arXiv subjects

S. Chowdhury

Publications and source records attributed to S. Chowdhury.

18 recordsLinked to original sources

arXiv2025

Controls Abstraction Towards Accelerator Physics: A Middle Layer Python Package for Particle Accelerator Control

Control system middle layers act as a co-ordination and communication bridge between end users, including operators, system experts, scientists, and experimental users, and the low-level control system interface. This article describes a Python package -- Controls Abstraction Towards Acclerator Physics (CATAP) -- which aims to build on previous experience and provide a modern Python-based middle layer with explicit abstraction, YAML-based configuration, and procedural code generation. CATAP provides a structured and coherent interface to a control system, allowing researchers and operators to centralize higher-level control logic and device information. This greatly reduces the amount of code that a user must write to perform a task, and codifies system knowledge that is usually anecdotal. The CATAP design has been deployed at two accelerator facilities, and has been developed to produce a procedurally generated facility-specific middle layer package from configuration files to enable its wider dissemination across other machines.

physics.acc-ph↗

arXiv2025

Combining the second data release of the European Pulsar Timing Array with low-frequency pulsar data

Low-frequency radio data improve the sensitivity of pulsar timing arrays (PTAs) to propagation effects such as dispersion measure (DM) variations, enabling better noise characterization essential for detecting the stochastic gravitational wave background (GWB). We combined LOFAR (100-200 MHz) and NenuFAR (30-90 MHz) observations with the recent European and Indian PTA release (DR2new+) into a new dataset, DR2low, spanning ~11 years for 12 pulsars. DR2low allows updated noise models, increasing PTA sensitivity to the GWB. Using Libstempo and Enterprise, we applied standard noise models including red noise (RN) and time-variable DM (DMv) as power laws, and performed Bayesian model selection over RN, DMv, and an additional chromatic noise term (CN4). Compared to DR2new+, DR2low improves DM constraints and separates DM and RN contributions. We found that the RN is required in the final model for 10 out of 12 pulsars, compared to only 5 in the DR2new+ dataset. The improved sensitivity to plasma effects provided by DR2low also favors the identification of significant CN4 in eight pulsars, while none showed such evidence in DR2new+. The analysis also reveals unmodelled solar wind effects, particularly near solar conjunction, with residual delays absorbed into the DM component, highlighting the importance of accurately modelling the solar wind in PTA datasets.

astro-ph.HE↗

arXiv2024

Real-time steerable frequency-stepped Doppler Backscattering (DBS) System for local helicon wave electric field measurements on the DIII-D tokamak

A new frequency-stepped Doppler backscattering (DBS) system has been integrated with a real-time steerable electron cyclotron heating launcher to probe local background turbulence (f<10 MHz) and high-frequency (20-550 MHz) density fluctuations in the DIII-D tokamak. The launcher enables 2D steering (horizontal and vertical) over wide angular ranges to optimize probe location and wavenumber response, with vertical steering adjustable in real time during discharges. The DBS system utilizes a programmable frequency synthesizer with adjustable dwell time, capable of stepping across the E-band frequency range (60-90 GHz) in real time, launching either O or X-mode polarized millimeter waves. This setup facilitates diagnosis of the complex spatial structure of high-power (>200 kW) helicon waves (476 MHz) during current drive experiments. Real-time scans reveal broadband density fluctuations around the helicon frequency, attributed to backscattering of the DBS millimeter wave probe from plasma turbulence modulated by the helicon wave. These fluctuations appear as high-frequency sidebands in the turbulence spectrum, effectively 'tagging' the background turbulence with the helicon wave's electric field. This method allows for monitoring local helicon wave amplitude by comparing high-frequency signal amplitude to background turbulence. Coupled with real-time scanning of measurement location and wavenumber, this allows for rapid helicon wave power distribution determination during steady-state plasma operation, potentially validating predictive models like GENRAY or AORSA for helicon current drive in DIII-D plasmas.

physics.plasm-ph↗

arXiv2024

On the Existence of "Maia variables"

