will enable it to measure the linear polarization (Stokes parameters $q$ and $u$), in a single exposure, of all sources in a field of view (FoV) of $35\times35$ arcminutes-squared in the SDSS-r broadband and narrowband filters between 500-750 nm with 0.1 % polarization accuracy. The unique goals of the instrument place very stringent systems engineering goals, including on the performance of the optical, polarimetric, optomechanical, and electronic subsystems. All the subsystems have been designed carefully to meet the overall instrument performance goals. As of May 2024, all the instrument optical and mechanical subsystems have been assembled and are currently getting tested and integrated. The complete testing and characterization of the instrument in the lab is expected to be completed by August 2024. In this paper, we will present (a) the design and development of the entire instrument and its major subsystems, focusing on the opto-mechanical design which has not been reported before, and (b) assembly and integration of the instrument in the lab and early results from lab characterization of the instrument.

astro-ph.IM↗

arXiv2024

Statistical properties of Galactic synchrotron temperature and polarization maps -- a multi-frequency comparison

Understanding the statistical properties of synchrotron emission from our Galaxy is valuable from the perspective of observations targeting signals of cosmological origin, as well as for understanding physical processes in our Galaxy. In this work, we extend the analysis of arXiv:2104.00419v2 to -- (a) all-sky observed maps of total foreground emissions at different frequencies provided by WMAP, Planck and Stockert-Villa, (b) component separated synchrotron temperature maps provided by WMAP, Planck and BeyondPlanck, and (c) component separated polarization maps provided by WMAP and Planck. The tools we use are Minkowski functionals and tensors. Our main goals are twofold. First, we determine the variation of morphological properties of the total foreground maps with observing frequency and compare with simulations. This elucidates how the morphology varies due to the relative dominance of different foreground components at different frequencies. Next, we analyze the nature of non-Gaussianity and statistical isotropy of synchrotron fluctuations on smaller scales using various component separated synchrotron temperature and polarization maps. We find that all maps exhibit kurtosis non-Gaussianity, in agreement with the Haslam map. This result can be an important input for the modelling of small-scale synchrotron fluctuations for component separation pipelines. This also suggests that residual synchrotron contamination in CMB will manifest as kurtosis. Studying different component separated maps, we find that BeyondPlanck and WMAP MCMC-e agree well with Haslam at all scales. The other maps show differences of varying statistical significance. Our analysis suggests a combination of residual AME and/or free-free emissions and point sources as contributing to these differences, and underscores the need for further improvement of the pipelines.

astro-ph.CO↗

arXiv2023

Orbital Architectures of Kepler Multis From Dynamical Instabilities

The high-multiplicity exoplanet systems are generally more tightly packed when compared to the solar system. Such compact multi-planet systems are often susceptible to dynamical instability. We investigate the impact of dynamical instability on the final orbital architectures of multi-planet systems using N-body simulations. Our models initially consist of six to ten planets placed randomly according to a power-law distribution of mutual Hill separations. We find that almost all of our model planetary systems go through at least one phase of dynamical instability, losing at least one planet. The orbital architecture, including the distributions of mutual Hill separations, planetary masses, orbital periods, and period ratios, of the transit-detectable model planetary systems closely resemble those for the multi-planet systems detected by Kepler. We find that without any formation-dependent input, a dynamically active past can naturally reproduce important observed trends including multiplicity-dependent eccentricity distribution, smaller eccentricities for larger planets, and intra-system uniformity. On the other hand, our transit-detectable planet populations lack the observed sub-population of eccentric single-transiting planets, pointing towards the Kepler dichotomy. These findings indicate that dynamical instabilities may have played a vital role in the final assembly of sub-Jovian planets.

astro-ph.EP↗

arXiv2023

Importance of high-frequency bands for thermal dust removal in ECHO

The Indian Consortium of Cosmologists has proposed a cosmic microwave background (CMB) space mission, Exploring Cosmic History and Origin (ECHO). A major scientific goal of the mission is to detect the primordial B-mode signal of CMB polarization. The detection of the targeted signal is very challenging as it is deeply buried under the dominant astrophysical foreground emissions of the thermal dust and the Galactic synchrotron. To facilitate the adequate subtraction of thermal dust, the instrument design of ECHO has included nine dust-dominated high-frequency bands over the frequency range of 220-850 GHz. In this work, we closely reexamine the utility of the high-frequency ECHO bands in foreground subtraction using the Needlet Internal Linear Combination component separation method. We consider three dust models: a physical dust model, a dust spectral energy distribution (SED) with a single modified black body (MBB) emission law and a multilayer dust model with frequency-frequency decorrelation. We consider eleven ECHO bands in the 28-190 GHz range as our baseline configuration and investigate the changes in the level foreground and noise residuals as subsequent dust-dominated high-frequency bands are added. We find that adding the high-frequency bands leads to a consistent decrease in the level of residual foreground and noise, and the sensitivity of r measurement improves. Most of the reduction in both residual levels and enhancement in the sensitivity is achieved in the 28-600 GHz frequency range. Negligible change in residual levels is seen by extending the frequency range from 600 GHz to 850 GHz.

