arXiv subjects

Abhirup Datta

Publications and source records attributed to Abhirup Datta.

At least 91 records · Page 5Linked to original sources

arXiv2023

Interpreting the HI 21-cm cosmology maps through Largest Cluster Statistics -- I: Impact of the synthetic SKA1-Low observations

We analyse the evolution of the largest ionized region using the topological and morphological evolution of the redshifted 21-cm signal coming from the neutral hydrogen distribution during the different stages of reionization. For this analysis, we use the "Largest Cluster Statistics" - LCS. We mainly study the impact of the array synthesized beam on the LCS analysis of the 21-cm signal considering the upcoming low-frequency Square Kilometer Array (SKA1-Low) observations using a realistic simulation for such observation based on the 21cmE2E-pipeline using OSKAR. We find that bias in LCS estimation is introduced in synthetic observations due to the array beam. This in turn shifts the apparent percolation transition point towards the later stages of reionization. The biased estimates of LCS, occurring due to the effect of the lower resolution (lack of longer baselines) and the telescope synthesized beam will lead to a biased interpretation of the reionization history. This is important to note while interpreting any future 21-cm signal images from upcoming or future telescopes like the SKA, HERA, etc. We conclude that one may need denser $uv$-coverage at longer baselines for a better deconvolution of the array synthesized beam from the 21-cm images and a relatively unbiased estimate of LCS from such images.

astro-ph.CO↗

arXiv2023

Snapshot Averaged Matrix Pencil Method (SAM) For Direction of Arrival Estimation

The estimation of the direction of electromagnetic (EM) waves from a radio source using electrically short antennas is one of the challenging problems in the field of radio astronomy. In this paper we have developed an algorithm which performs better in direction and polarization estimations than the existing algorithms. Our proposed algorithm Snapshot Averaged Matrix Pencil Method (SAM) is a modification to the existing Matrix Pencil Method (MPM) based Direction of Arrival (DoA) algorithm. In general, MPM estimates DoA of the incoherent EM waves in the spectra using unitary transformations and least square method (LSM). Our proposed SAM modification is made in context to the proposed Space Electric and Magnetic Sensor (SEAMS) mission to study the radio universe below 16 MHz. SAM introduces a snapshot averaging method to improve the incoherent frequency estimation improving the accuracy of estimation. It can also detect polarization to differentiate between Right Hand Circular Polarlization (RHCP), Right Hand Elliptical Polarlization (RHEP), Left Hand Circular Polarlization (LHCP), Left Hand Elliptical Polarlization (LHEP) and Linear Polarlization (LP). This paper discusses the formalism of SAM and shows the initial results of a scaled version of a DoA experiment at a resonant frequency of ~72 MHz.

astro-ph.IM↗

arXiv2023

A dynamic spatial filtering approach to mitigate underestimation bias in field calibrated low-cost sensor air-pollution data

Low-cost air pollution sensors, offering hyper-local characterization of pollutant concentrations, are becoming increasingly prevalent in environmental and public health research. However, low-cost air pollution data can be noisy, biased by environmental conditions, and usually need to be field-calibrated by collocating low-cost sensors with reference-grade instruments. We show, theoretically and empirically, that the common procedure of regression-based calibration using collocated data systematically underestimates high air pollution concentrations, which are critical to diagnose from a health perspective. Current calibration practices also often fail to utilize the spatial correlation in pollutant concentrations. We propose a novel spatial filtering approach to collocation-based calibration of low-cost networks that mitigates the underestimation issue by using an inverse regression. The inverse-regression also allows for incorporating spatial correlations by a second-stage model for the true pollutant concentrations using a conditional Gaussian Process. Our approach works with one or more collocated sites in the network and is dynamic, leveraging spatial correlation with the latest available reference data. Through extensive simulations, we demonstrate how the spatial filtering substantially improves estimation of pollutant concentrations, and measures peak concentrations with greater accuracy. We apply the methodology for calibration of a low-cost PM2.5 network in Baltimore, Maryland, and diagnose air pollution peaks that are missed by the regression-calibration.