There are different classes of pulsating stars in the H-R diagram. While many of those classes are undisputed, some remain a mystery such as the objects historically called "Maia variables". Whereas the presence of such a class was suggested seven decades ago, no pulsational driving mechanism is known that could excite short-period oscillations in these late B to early A-type stars. Alternative hypotheses that would render the reports of variability of those stars erroneous have been proposed such as incorrect effective temperatures, binarity or rapid rotation, but no certain conclusions have been reached yet. Therefore the existence of these variables as a homogeneous class of pulsating star is still under discussion. Meanwhile, many new candidates of these variables have been claimed especially by using photometric observations of space telescopes. In this study, we examined 31 objects that are alleged members of this hypothetical group and carried out detailed spectroscopic and photometric analyses to test the proposed hypotheses for their cause of variability. The Teff, log g, v sin i, and chemical abundances of the targets were determined and the TESS photometric data were examined. As a result, we found that most of these targets are located inside the delta Scuti, beta Cep, or SPB star instability strips, a few show evidence for binarity and others for rapid rotation. We give arguments that none of the apparently rapid pulsations in our targets is caused by a star outside any known instability strip. By extrapolation, we argue that most stars proposed as pulsators outside well-established instability domains are misclassified. Hence there is no sufficient evidence justifying the existence of a class of pulsating stars formerly known as the "Maia variables".

astro-ph.SR↗

arXiv2023

Thermal effect on microwave pulse driven magnetization switching of Stoner particle

Recently it has been demonstrated that the cosine chirp microwave pulse (CCMP) is capable of achieving fast and energy-efficient magnetization-reversal of a nanoparticle with zero-Temperature. However, we investigate the finite temperature, $T$ effect on the CCMP-driven magnetization reversal using the framework of the stochastic Landau Lifshitz Gilbert equation. At finite Temperature, we obtain the CCMP-driven fast and energy-efficient reversal and hence estimate the maximal temperature, $T_{max}$ at which the magnetization reversal is valid. $T_{max}$ increases with increasing the nanoparticle cross-sectional area/shape anisotropy up to a certain value, and afterward $T_{max}$ decreases with the further increment of nanoparticle cross-sectional area/shape anisotropy. This is because of demagnetization/shape anisotropy field opposes the magnetocrystalline anisotropy, i.e., reduces the energy barrier which separates the two stable states. For smaller cross-sectional area/shape anisotropy, the controlling parameters of CCMP show decreasing trend with temperature. We also find that with the increment easy-plane shape-anisotropy, the required initial frequency of CCMP significantly reduces. For the larger volume of nanoparticles, the parameters of CCMP remains constant for a wide range of temperature which are desired for the device application. Therefore, The above findings might be useful to realize the CCMP-driven fast and energy-efficient magnetization reversal in realistic conditions.

cond-mat.mes-hall↗

arXiv2023

Internal Gravity Waves in Massive Stars II: Frequency Analysis Across Stellar Mass

Stars with masses above 1.6 solar masses generally possess convective cores and radiative envelopes, which allows the propagation of outward-travelling internal gravity waves. We have studied the generation and propagation of IGWs in such stars using two-dimensional, fully nonlinear hydrodynamical simulations with realistic stellar reference states from the one-dimensional stellar evolution code, Modules for Stellar Astrophysics. Compared to previous similar works, this study utilises radius-dependent thermal diffusivity profiles for 5 different stellar masses at the middle of main sequence: 3 - 13 solar masses. From the simulations, we find that the surface perturbations are larger for higher masses, but no noticeable trends are observed for the frequency slopes with different stellar masses. The slopes are also similar to the results from previous works. We compare our simulation results with stellar photometric data from a recent survey and find that for frequency intervals above 8 microHz, there is a good agreement between the temperature frequency slopes from the simulations and the surface brightness variations of these observed stars, indicating that the brightness variations are caused by core-generated IGWs.

astro-ph.SR↗

arXiv2023

A novel Doppler backscattering (DBS) system to simultaneously monitor radio frequency plasma fluctuations and low frequency turbulence