astro-ph.CO↗

arXiv2023

An inverse boundary value problem for the inhomogeneous porous medium equation

In this paper we establish uniqueness in the inverse boundary value problem for the two coefficients in the inhomogeneous porous medium equation $ε\partial_tu-\nabla\cdot(γ\nabla u^m)=0$, with $m>1$, in dimension 3 or higher, which is a degenerate parabolic type quasilinear PDE. Our approach relies on using a Laplace transform to turn the original equation into a coupled family of nonlinear elliptic equations, indexed by the frequency parameter ($1/h$ in our definition) of the transform. A careful analysis of the asymptotic expansion in powers of $h$, as $h\to\infty$, of the solutions to the transformed equation, with special boundary data, allows us to obtain sufficient information to deduce the uniqueness result.

math.AP↗

arXiv2023

A Reduction of the Fractional Calderón Problem to the Local Calderón Problem by Means of the Caffarelli-Silvestre Extension

We relate the (anisotropic) variable coefficient local and nonlocal Calderón problems by means of the Caffarelli-Silvestre extension. In particular, we prove that (partial) Dirichlet-to-Neumann data for the fractional Calderón problem in three and higher dimensions determine the (full) Dirichlet-to-Neumann data for the local Calderón problem. As a consequence, any (variable coefficient) uniqueness result for the local problem also implies a uniqueness result for the nonlocal problem. Moreover, our approach is constructive and associated Tikhonov regularization schemes can be used to recover the data. Finally, we highlight obstructions for reversing this procedure, which essentially consist of two one-dimensional averaging processes.

math.AP↗

arXiv2023

Is Cosmic Birefringence model-dependent?

Exciting clues to isotropic cosmic birefringence have recently been detected in the $EB$ cross-power spectra of the polarization data of the cosmic microwave background (CMB). Early Dark Energy (EDE) models with a pseudoscalar field coupled to photons via a Chern-Simons term can be used to explain this phenomenon, and can also potentially be used to simultaneously resolve the $H_0$ tension. In this work we incorporate an early dark energy scalar field, including a Chern-Simons coupling, into an existing Boltzmann solver and numerically recover the $EB$ cross-power spectrum for two models in the literature; the $α$-attractor, and the Rock `n' Roll field. We find that both the models fit the $EB$ spectra, and the $EB$ spectra alone do not possess sufficient constraining power to distinguish the two models based on current data.

astro-ph.CO↗

arXiv2022

Effects of Planetesimal Scattering: Explaining the Observed Offsets from Period Ratios 3:2 and 2:1

The observed deficit and excess of adjacent planet pairs with period ratios narrow and wide of 3:2 and 2:1, the nominal values for the corresponding mean motion resonances (MMRs), have intrigued many. Previously, using a suite of simulations, Chatterjee & Ford (2015) showed that the excess above the 2:1 MMR can be naturally explained if planet pairs, initially trapped in the 2:1 MMR, dynamically interact with nearby planetesimals in a disk. We build on this work by: a) updating the census of discovered planet pairs, b) extending the study to initially non-resonant as well as resonant planet pairs, c) using initial planet and orbital properties directly guided by those observed, and d) extending the initial period ratios to include both 2:1 and 3:2. We find that 1) interactions with planetesimals typically increase the period ratios of both initially resonant and non-resonant planet pairs; 2) starting from an initially flat period ratio distribution for systems across 3:2 and 2:1, these interactions can naturally create the deficits observed narrow of these period ratios; 3) contribution from initially resonant planet pairs is needed to explain the observed levels of excess wide of 3:2; 4) a mixture model where about 25% (1%) planet pairs were initially trapped into 3:2 (2:1) MMRs is favored to explain both the observed deficit and excess of systems across these period ratios, however, up to a few percent of planet pairs are expected to remain in MMR today.