stat.AP↗

arXiv2023

An ANN Approach in Predicting Solar and Geophysical Indices from Ionospheric TEC Over Indore

In this paper, preliminary results from the artificial neural network (ANN) based model developed at IIT Indore has been presented. One year's hourly total electron content (TEC) database has been created from the International Reference Ionosphere (IRI) 2016 model. For the first time, a reverse problem has been addressed, wherein the training has been performed for predicting the three indices: 13-month running sunspot number, ionospheric index, and daily solar radio flux also called targets to the network when hourly TEC values are the inputs. The root mean square errors (RMSEs) of these targets have been compared and minimized after several training of the dataset using different sets of combinations. Unknown data fed to the network yielded 0.99%, 3.12%, and 0.90% errors for Rz12, IG12, and F10.7 radio flux, respectively, thus signifying ~97% prediction accuracy of the model.

physics.space-ph↗

arXiv2023

Impact of Intense Geomagnetic Storm on NavIC Signals Over Indore

Intense geomagnetic storms can have a strong impact on the signals (termed ionospheric scintillations) emitted by any global navigation satellite system (GNSS). The paper reports the first studies of scintillations at the Indore region on the NavIC signals due to the impact of the intense geomagnetic storm event reported on September 8, 2017, at 0151 and 1304 UT. The variation of the planetary indices as well as the DST index which dropped to a value of -124 nT on September 8, 2017, indicates the occurrence of an intense geomagnetic storm on September 8, 2017. The observations presented are carried out at Indore, which is located at the equatorial anomaly crest. The S4 index measurements of colocated GNSS receiver showed values of 0.5 or above on the disturbed day between 15 and 18 UT. The analysis presented clearly signifies the degradation of the carrier to noise measurements of the NavIC L5 signal during the same time, which in turn affected the positional accuracy of NavIC, an important consideration for performance.

physics.space-ph↗

arXiv2023

Study of Low-latitude Ionospheric Scintillation using NavIC

Equatorial ionospheric irregularities have been studied in the past and have produced interesting insights into ionospheric physics and processes. Here, we present the initial results of a long-term study of the ionosphere near the Equatorial Ionization Anomaly (EIA) using Navigation with the Indian Constellation (NavIC). We have characterized the ionospheric irregularities in terms of the power spectral density at different dynamical frequencies. The formalism is similar to as suggested by earlier works using the phase screen modeling of the ionosphere. The observations of the C/N 0 (dB-Hz) variation have been taken by utilizing the L5 (1176.45 MHz) signal of NavIC over Indore located near the northern crest of EIA. We show some initial results as a proof of concept study from a single day (December 4, 2017) of scintillation observations. This is a first-of-its-kind study in this region with NavIC. From the power spectral density analysis, we have demonstrated that NavIC is capable of detecting such irregularities over long periods over this region and has implications for forecasting such events in the future.

physics.space-ph↗

arXiv2023

First results of Low-latitude Ionospheric Irregularities measured by NavIC and GPS near the Anomaly Crest and the Magnetic Equator

Ionospheric irregularity studies are important aspects for understanding ionospheric physics and related processes, especially near the low-latitude regions. However, simultaneous measurements (utilizing the L-band signals of NavIC and GPS) of irregularity scale sizes over the Indian longitude sector, has not been addressed extensively. To address this problem, the paper presents simultaneous characterization of low-latitude ionospheric irregularities over a location near the northern EIA crest (Indore: 22.52$^\circ$N, 75.92$^\circ$E geographic and magnetic dip of 32.23$^\circ$N) and a location (Hyderabad: 17.41$^\circ$N, 78.55$^\circ$E geographic and magnetic dip of 21.69$^\circ$N) between the crest and the magnetic equator, utilizing the Indian navigation system, NavIC and GPS L5 signal C/N$_o$ variations to determine the range of the ionospheric irregularity scale sizes using Power Spectral Density (PSD) analysis. The study period spans from September 2017- September 2019, covering both disturbed and quiet-time conditions in the declining phase of solar cycle 24. Observations show that the irregularity scale size ranges from about 500 m to 6 km. This study for the first time, shows the nature of the temporal PSD for ionospheric scintillation during varying solar and geophysical conditions, by measuring the irregularity scale sizes utilizing simultaneous observations from NavIC and GPS from locations near the northern crest of the EIA and in between crest and the magnetic equator, ensuring proper characterization of ionosphere over the geosensitive Indian subcontinent.