A novel quadrature Doppler Backscattering (DBS) system has been developed and optimized for the E-band (60-90GHz) frequency range using either O-mode or X-mode polarization in DIII-D plasmas. In general, DBS measures the amplitude of density fluctuations and their velocity in the lab frame. The system can simultaneously monitor both low-frequency turbulence (f < 10MHz) and radiofrequency plasma density fluctuations over a selectable frequency range (20-500 MHz). Detection of high-frequency fluctuations has been demonstrated for low harmonics of the ion cyclotron frequency (e.g., 2fci~23MHz) and externally driven high-frequency helicon waves (f = 476MHz) using an adjustable frequency down conversion system. Importantly, this extends the application of DBS to a high-frequency spectral domain while maintaining important turbulence and flow measurement capabilities. This unique system has low phase noise, good temporal resolution (sub-millisecond) and excellent wavenumber coverage (k_θ ~ 1-20cm^{-1} and k_r ~ 20-30cm^{-1}). As a demonstration, localized internal DIII-D plasma measurements are presented from turbulence (f <= 5MHz), Alfvenic waves (f~6.5MHz), ion cyclotron waves (f >= 20MHz) as well as fluctuations around 476MHz driven by an external high-power 476 MHz helicon wave antenna. In the future, helicon measurements will be used to validate GENRAY and AORSA modeling tools for prediction of helicon wave propagation, absorption and current drive location for the newly installed helicon current drive system on DIII-D.

physics.plasm-ph↗

arXiv2022

Exchange bias in Sm$ _{2} $NiMnO$ _{6}$/BaTiO$ _{3}$ ferromagnetic-diamagnetic heterostructure thin films

Exchange bias (EB) shifts are commonly reported for the ferromagnetic (FM)/antiferromagnetic (AFM) bilayer systems. While stoichiometric ordered Sm$_{2}$NiMnO$_{6}$ (SNMO) and BaTiO$_{3}$ (BTO) are known to possesses FM and diamagnetic orderings respectively, here we have demonstrated the cooling field dependent EB and training effects in epitaxial SNMO/BTO/SNMO (SBS) heterostructure thin films. The polarized Raman spectroscopy and magnetometric studies reveal the presence of anti-site cation disorders in background of ordered lattice in SNMO layers, which introduces Ni-O-Ni or Mn-O-Mn local AFM interactions in long range Ni-O-Mn FM ordered host matrix. We have also presented growth direction manipulation of the degree of cation disorders in the SNMO system. Polarization dependent X-ray absorption measurements, duly combined with configuration interaction simulations suggest charge transfer from Ni/Mn 3\textit{d} to Ti 3\textit{d} orbitals through O 2\textit{p} orbitals across the SNMO/BTO (SB) interfaces, which can induce magnetism in the BTO spacer layer. The observed exchange bias in SBS heterostructures is discussed considering the pinning of moments due to exchange coupling at SB (or BTO/SNMO) sandwich interface.

cond-mat.mtrl-sci↗

arXiv2022

Mapping the magnetic state as a function of anti-site disorder in Sm$ _{2} $NiMnO$ _{6} $ double perovskite thin films

The predictability of any characteristic functional aspect in a double perovskite system has always been compromised by its strong dependence over the inevitably present anti-site disorders (ASD). Here, we aim to precisely map the quantitative and qualitative nature of ASD with the corresponding modifications in observables describing the magnetic and electronic state in epitaxial Sm$ _{2} $NiMnO$ _{6} $ (SNMO) double perovskite thin films. The concentration and distribution patterns of ASD are effectively controlled by optimizing growth conditions and estimated on both local and global scales utilizing extended X-ray absorption fine structure and bulk magnetometry. Depending upon the defect densities, the nature of disorder distribution can vary from homogeneous to partially segregated patches. Primarily, the effect of varying B-site cationic arrangement in SNMO is reflected as the competition of long range ferromagnetic (FM) and short scale antiferromagnetic (AFM) interactions originated from ordered Ni-O-Mn and disordered Ni-O-Ni or Mn-O-Mn bonds, respectively, which leads to systematic shift in magnetic transition temperature and drastic drop in saturation magnetization. In addition, we have observed that the gradual increment in density of ASD leads to significant deviation from uniaxial anisotropy character, reduction in anisotropy energy and enhancement of moment pinning efficiency. However, the observed signatures of $ Ni^{2+}+Mn^{4+} \longrightarrow Ni^{3+}+Mn^{3+} $ charge disproportionation is found to be independent of cation disorder densities. This work serves as a basic route-map to tune the characteristic magnetic anisotropy, magnetic phase transitions, and magnetization reversal mechanism by controlling ASD in a general double perovskite system.

cond-mat.mtrl-sci↗

arXiv2021

Study of Chemically Peculiar Stars-I : High-resolution Spectroscopy and K2 Photometry of Am Stars in the Region of M44

