arXiv subjects

Susmita Das

Publications and source records attributed to Susmita Das.

At least 37 records · Page 2Linked to original sources

arXiv2024

Bridging theory and observations in stellar pulsations: The impact of convection and metallicity on the instability strips of Classical and Type-II Cepheids

The effect of metallicity on the theoretical and empirical period-luminosity (PL) relations of Cepheid variables is not well understood and remains a highly debated issue. Here, we examine empirical colour-magnitude diagrams (CMDs) of Classical and Type-II Cepheids in the Magellanic Clouds and compare those with the theoretically predicted instability strip (IS) edges. We explore the effects of incorporating turbulent flux, turbulent pressure, and radiative cooling into the convection theory on the predicted IS at various metallicities using MESA-RSP. We find that the edges become redder with the increasing complexity of convection physics incorporated in the fiducial convection sets, and are similarly shifted to the red with increasing metallicity. The inclusion of turbulent flux and pressure improves the agreement of the red edge of the IS, while their exclusion leads to better agreement with observations of the blue edge. About 90% of observed stars are found to fall within the predicted bluest and reddest edges across the considered variations of turbulent convection parameters. Furthermore, we identify and discuss discrepancies between theoretical and observed CMDs in the low effective temperature and high luminosity regions for stars with periods greater than ~ 20 days. These findings highlight the potential for calibrating the turbulent convection parameters in stellar pulsation models or the prediction of a new class of rare, long-period, 'red Cepheids', thereby improving our understanding of Cepheids and their role in cosmological studies.

astro-ph.SR↗

arXiv2024

A Survey on Security of Ultra/Hyper Reliable Low Latency Communication: Recent Advancements, Challenges, and Future Directions

Ultra-reliable low latency communication (URLLC) is an innovative service offered by fifth-generation (5G) wireless systems. URLLC enables various mission-critical applications by facilitating reliable and low-latency signal transmission to support extreme Quality of Service (QoS) requirements. Apart from reliability and latency, ensuring secure data transmission for URLLC has been a prominent issue for researchers in recent years. Using finite blocklength signals to achieve the stringent reliability and latency criteria in URLLC eliminates the possibility of using conventional complex cryptographic security enhancement techniques based on encoding and decoding of secret keys. Thus, the development of lightweight security mechanisms is of paramount importance for URLLC. Recently, Physical-Layer Security (PLS) techniques have emerged as a powerful alternative to the complex cryptography-based security approaches for facilitating secure URLLC by exploiting the randomness of the wireless channel. Therefore, in this survey, we present a comprehensive and in-depth review of the state-of-the-art PLS enhancements utilized to unleash secure URLLC while analyzing the impact of various system design parameters on its performance. Moreover, the survey incorporates a detailed overview of the recent advancements in ensuring secure URLLC using PLS in various mission-critical applications, and 5G URLLC enabling technologies like non-orthogonal multiple access (NOMA), multi-antenna systems, cooperative communication using unmanned aerial vehicles (UAV), and intelligent reflective surfaces (IRS). Apart from this, we briefly discuss the role of advanced Machine Learning (ML) techniques in designing robust and intelligent PLS schemes for URLLC service.

cs.CR↗

arXiv2024

A theoretical framework for BL Her stars -- II. New period-luminosity relations in the Gaia passbands

We present new theoretical period-luminosity (PL) and period-Wesenheit (PW) relations for a fine grid of convective BL Her, the shortest period T2Cs, models computed using MESA-RSP and compare our results with the empirical relations from Gaia DR3. We use the state-of-the-art 1D non-linear radial stellar pulsation tool MESA-RSP to compute models of BL Her stars over a wide range of input parameters - metallicity (-2.0 dex $\leq$ [Fe/H] $\leq$ 0.0 dex), stellar mass (0.5M$_{\odot}$-0.8M$_{\odot}$), stellar luminosity (50L$_{\odot}$-300L$_{\odot}$) and effective temperature (full extent of the instability strip; in steps of 50K). The BL Her stars in the All Sky region exhibit statistically different PL slopes compared to the theoretical PL slopes computed using the four sets of convection parameters. We find the empirical PL and PW slopes from BL Her stars in the Magellanic Clouds to be statistically consistent with the theoretical relations computed using the different convection parameter sets in the Gaia passbands. There is negligible effect of metallicity on the PL relations in the individual Gaia passbands. However, there exists a small but significant negative coefficient of metallicity in the PWZ relations for the BL Her models using the four sets of convection parameters. This could be attributed to the increased sensitivity of bolometric corrections to metallicities at wavelengths shorter than the V band. Our BL Her models also suggest a dependence of the mass-luminosity relation on metallicity. We found the observed Fourier parameter space to be covered well by our models. Higher mass models (> 0.6M$_{\odot}$) may be needed to reliably model the observed light curves of BL Her stars in the All Sky region. We also found the theoretical light curve structures (especially the Fourier amplitude parameters) to be affected by the choice of convection parameters.