physics.space-ph↗

arXiv2023

Towards $21$-cm intensity mapping at $z=2.28$ with uGMRT using the tapered gridded estimator II: Cross-polarization power spectrum

Neutral hydrogen ($\rm{HI}$) $21$-cm intensity mapping (IM) offers an efficient technique for mapping the large-scale structures in the universe. We introduce the 'Cross' Tapered Gridded Estimator (Cross TGE), which cross-correlates two cross-polarizations (RR and LL) to estimate the multi-frequency angular power spectrum (MAPS) $C_{\ell}(Δν)$. We expect this to mitigate several effects like noise bias, calibration errors etc., which affect the 'Total' TGE which combines the two polarizations. Here we apply the Cross TGE on a $24.4 \,\rm{MHz}$ bandwidth uGMRT Band $3$ data centred at $432.8 \,\rm{MHz}$ aiming $\rm{HI}$ IM at $z=2.28$. The measured $C_{\ell}(Δν)$ is modelled to yield maximum likelihood estimates of the foregrounds and the spherical power spectrum $P(k)$ in several $k$ bins. Considering the mean squared brightness temperature fluctuations, we report a $2σ$ upper limit $Δ_{UL}^{2}(k) \le (58.67)^{2} \, {\rm mK}^{2}$ at $k=0.804 \, {\rm Mpc}^{-1}$ which is a factor of $5.2$ improvement on our previous estimate based on the Total TGE. Assuming that the $\rm{HI}$ traces the underlying matter distribution, we have modelled $C_{\ell}(Δν)$ to simultaneously estimate the foregrounds and $[Ω_{\rm{HI}} b_{\rm{HI}}] $ where $Ω_{\rm{HI}}$ and $b_{\rm{HI}}$ are the $\rm{HI}$ density and linear bias parameters respectively. We obtain a best fit value of $[Ω_{\rm{HI}}b_{\rm{HI}}]^2 = 7.51\times 10^{-4} \pm 1.47\times 10^{-3}$ which is consistent with noise. Although the $2σ$ upper limit $[Ω_{\rm{HI}}b_{\rm{HI}}]_{UL} \leq 0.061$ is $\sim 50$ times larger than the expected value, this is a considerable improvement over earlier works at this redshift.

astro-ph.CO↗

arXiv2022

Exploring Earth's Ionosphere and its effect on low radio frequency observation with the uGMRT and the SKA

The Earth's ionosphere introduces systematic effects that limit the performance of a radio interferometer at low frequencies ($\lesssim 1$\,GHz). These effects become more pronounced for severe geomagnetic activities or observations involving longer baselines of the interferometer. The uGMRT, a pathfinder for the Square Kilometre Array (SKA), is located in between the northern crest of the Equatorial Ionisation Anomaly (EIA) and the magnetic equator. Hence, this telescope is more prone to severe ionospheric conditions and is a unique radio interferometer for studying the ionosphere. Here, we present 235\,MHz observations with the GMRT, showing significant ionospheric activities over a solar minimum. In this work, we have characterised the ionospheric disturbances observed with the GMRT and compared them with ionospheric studies and observations with other telescopes like the VLA, MWA and LOFAR situated at different magnetic latitudes. We have estimated the ionospheric total electron content (TEC) gradient over the full GMRT array which shows an order of magnitude higher sensitivity compared to the Global Navigation Satellite System (GNSS). Furthermore, this article uses the ionospheric characteristics estimated from the observations with uGMRT, VLA, LOFAR and MWA to forecast the effects on the low-frequency observations with the SKA1-MID and SKA1-LOW in future.

astro-ph.EP↗

arXiv2022

Synthetic Observations with the Square Kilometre Array (SKA) -- development towards an end-to-end pipeline