astro-ph.EP↗

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↗

arXiv2022

$B$-mode forecast of CMB-Bh$\overline{a}$rat

Exploring Cosmic History and Origins (ECHO), popularly known as `CMB-Bh$\overline{a}$rat', is a space mission that has been proposed to the Indian Space Research Organisation (ISRO) for the scientific exploitation of the Cosmic Microwave Background (CMB) at the next level of precision and accuracy. The quest for the CMB polarization $B$-mode signals, generated by inflationary gravitational waves in the very early universe, is one of the key scientific goals of its experimental design. This work studies the potential of the proposed ECHO instrumental configuration to detect the target tensor-to-scalar ratio $r \sim 10^{-3}$ at $3σ$ significance level, which covers the predictions of a large class of inflationary models. We investigate the performance of two different component separation pipelines, NILC and Commander, for the measurement of $r$ in presence of different physically motivated models of astrophysical foregrounds. For a simplistic foreground model (only polarized dust and synchrotron), both component separation pipelines can achieve the desired sensitivity of ECHO, i.e. $σ(r =0) \sim (0.4 - 0.7)\times 10^{-3}$. NILC performs better than Commander in terms of bias on recovered $r$ for complex spectral models (power-law and curved power-law) of the synchrotron emission and complex dust models (dust decorrelation). Assuming 84 % delensing, we can achieve an improvement of $σ(r = 0)$ by approximately 50 % as compared to the results obtained for the same configuration without any lensing correction.

astro-ph.CO↗

arXiv2022

Magnetic fields and young stellar objects in cometary cloud LDN 1616

LDN 1615/1616 and CB 28 (hereafter, L1616) together form a cometary globule located at an angular distance of about 8 degrees west of the Orion OB1 association, aligned roughly along the east-west direction, and showing a distinct head-tail structure. The presence of massive stars in the Orion belt has been considered to be responsible for the radiation driven implosion mode of star formation in L1616. Based on the latest Gaia EDR3 measurements of the previously known young stellar objects (YSOs) associated with L1616, we find the distance to this cloud of 384$\pm$5 pc. We present optical polarimetry towards L1616 that maps the plane-of-sky component of the ambient magnetic field (B$_{POS}$) geometry. Based on the proper motion of the YSOs associated with L1616, we investigate their plane-of-sky motion relative to the exciting star $ε$ Ori. Using the Gaia EDR3 measurements of the distances and proper motions of the YSOs, we find two additional sources comoving with the known YSOs. One comoving source is HD33056, a B9 star and the other might be a young pre-main sequence star not reported in previous studies. The mean direction of B$_{POS}$ is found to follow the cloud structure. This could be the effect of dragging of the magnetic field lines by the impact of the ionizing radiation from $ε$ Ori. Based on the pressure exerted on L1616, and the ages of the associated YSOs, we show that it could possibly be the main source of ionization in L1616, and thus the star formation in it.

astro-ph.GA↗

arXiv2021

An inverse problem for the porous medium equation with partial data and a possibly singular absorption term

In this paper we prove uniqueness in the inverse boundary value problem for the three coefficient functions in the porous medium equation with an absorption term $ε\partial_t u-\nabla\cdot(γ\nabla u^m)+λu^q=0$, with $m>1$, $m^{-1}<q<\sqrt{m}$, with the space dimension 2 or higher. This is a degenerate parabolic type quasilinear PDE which has been used as a model for phenomena in fields such as gas flow (through a porous medium), plasma physics, and population dynamics. In the case when $γ=1$ a priori, we prove unique identifiability with data supported in an arbitrarily small part of the boundary. Even for the global problem we improve on previous work by obtaining uniqueness with a finite (rather than infinite) time of observation and also by introducing the additional absorption term $λu^q$.

math.AP↗

arXiv2021

Fractional anisotropic Calderón problem on closed Riemannian manifolds

In this paper we solve the fractional anisotropic Calderón problem on closed Riemannian manifolds of dimensions two and higher. Specifically, we prove that the knowledge of the local source-to-solution map for the fractional Laplacian, given on an arbitrary small open nonempty a priori known subset of a smooth closed connected Riemannian manifold, determines the Riemannian manifold up to an isometry. This can be viewed as a nonlocal analog of the anisotropic Calderón problem in the setting of closed Riemannian manifolds, which is wide open in dimensions three and higher.

math.AP↗

arXiv2021

The Calderón problem for nonlocal operators

We study the inverse problem of determining the coefficients of the fractional power of a general second order elliptic operator given in the exterior of an open subset of the Euclidean space. We show the problem can be reduced into determining the coefficients from the boundary Cauchy data of the elliptic operator on the open set, the Calderón problem. As a corollary we establish several new results for nonlocal inverse problems by using the corresponding results for the local inverse problems. In particular the isotropic nonlocal Calderón problem can be resolved completely, assuming some regularity assumptions on the coefficients, and the anisotropic Calderón problem modulo an isometry which is the identity at the boundary for real-analytic anisotropic conductivities in dimension greater than two and bounded and measurable anisotropic conductivities in two dimensions.

math.AP↗

arXiv2021

An Inverse Problem on Determining Second Order Symmetric Tensor for Perturbed Biharmonic Operator