We present a study based on the high-resolution spectroscopy and K2 space photometry of five chemically peculiar stars in the region of the open cluster M44. The analysis of the high-precision photometric K2 data reveals that the light variations in HD 73045 and HD 76310 are rotational in nature and caused by spots or cloud-like co-rotating structures, which are non-stationary and short-lived. The time-resolved radial velocity measurements, in combination with the K2 photometry, confirm that HD 73045 does not show any periodic variability on timescales shorter than 1.3 d, contrary to previous reports in the literature. In addition to these new rotational variables, we discovered a new heartbeat system, HD 73619, where no pulsational signatures are seen. The spectroscopic and spectropolarimetric analyses indicate that HD 73619 belongs to the peculiar Am class, with either a weak or no magnetic field considering the 200 G detection limit of our study. The Least-Squares Deconvolution (LSD) profiles for HD 76310 indicate a complex structure in its spectra suggesting that this star is either part of a binary system or surrounded by a cloud shell. When placed in the Hertzsprung-Russell diagram, all studied stars are evolved from main-sequence and situated in the $δ$ Scuti instability strip. The present work is relevant for further detailed studies of CP stars, such as inhomogeneities (including spots) in the absence of magnetic fields and the origin of the pulsational variability in heartbeat systems.

astro-ph.SR↗

arXiv2021

Proton light yield of fast plastic scintillators for neutron imaging

Plastic organic scintillators have been tailored in composition to achieve ultra-fast temporal response, thereby enabling the design and development of fast neutron detection systems with high timing resolution. Eljen Technology's plastic organic scintillators -- EJ-230, EJ-232, and EJ-232Q -- are prospective candidates for use in emerging neutron imaging systems, where fast timing is paramount. To support the neutron response characterization of these materials, the relative proton light yields of EJ-230, EJ-232, and EJ-232Q were measured at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Using a broad-spectrum neutron source and a double time-of-flight technique, the proton light yield relations were obtained over a proton recoil energy range of approximately 300 keV to 4 MeV. The EJ-230, EJ-232, and EJ-232Q scintillators exhibited similar proton light yield relations to each other as well as to other plastic scintillators with the same polymer base material. A comparison of the relative proton light yield of different sized cylindrical EJ-232 and EJ-232Q scintillators also revealed consistent results. This work provides key input data for the realistic computational modeling of neutron detection technologies employing these materials, thereby supporting new capabilities in near-field radionuclide detection for national security applications.

physics.ins-det↗

arXiv2020

Tidally Trapped Pulsations in a close binary star system discovered by TESS

It has long been suspected that tidal forces in close binary stars could modify the orientation of the pulsation axis of the constituent stars. Such stars have been searched for, but until now never detected. Here we report the discovery of tidally trapped pulsations in the ellipsoidal variable HD 74423 in TESS space photometry data. The system contains a Delta Scuti pulsator in a 1.6-d orbit, whose pulsation mode amplitude is strongly modulated at the orbital frequency, which can be explained if the pulsations have a much larger amplitude in one hemisphere of the star. We interpret this as an obliquely pulsating distorted dipole oscillation with a pulsation axis aligned with the tidal axis. This is the first time that oblique pulsation along a tidal axis has been recognized. It is unclear whether the pulsations are trapped in the hemisphere directed towards the companion or in the side facing away from it, but future spectral measurements can provide the solution. In the meantime, the single-sided pulsator HD 74423 stands out as the prototype of a new class of obliquely pulsating stars in which the interactions of stellar pulsations and tidal distortion can be studied.

astro-ph.SR↗

arXiv2019

Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr2O4 spinel thin films

Cubic spinel CoCr2O4 has attained recent attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co1-xNixCr2O4 (x = 0, 0.5) epitaxial thin films deposited on MgAl2O4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was confirmed through X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A1g and F2g modes of the CoCr2O4 with increase in the Ni content. Atomic force microscopy (AFM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature (Tc) of the CoCr2O4 in thin film grown on MgAl2O4 (100) and MgO (100) substrates were found to be 100.6 +/- 0.5 K and 93.8 +/- 0.2 K, respectively. With Ni substitution the transition temperatures significantly get enhanced from that of CoCr2O4. X-ray photoelectron spectroscopy (XPS) suggests Cr3+ oxidation states in the films, while Co ions are present in a mixed Co2+/Co3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.

cond-mat.mtrl-sci↗

arXiv2019

The first view of $δ$ Scuti and $γ$ Doradus stars with the TESS mission

We present the first asteroseismic results for $δ$ Scuti and $γ$ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilise the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung-Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, $γ$ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74% for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young $δ$ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler.