Detection of the redshifted 21-cm signal of neutral hydrogen from the Cosmic Dawn and the Epoch of Reionization is one of the final frontiers of modern observational cosmology. The inherently faint signal makes it susceptible to contamination by several sources like astrophysical foregrounds and instrumental systematics. Nevertheless, developments achieved in the recent times will combine to make signal detection possible with the upcoming Square Kilometer Array (SKA), both statistically and via tomography. This review describes an indigenously developed end-to-end pipeline that simulates sensitive interferometric observations. It mainly focuses on the requirements for \hi detection in interferometers. In its present form, it can mimic the effects of realistic point source foregrounds and systematics- calibration error and position error on 21-cm observations. The performance of the pipeline is demonstrated for test cases with 0.01\% calibration error and position error. Its performance is consistent across telescope, foreground, and signal models. The focus of the simulation pipeline during the initial stages was for EoR science. But since this is a general interferometric simulation pipeline, it will be helpful to the entire SKA user community, irrespective of the science goals.

astro-ph.IM↗

arXiv2022

Exploring diffuse radio emission in galaxy clusters and groups with the uGMRT and the SKA

Diffuse radio emission has been detected in a considerable number of galaxy clusters and groups, revealing the presence of pervasive cosmic magnetic fields, and of relativistic particles in the large-scale structure (LSS) of the Universe. Since cluster radio emission is faint and steep spectrum, its observations are largely limited by the instrument sensitivity and frequency of observation, leading to a dearth of information, more so for lower-mass systems. The unprecedented sensitivity of recently commissioned low-frequency radio telescope arrays, aided by the development of advanced calibration and imaging techniques, have helped in achieving unparalleled image quality. At the same time, the development of sophisticated numerical simulations and the availability of supercomputing facilities have paved the way for high-resolution numerical modeling of radio emission, and the structure of the cosmic magnetic fields in LSS, leading to predictions matching the capabilities of observational facilities. In view of these rapidly-evolving scenerio in modeling and observations, in this review, we summarise the role of the new telescope arrays and the development of advanced imaging techniques and discuss the detections of various kinds of cluster radio sources. In particular, we discuss observations of the cosmic web in the form of supercluster filaments, studies of emission in poor clusters and groups of galaxies, and of ultra-steep spectrum sources. We also review the current theoretical understanding of various diffuse cluster radio sources and the associated magnetic field and polarization. As the statistics of detections improve along with our theoretical understanding, we update the source classification schemes based on their intrinsic properties. We conclude by summarising the role of the upgraded GMRT and our expectations from the upcoming Square Kilometre Array (SKA) observatories.

astro-ph.CO↗

arXiv2022

Plans for building a prototype SKA regional centre in India

In order to deliver the full science potential of the Square Kilometer Array (SKA) telescope, several SKA Regional Centres (SRCs) will be required to be constructed in different SKA member countries around the world. These SRCs will provide high performance compute and storage for the generation of advanced science data products from the basic data streams generated by the SKA Science Data Handling and Processing system, critically necessary to the success of the key science projects to be carried out by the SKA user community. They will also provide support to astronomers to enable them to carry out analysis on very large SKA datasets. Construction of such large data centres is a technical challenge for all SKA member nations. In such a situation, each country plans to construct a smaller SRC over the next few years (2022 onwards), known as a proto-SRC. In India, we propose to construct a proto-SRC which will be used for the analysis of data from SKA pathfinders and precursors with strong Indian involvement such as uGMRT, Meerkat and MWA. We describe our thinking on some aspects of the the storage, compute and network of the proto-SRC and how it will be used for data analysis as well as for carrying out various simulations related to SKA key science projects led by Indian astronomers. We also present our thoughts on how the proto-SRC plans to evaluate emerging hardware and software technologies and to also begin software development in areas of relevance to SKA data processing and analysis such as algorithm implementation, pipeline development and data visualisation software.

astro-ph.IM↗

arXiv2022

Improved fMRI-based Pain Prediction using Bayesian Group-wise Functional Registration