astro-ph.SR↗

arXiv2023

A study of the stellar photosphere-hydrogen ionization front interaction in $δ$ Scuti stars

Pulsating variable $δ$ Scuti stars are intermediate-mass stars with masses in the range of $1-3$ $M_{\odot}$ and spectral types between $A2$ and $F2$. They can be found at the intersection of the Cepheid instability strip with the main sequence. They can be used as astrophysical laboratories to test theories of stellar evolution and pulsation. In this contribution, we investigate the observed period-colour and amplitude-colour (PCAC) relations at maximum/mean/minimum light of Galactic bulge and Large Magellanic Cloud $δ$ Scuti stars for the first time and test the hydrogen ionization front (HIF)-photosphere interaction theory using the MESA- RSP code. The PCAC relations, as a function of pulsation phase, are crucial probes of the structure of the outer stellar envelope and provide insight into the physics of stellar pulsation and evolution. The observed behaviour of the $δ$ Scuti PCAC relations is consistent with the theory of the interaction between the HIF and the stellar photosphere.

astro-ph.SR↗

arXiv2023

A multiphase study of classical Cepheids in the Magellanic Clouds- Models and Observations

This work presents the study of multiphase relations of classical Cepheids in the Magellanic Clouds for short periods (log P < 1) and long periods (log P > 1). From the analysis, it has been found that the multiphase relations obtained using the models as well as observations are highly dynamic with pulsational phase. The multiphase relations for short and long periods are found to display contrasting behaviour for both LMC and SMC. It has been observed that the multiphase relations obtained using the models agree better with the observations in the PC plane in most phases in comparison to the PL plane. Multiphase relations obtained using the models display a clear distinction among different convection sets in most phases. Comparison of models and observations in the multiphase plane is one way to test the models with the observations and to constrain the theory of stellar pulsation.

astro-ph.GA↗

arXiv2023

Completely non unitary contractions and Analyticity

We study completely non-unitary contractions $T$ with finite dimensional defect spaces $\mathcal{D}_T$ and $\mathcal{D}_{T^*}$. We present a complete classification of all such contractions $T$ that satisfy a generalized property of Hardy shift operator: $\mathcal{D}_T\subseteq\mathcal{D}_{T^*}$. We show that $T$ is analytic if and only if it has no non-zero eigenvalue. Furthermore, we characterize the hyponormality of all those $T$.

math.FA↗

arXiv2023

Correlated Short-Timescale Hard-Soft X-ray Variability of the Blazars Mrk 421 and 1ES 1959+650 using $\textit{AstroSat}$

We study simultaneous soft ($0.7 - 7$ keV) and hard ($7 - 20$ keV) X-ray light curves at a total of eight epochs during $2016 - 2019$ of two TeV blazars Mrk 421 and 1ES 1959+650 observed by the SXT and LAXPC instruments onboard AstroSat. The light curves are $45 - 450$ ks long and may be sampled with time bins as short as $600 - 800$ sec with high signal to noise ratio. The blazars show a harder when brighter trend at all epochs. Discrete cross-correlation functions indicate that the hard and soft X-ray variability are strongly correlated. The time lag is consistent with zero in some epochs, and indicates hard or soft lag of a few hours in the rest. In the leptonic model of blazar emission, soft lag may be due to slower radiative cooling of lower energy electrons while hard lag may be caused by gradual acceleration of the high energy electrons emitting at the hard X-ray band. Assuming the above scenario and the value of the Doppler factor ($δ$) to be $10 - 20$, the hard and soft lags may be used to estimate the magnetic field to be $\sim 0.1$ Gauss and the acceleration parameter to be $\sim 10^4$ in the emission region. Due to the availability of the high time resolution ($\sim$ minutes to hours) light curves from AstroSat, the value of the illusive acceleration parameter could be estimated, which provides a stringent constraint on the theories of particle acceleration in blazar jets.

astro-ph.HE↗

arXiv2023

Predicting light curves of RR Lyrae variables using artificial neural network based interpolation of a grid of pulsation models