astro-ph.SR↗

arXiv2019

MOBSTER - II. Identification of rotationally variable A stars observed with \emph{TESS} in Sectors 1 to 4

Recently, high-precision optical 2~min cadence light curves obtained with \emph{TESS} for targets located in the mission's defined first four sectors have been released. The majority of these high-cadence and high-precision measurements currently span $\sim28\,{\rm d}$, thereby allowing periodic variability occurring on timescales $\lesssim14\,{\rm d}$ to potentially be detected. Magnetic chemically peculiar (mCP) A-type stars are well known to exhibit rotationally modulated photometric variability that is produced by inhomogeneous chemical abundance distributions in their atmospheres. While mCP stars typically exhibit rotation periods that are significantly longer than those of non-mCP stars, both populations exhibit typical periods $\lesssim10\,{\rm d}$; therefore, the early \emph{TESS} releases are suitable for searching for rotational modulation of the light curves of both mCP and non-mCP stars. We present the results of our search for A-type stars that exhibit variability in their \emph{TESS} light curves that is consistent with rotational modulation based on the first two data releases obtained from sectors 1 to 4. Our search yielded $134$ high-probability candidate rotational variables -- $60$ of which have not been previously reported. Approximately half of these stars are identified in the literature as Ap (mCP) stars. Comparisons between the subsample of high-probability candidate rotationally variable Ap stars and the subsample of stars that are not identified as Ap reveal that the latter subsample exhibits statistically (i) shorter rotation periods and (ii) significantly lower photometric amplitudes.

astro-ph.SR↗

arXiv2019

Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties (MOBSTER) - I. First-light observations of known magnetic B and A stars

In this paper we introduce the MOBSTER collaboration and lay out its scientific goals. We present first results based on the analysis of nineteen previously known magnetic O, B and A stars observed in 2-minute cadence in sectors 1 and 2 of the Transiting Exoplanet Survey Satellite (TESS) mission. We derive precise rotational periods from the newly obtained light curves and compare them to previously published values. We also discuss the overall photometric phenomenology of the known magnetic massive and intermediate-mass stars and propose an observational strategy to augment this population by taking advantage of the high-quality observations produced by TESS.

astro-ph.SR↗

arXiv2019

Rotational modulation in TESS B stars

Light curves and periodograms of 160 B stars observed by the TESS space mission and 29 main-sequence B stars from Kepler and K2 were used to classify the variability type. There are 114 main-sequence B stars in the TESS sample, of which 45 are classified as possible rotational variables. This confirms previous findings that a large fraction (about 40 percent) of A and B stars may exhibit rotational modulation. Gaia DR2 parallaxes were used to estimate luminosities, from which the radii and equatorial rotational velocities can be deduced. It is shown that observed values of the projected rotational velocities are lower than the estimated equatorial velocities for nearly all the stars, as they should be if rotation is the cause of the light variation. We conclude that a large fraction of main-sequence B stars appear to contain surface features which cannot likely be attributed to abundance patches.

astro-ph.SR↗

arXiv2015

Complex stellar system ESO65SC03: Open cluster or remnant?

We present a complete spatial and dynamical study of the poorly populated stellar system ESO65SC03. The radial distribution of the system gives a core and cluster radii of 1.10+/-0.63 arcmin and 5.36+/-0.24 arcmin, respectively. The surface number density profile (SNDP) does not show any clear enhancement of the surface stellar number density between the stars of the system and the field regions. We derive the optimum isochrone solution for a particular grid size in the colour-magnitude diagram (CMD) using the statistical cleaning procedure. Using the statistically cleaned CMDs, we find the distance modulus, (m-M)_0, and reddening, E({B-V}), of the system to be 11.8+/-0.2 mag and 0.45 mag, respectively. The mean proper motion of this system is -5.37+/-0.81 mas/yr and 0.31+/-0.40 in RA and DEC directions, respectively. The mean proper motion of this system is found to be almost similar to the field region. The mass function for the brighter stars is found to be too high for the system to be an open cluster. These combined results place constraints on whether stellar system ESO65SC03 is a possible open star cluster remnant (POCR) or an Asterism. Our understanding is that the ESO65SC03 is in a stage of POCR by loosing their main sequence stars in the dynamic evolution processes.

astro-ph.SR↗