In recent years, neuroimaging has undergone a paradigm shift, moving away from the traditional brain mapping approach toward developing integrated, multivariate brain models that can predict categories of mental events. However, large interindividual differences in brain anatomy and functional localization after standard anatomical alignment remain a major limitation in performing this analysis, as it leads to feature misalignment across subjects in subsequent predictive models.

stat.AP↗

arXiv2022

Systematic study of ionospheric scintillation over the indian low-latitudes during low solar activity conditions

A systematic study of ionospheric scintillation at the low-latitudes, especially around the Equatorial Ionization Anomaly (EIA) and the magnetic equator, is essential in understanding the dynamics of ionospheric variation and related physical processes. Our study involves NavIC $S_{4_C}$ observations over Indore and Hyderabad. Additionally, GPS $S_{4_C}$ observations over Indore were analyzed, under disturbed as well as quiet time ionospheric conditions from September 2017 through 2019, falling in the declining phase of the solar cycle 24. The $S_{4_C}$ observations were further analyzed using proxy parameters: ROT and ROTI. These results have been obtained from three satellites of the NavIC constellation (PRNs 2, 5, and 6). The onset times of scintillations \textbf{were} observed to be around 19:30 LT (h) and 20:30 LT (h) for Hyderabad and Indore respectively, while the $S_{4_C}$ peak values occurred between 22:00 LT (h) and 23:00 LT (h). The reliability of NavIC was evaluated using scattering coefficients that revealed a good correlation across the pair of signals during quiet time ionospheric conditions. The observations clearly show that the amplitude scintillation of the NavIC signal follows the Nakagami-m distribution along with the $α-μ$ distribution as a depiction of the deep power fades caused by scintillation on these signals. This paper shows the impact of such systematic studies near these locations for the first time, in improving the understanding of the dynamic nature of low-latitude ionosphere under low solar activity conditions.

physics.space-ph↗

arXiv2022

Towards 21-cm Intensity Mapping at $z=2.28$ with uGMRT using the Tapered Gridded Estimator I: Foreground Avoidance

The post-reionization $(z \le 6)$ neutral hydrogen (HI) 21-cm intensity mapping signal holds the potential to probe the large scale structures, study the expansion history and constrain various cosmological parameters. Here we apply the Tapered Gridded Estimator (TGE) to estimate $P(k_{\perp},k_{\parallel})$ the power spectrum of the $z = 2.28$ $(432.8\, {\rm MHz})$ redshifted 21-cm signal using a $24.4\,{\rm MHz}$ sub-band drawn from uGMRT Band 3 observations of European Large-Area ISO Survey-North 1 (ELAIS-N1). The TGE allows us to taper the sky response which suppresses the foreground contribution from sources in the periphery of the telescope's field of view. We apply the TGE on the measured visibility data to estimate the multi-frequency angular power spectrum (MAPS) $C_{\ell}(Δν)$ from which we determine $P(k_{\perp},k_{\parallel})$ using maximum-likelihood which naturally overcomes the issue of missing frequency channels (55 \% here). The entire methodology is validated using simulations. For the data, using the foreground avoidance technique, we obtain a $2\,σ$ upper limit of $Δ^2(k) \le (133.97)^2 \, {\rm mK}^{2}$ for the 21-cm brightness temperature fluctuation at $k = 0.347 \, \textrm{Mpc}^{-1}$. This corresponds to $[Ω_{\rm HI}b_{\rm HI}] \le 0.23$, where $Ω_{\rm HI}$ and $b_{\rm HI}$ respectively denote the cosmic \HI mass density and the \HI bias parameter. A previous work has analyzed $8 \, {\rm MHz}$ of the same data at $z=2.19$, and reported $Δ^{2}(k) \le (61.49)^{2} \, {\rm mK}^{2}$ and $[Ω_{\rm HI} b_{\rm HI}] \le 0.11$ at $k=1 \, {\rm Mpc}^{-1}$. The upper limits presented here are still orders of magnitude larger than the expected signal corresponding to $Ω_{\rm HI} \sim 10^{-3}$ and $b_{\rm HI} \sim 2 $.