We present a new technique to generate the light curves of RRab stars in different photometric bands ($I$ and $V$ bands) using Artificial Neural Networks (ANN). A pre-computed grid of models was used to train the ANN, and the architecture was tuned using the $I$ band light curves. The best-performing network was adopted to make the final interpolators in the $I$ and $V$ bands. The trained interpolators were used to predict the light curve of RRab stars in the Magellanic Clouds, and the distances to the LMC and SMC were determined based on the reddening independent Wesenheit index. The estimated distances are in good agreement with the literature. The comparison of the predicted and observed amplitudes, and Fourier amplitude ratios showed good agreement, but the Fourier phase parameters displayed a few discrepancies. To showcase the utility of the interpolators, the light curve of the RRab star EZ Cnc was generated and compared with the observed light curve from the Kepler mission. The reported distance to EZ Cnc was found to be in excellent agreement with the updated parallax measurement from Gaia EDR3. Our ANN interpolator provides a fast and efficient technique to generate a smooth grid of model light curves for a wide range of physical parameters, which is computationally expensive and time-consuming using stellar pulsation codes.

astro-ph.SR↗

arXiv2023

A multiphase study of theoretical and observed light curves of classical Cepheids in the Magellanic Clouds

We present an analysis of the theoretical and observed light curve parameters of the fundamental mode (FU) classical Cepheids in the Magellanic Clouds in $V$- and $I$- photometric bands. The state-of-the-art 1D non-linear radial stellar pulsation (RSP) code in MESA (\textsc{mesa-rsp}) has been utilized to generate the theoretical light curves using four sets of convection parameters. Theoretical light curves with two chemical compositions: $Z=0.008$ and $Z=0.004$ appropriate for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), respectively, covered a wide range of periods ($3 1$) and all periods. The multiphase relations obtained from theoretical and observed light curves in the PL/PC/AC plane are found to be dynamic in nature, with the effect more pronounced at $Φ\sim 0.75-0.85$. Furthermore, a contrasting behaviour of the theoretical/observed multiphase PL and PC relations between the short and long periods has been found for both LMC and SMC. The analysis shows that multiphase PL relations are more stringent to test the models with observations over the FPs. Distances to the LMC/SMC determined using long period Cepheids are found to be in good agreement with the literature values when the term $R_{21}$ is added to the PL relation.

astro-ph.SR↗

arXiv2022

Period-Colour and Amplitude-Colour relations for OGLE-$δ$ Scuti stars in the Galactic Bulge and LMC

We present an analysis on the behaviour of the Galactic bulge and the Large Magellanic Cloud (LMC) $δ$ Scuti stars in terms of period-colour and amplitude-colour (PCAC) relations at maximum, mean and minimum light. The publicly available Optical Gravitational Lensing Experiment-IV (OGLE-IV) light curves for Galactic bulge and OGLE-III light curves for LMC $δ$ Scuti stars are exploited for the analysis. It has been found that the Galactic bulge $δ$ Scuti stars obey flat PC relations at maximum/mean/minimum light while the LMC $δ$ Scutis have sloped/sloped/flat PC relations at maximum/mean/minimum light. Both the Galactic bulge and the LMC $δ$ Scutis have sloped/flat/sloped AC relations at maximum/mean/minimum. These relations also show that Galactic $δ$ Scutis are hotter as compared to their LMC counterparts. The period-amplitude (PA) relations for $δ$ Scutis exhibit different behaviour in the Galactic bulge and the LMC. The LMC variables are found to have higher amplitudes at a given period. The amplitude of the Galactic bulge $δ$ Scuti shows a bimodal distribution which can be modelled using a two-component Gaussian Mixture Model: one component with a lower amplitude and another with a higher amplitude. The observed behaviour of the $δ$ Scuti PCAC relations can be explained using the theory of the interaction of hydrogen ionization front (HIF) and stellar photosphere as well as the PA diagram. We use MESA-RSP to calculate theoretical non-linear hydrodynamical pulsation models for $δ$ Scuti stars with input metallicities of $Z=0.02$ and $Z=0.008$ appropriate for the Galactic bulge and LMC, respectively. The observed PCAC relations and theoretical calculations support the HIF-photosphere interaction theory.

astro-ph.SR↗

arXiv2022

PathologyBERT -- Pre-trained Vs. A New Transformer Language Model for Pathology Domain