This article offers a study of the Calderón type inverse problem of determining up to second order coefficients of the higher order elliptic operator. Here we show that it is possible to determine an anisotropic second order perturbation given by a symmetric matrix, along with a first order perturbation given by a vector field and a zero-th order potential function inside a bounded domain by measuring the Dirichlet to Neumann map of the perturbed biharmonic operator on the boundary of that domain.

math.AP↗

arXiv2021

The nature of non-Gaussianity and statistical isotropy of the 408 MHz Haslam synchrotron map

Accurate component separation of full-sky maps in the radio and microwave frequencies, such as the cosmic microwave background (CMB), relies on a thorough understanding of the statistical properties of the Galactic foreground emissions. Using scalar Minkowski functionals and their tensorial generalization known as Minkowski tensors, we analyze the statistical properties of one of the major foreground components, namely the Galactic synchrotron given by the full sky 408 MHz Haslam map. We focus on understanding the nature of non-Gaussianity and statistical isotropy of the cooler regions of the map as a function of angular scale. We find that the overall level of the non-Gaussian deviations does decrease as more high emission regions are masked and as we go down to smaller scales, in agreement with the results obtained in earlier works. However, they remain significantly high, of order 3.3$σ$, at the smallest angular scales relevant for the Haslam map. We carry out a detailed examination of the non-Gaussian nature using the generalized skewness and kurtosis cumulants that arise in the perturbative expansion of Minkowski functionals for weakly non-Gaussian fields. We find that the leading sources of non-Gaussianity are the kurtosis terms which are considerably larger than the skewness terms at all angular scales. Further, for the cooler regions of the Haslam map, we find that the non-Gaussian deviations of the Minkowski functionals can be well explained by the perturbative expansion up to second-order (up to kurtosis terms), with first-order terms being sub-dominant. Lastly, we test the statistical isotropy of the Haslam map and find that it becomes increasingly more isotropic at smaller scales.

astro-ph.CO↗

arXiv2021

Tracing the magnetic field morphology of the LDN 1172/1174 cloud complex

The LDN 1172/1174 cloud complex in the Cepheus Flare region presents a hub-filament structure with the reflection nebula, NGC 7023, illuminated by a Herbig Be star, HD 200775, which consists of the hub with a $\sim$5 pc long narrow filament attached to it. Formation of a sparse cluster of low- and intermediate-mass stars is presently taking place in the hub. The aim of this work is to map the magnetic field geometry of LDN 1172/1174 to understand the role played by the field lines in the formation of the molecular cloud. We made R-band polarization measurements of 249 stars projected on the entire LDN 1172/1174 cloud complex to map the geometry of the magnetic field of this region. The magnetic field geometry constructed from our R-band polarization measurements is found to be parallel to the elongated structure inferred from the column density distribution of the cloud produced using the Herschel images. Our R-band polarization measurements are found to be in good agreement with those obtained from Planck. There is evidence of a possible distortion of the magnetic fields toward the northwestern part of the cloud by HD 200775. The magnetic field strength is estimated as $\sim$30 $μ$G. The estimated star formation rate (SFR)/mass of 2.0$\pm$1.3 \%Myr$^{-1}$ and 0.4$\pm$0.3 \%Myr$^{-1}$ for LDN 1172/1174 and the neighboring cloud complex, LDN 1147/1158, respectively, are found to be consistent with the mean SFR/mass found for the clouds with magnetic field orientations parallel and perpendicular to their elongated structures, respectively. These results support earlier findings that the clouds with magnetic field lines parallel to their long axes seem to have higher SFRs compared to those with the magnetic field orientation perpendicular to the cloud elongation.

astro-ph.GA↗

arXiv2021

Application of Contour Minkowski Tensor and $\mathcal{D}$ Statistic to the Planck $E$-mode data

We test the statistical isotropy (SI) of the $E$-mode polarization of the Cosmic Microwave Background (CMB) radiation observed by the Planck satellite using two statistics, namely, the $α$ estimator that is derived from the contour Minkowski Tensor (CMT), and the Directional statistic ($\mathcal{D}$ statistic). The $α$ estimator obtained from the CMT provides information about the alignment of structures and can be used to infer statistical properties such as Gaussianity and SI of random fields. The $\mathcal{D}$ statistic is based on detecting preferred directionality shown by vectors defined by the field. We compute $α$ and $\mathcal{D}$ statistic for the low resolution component separated SMICA $E$-mode map of CMB polarization, and compare with the values calculated using FFP10 SMICA simulations. We find good agreement between the Planck data and SMICA simulations for both $α$ estimator and $\mathcal{D}$ statistic.

astro-ph.CO↗