astro-ph.CO↗

arXiv2022

A study on the Clustering Properties of Radio-Selected sources in the Lockman Hole Region at 325 MHz

Studying the spatial distribution of extragalactic source populations is vital in understanding the matter distribution in the Universe. It also enables understanding the cosmological evolution of dark matter density fields and the relationship between dark matter and luminous matter. Clustering studies are also required for EoR foreground studies since it affects the relevant angular scales. This paper investigates the angular and spatial clustering properties and the bias parameter of radio-selected sources in the Lockman Hole field at 325 MHz. The data probes sources with fluxes $\gtrsim$0.3 mJy within a radius of 1.8$^\circ$ around the phase center of a $6^\circ \times 6^\circ$ mosaic. Based on their radio luminosity, the sources are classified into Active Galactic Nuclei (AGNs) and Star-Forming Galaxies (SFGs). Clustering and bias parameters are determined for the combined populations and the classified sources. The spatial correlation length and the bias of AGNs are greater than SFGs -- indicating that more massive haloes host the former. This study is the first reported estimate of the clustering property of sources at 325 MHz, intermediate between the preexisting studies at high and low-frequency bands. It also probes a well-studied deep field at an unexplored frequency with moderate depth and area. Clustering studies require such observations along different lines of sight, with various fields and data sets across frequencies to avoid cosmic variance and systematics. Thus, an extragalactic deep field has been studied in this work to contribute to this knowledge.

astro-ph.CO↗

arXiv2022

Magnetic field measurement in TMC-1C using 22.3 GHz CCS Zeeman splitting

Measurement of magnetic fields in dense molecular clouds is essential for understanding the fragmentation process prior to star formation. Radio interferometric observations of CCS 22.3 GHz emission, from the starless core TMC-1C, have been carried out with the Karl G. Jansky Very Large Array to search for Zeeman splitting of the line in order to constrain the magnetic field strength. Toward a region offset from the dust peak, we report a detection of the Zeeman splitting of the CCS 2_1 - 1_0 transition, with an inferred magnetic field of ~2 mG. If we interpret the dust peak to be the core of TMC-1C, and the region where we have made a detection of the magnetic field to be the envelope, then our observed value for the magnetic field is consistent with a subcritical mass-to-flux ratio envelope around a core with supercritical mass-to-flux ratio. The ambipolar diffusion timescale for the formation of the core is consistent with the relevant timescale based on chemical modeling of the TMC-1C core. This work demonstrates the potential of deep CCS observation to carry out future measurements of magnetic field strengths in dense molecular clouds and, in turn, understand the role of the magnetic field in star formation.

astro-ph.GA↗

arXiv2022

Observing the Reionization : Effect of Calibration and Position Errors on Realistic Observation Conditions

Observation of the redshifted 21-cm signal from Cosmic Dawn and Epoch of Reionization is a challenging endeavor in observational cosmology. Presence of orders of magnitude brighter astrophysical foregrounds and various instrumental systematics increases the complexity of these observations. This work presents an end-to-end pipeline dealing with synthetic interferometric data of sensitive radio observations . The mock sky model includes the redshifted 21-cm signal and astrophysical foregrounds. The effects of calibration error and position error in the extraction of the redshifted 21-cm power spectrum has been simulated. The effect of the errors in the image plane detection of the cosmological signal has also been studied. A comparative analysis for array configurations like the SKA1-Low, MWA and HERA has been demonstrated. The calibration error tolerance of the arrays, under some assumptions about the nature of the systematic components, is optimally found to be $\sim 0.01\%$ for the detection of the signal. For position errors, an offset of $\gtrapprox 5\arcsec$ makes the residual foregrounds obscure the target signal. These simulations also imply that in the SKA-1 Low performs marginally better than the others in the image domain, while the same is true for MWA in the power spectrum domain. This is one of the first studies that compares performance of various radio telescopes operating under similar observing conditions towards detecting the cosmological signal. This end-to-end pipeline can also be extended to study effects of chromatic primary beam, radio frequency inferences, foregrounds with spectral features, etc.

astro-ph.CO↗