Pathology text mining is a challenging task given the reporting variability and constant new findings in cancer sub-type definitions. However, successful text mining of a large pathology database can play a critical role to advance 'big data' cancer research like similarity-based treatment selection, case identification, prognostication, surveillance, clinical trial screening, risk stratification, and many others. While there is a growing interest in developing language models for more specific clinical domains, no pathology-specific language space exist to support the rapid data-mining development in pathology space. In literature, a few approaches fine-tuned general transformer models on specialized corpora while maintaining the original tokenizer, but in fields requiring specialized terminology, these models often fail to perform adequately. We propose PathologyBERT - a pre-trained masked language model which was trained on 347,173 histopathology specimen reports and publicly released in the Huggingface repository. Our comprehensive experiments demonstrate that pre-training of transformer model on pathology corpora yields performance improvements on Natural Language Understanding (NLU) and Breast Cancer Diagnose Classification when compared to nonspecific language models.

cs.CL↗

arXiv2022

Tridiagonal shifts as compact + isometry

Let $\{a_n\}_{n\geq 0}$ and $\{b_n\}_{n\geq 0}$ be sequences of scalars. Suppose $a_n \neq 0$ for all $n \geq 0$. We consider the tridiagonal kernel (also known as band kernel with bandwidth one) as \[ k(z, w) = \sum_{n=0}^\infty ((a_n + b_n z)z^n) \overline{(({a}_n + {b}_n {w}) {w}^n)} \qquad (z, w \in \mathbb{D}), \] where $\mathbb{D} = \{z \in \mathbb{C}: |z| < 1\}$. Denote by $M_z$ the multiplication operator on the reproducing kernel Hilbert space corresponding to the kernel $k$. Assume that $M_z$ is left-invertible. We prove that $M_z =$ compact $+$ isometry if and only if $|\frac{b_n}{a_n}-\frac{b_{n+1}}{a_{n+1}}|\rightarrow 0$ and $|\frac{a_n}{a_{n+1}}| \rightarrow 1$.

math.FA↗

arXiv2022

Left-invertibility of rank-one perturbations

For each isometry $V$ acting on some Hilbert space and a pair of vectors $f$ and $g$ in the same Hilbert space, we associate a nonnegative number $c(V;f,g)$ defined by \[ c(V; f,g) = (\|f\|^2 - \|V^*f\|^2) \|g\|^2 + |1 + \langle V^*f , g\rangle|^2. \] We prove that the rank-one perturbation $V + f \otimes g$ is left-invertible if and only if \[ c(V;f,g) \neq 0. \] We also consider examples of rank-one perturbations of isometries that are shift on some Hilbert space of analytic functions. Here, shift refers to the operator of multiplication by the coordinate function $z$. Finally, we examine $D + f \otimes g$, where $D$ is a diagonal operator with nonzero diagonal entries and $f$ and $g$ are vectors with nonzero Fourier coefficients. We prove that $D + f\otimes g$ is left-invertible if and only if $D+f\otimes g$ is invertible.

math.FA↗

arXiv2021

Invariant subspaces of analytic perturbations

By analytic perturbations, we refer to shifts that are finite rank perturbations of the form $M_z + F$, where $M_z$ is the unilateral shift and $F$ is a finite rank operator on the Hardy space over the open unit disc. Here shift refers to the multiplication operator $M_z$ on some analytic reproducing kernel Hilbert space. In this paper, we first isolate a natural class of finite rank operators for which the corresponding perturbations are analytic, and then we present a complete classification of invariant subspaces of those analytic perturbations. We also exhibit some instructive examples and point out several distinctive properties (like cyclicity, essential normality, hyponormality, etc.) of analytic perturbations.

math.FA↗

arXiv2021

Tridiagonal kernels and left-invertible operators with applications to Aluthge transforms

Given scalars $a_n (\neq 0)$ and $b_n$, $n \geq 0$, the tridiagonal kernel or band kernel with bandwidth $1$ is the positive definite kernel $k$ on the open unit disc $\mathbb{D}$ defined by \[ k(z, w) = \sum_{n=0}^\infty \Big((a_n + b_n z)z^n\Big) \Big((\bar{a}_n + \bar{b}_n \bar{w}) \bar{w}^n \Big) \qquad (z, w \in \mathbb{D}). \] This defines a reproducing kernel Hilbert space $\mathcal{H}_k$ (known as tridiagonal space) of analytic functions on $\mathbb{D}$ with $\{(a_n + b_nz) z^n\}_{n=0}^\infty$ as an orthonormal basis. We consider shift operators $M_z$ on $\mathcal{H}_k$ and prove that $M_z$ is left-invertible if and only if $\{|{a_n}/{a_{n+1}}|\}_{n\geq 0}$ is bounded away from zero. We find that, unlike the case of weighted shifts, Shimorin's models for left-invertible operators fail to bring to the foreground the tridiagonal structure of shifts. In fact, the tridiagonal structure of a kernel $k$, as above, is preserved under Shimorin model if and only if $b_0=0$ or that $M_z$ is a weighted shift. We prove concrete classification results concerning invariance of tridiagonality of kernels, Shimorin models, and positive operators. We also develop a computational approach to Aluthge transforms of shifts. Curiously, in contrast to direct kernel space techniques, often Shimorin models fails to yield tridiagonal Aluthge transforms of shifts defined on tridiagonal spaces.

math.FA↗

arXiv2021

Short-Timescale Variability of the Blazar Mrk 421 from AstroSat and Simultaneous Multi-Wavelength Observations

We study the multi-wavelength variability of the blazar Mrk 421 at minutes to days timescales using simultaneous data at $γ$-rays from Fermi, 0.7-20 keV energies from AstroSat, and optical and near-infrared (NIR) wavelengths from ground-based observatories. We compute the shortest variability timescales at all of the above wavebands and find its value to be ~1.1 ks at the hard X-ray energies and increasingly longer at soft X-rays, optical and NIR wavelengths as well as at the GeV energies. We estimate the value of the magnetic field to be 0.5 Gauss and the maximum Lorentz factor of the emitting electrons ~1.6 x $10^5$ assuming that synchrotron radiation cooling drives the shortest variability timescale. Blazars vary at a large range of timescales often from minutes to years. These results, as obtained here from the very short end of the range of variability timescales of blazars, are a confirmation of the leptonic scenario and in particular the synchrotron origin of the X-ray emission from Mrk 421 by relativistic electrons of Lorentz factor as high as $10^5$. This particular mode of confirmation has been possible using minutes to days timescale variability data obtained from AstroSat and simultaneous multi-wavelength observations.

astro-ph.HE↗

arXiv2020

A theoretical framework of BL Her stars. I. Effect of metallicity and convection parameters on period-luminosity and period-radius relations

We present a new grid of convective BL Herculis models using the state-of-the-art 1D non-linear radial stellar pulsation tool MESA-RSP. We investigate the impact of metallicity and four sets of different convection parameters on multi-wavelength properties. Non-linear models were computed for periods typical for BL Her stars, i.e. $1 \leq \mathrm{P (days)} \leq 4$ covering a wide range of input parameters - metallicity ($-2.0\; \mathrm{dex} \leq \mathrm{[Fe/H]} \leq 0.0\; \mathrm{dex}$), stellar mass (0.5M$_{\odot}$-0.8M$_{\odot}$), luminosity (50L$_{\odot}$-300L$_{\odot}$) and effective temperature (full extent of the instability strip; in steps of 50K). The total number of BL Her models with full-amplitude stable pulsations used in this study is 10280 across the four sets of convection parameters. We obtain their multiband ($UBVRIJHKLL'M$) light curves and derive new theoretical period-luminosity ($PL$), period-Wesenheit ($PW$) and period-radius ($PR$) relations at mean light. We find that the models computed with radiative cooling show statistically similar slopes for $PL$, $PW$ and $PR$ relations. Most empirical relations match well with the theoretical $PL$, $PW$ and $PR$ relations from the BL Her models computed using the four sets of convection parameters. However, $PL$ slopes of the models with radiative cooling provide a better match to empirical relations for BL Her stars in the LMC in the $HK_S$ bands. For each set of convection parameters, the effect of metallicity is significant in $U$ and $B$-bands and negligible in infrared bands, which is consistent with empirical results. No significant metallicity effects are seen in the $PR$ relations.

astro-ph.SR↗

arXiv2020

A study of the stellar photosphere-hydrogen ionisation front interaction in pulsating variables using period-color relations

Period-color (PC) relations may be used to study the interaction of the stellar photosphere and the hydrogen ionization front (HIF). RR Lyraes (RRLs) and long period classical Cepheids (P > 10d) have been found to exhibit different PC behavior at minimum and maximum light which can be explained by the HIF-photosphere interaction based on their location on the HR diagram. In this work, we extend the study to include type II Cepheids (T2Cs) with an aim to test the HIF-photosphere interaction theory across a broad spectrum of variable star types. We find W Vir stars and BL Her stars to have similar PC relations as those from long period and short period classical Cepheids, respectively. We also use MESA to compute RRL, BL Her and classical Cepheid models to study the theoretical HIF-photosphere distance and find the results to be fairly consistent with the HIF-photosphere interaction theory.

astro-ph.SR↗