Browsing by Author "Choudhuri, Samir"

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    A measurement of Galactic synchrotron emission using MWA drift scan observations
    (Cambridge University Press, 2025) Chatterjee, Suman; Sarkar, Shouvik; Choudhuri, Samir
    Studying the diffuse Galactic synchrotron emission (hereafter, DGSE) at arc-minute angular scale is important to remove the foregrounds for the cosmological 21-cm observations. Statistical measurements of the large-scale DGSE can also be used to constrain the magnetic field and the cosmic ray electron density of our Galaxy's interstellar medium (ISM). Here, we have used the Murchison Widefield Array (MWA) drift scan observations at$154.2 \, {\rm MHz}$to measure the angular power spectrum$({\cal C}_{\ell})$of the DGSE of a region of the sky from right ascension (RA)$349^{\circ}$to$70.3^{\circ}$at the fixed declination$-26.7^{\circ}$ . In this RA range, we have chosen 24 pointing centers (PCs), for which we have removed all the bright point sources above$\sim430 \, {\rm mJy}\,(3σ)$ , and applied the Tapered Gridded Estimator (TGE) on residual data to estimate the${\cal C}_{\ell}$ . We use the angular multipole range$65 \le \ell \le 650$to fit the data with a model,${\cal C}^M_{\ell}=A\times \left(\frac{1000}{\ell}\right)^β+C$ , where we interpret the model as the combination of a power law$(\propto \ell^{-β})$nature of the DGSE and a constant part due to the Poisson fluctuations of the residual point sources. We are able to fit the model${\cal C}^M_{\ell}$for six PCs centered at$α=352.5^{\circ}, 353^{\circ}, 357^{\circ}, 4.5^{\circ}, 4^{\circ}$and$1^{\circ}$ . We run the Markov Chain Monte Carlo (MCMC) ensemble sampler to get the best-fit values of the parameters$A, β$and$C$for these PCs. We see that the values of$A$vary in the range$155$to$400$mK $^{2}$ , whereas the$β$varies in the range$0.9$to$1.7$ . We find that the value of$β$is consistent at$2-σ$level with the earlier measurement of the DGSE at similar frequency and angular scales
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    All-sky angular power spectrum – I. Estimating brightness temperature fluctuations using the 150-MHz TGSS survey
    (Oxford University Press, 2020) Ghosh, Abhik; Choudhuri, Samir; Roy, Nirupam
    Measurements of the Galactic synchrotron emission are important for the 21-cm studies of the epoch of reionization. The study of synchrotron emission is also useful for quantifying the fluctuations in the magnetic field and the cosmic-ray electron density of the turbulent interstellar medium (ISM) of our Galaxy. Here, we present the all-sky angular power spectrum (Cℓ) measurements of the diffuse synchrotron emission obtained using the TIFR GMRT Sky Survey (TGSS) at 150 MHz.
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    Patterns of primary beam non-redundancy in close-packed 21 cm array observations
    (Oxford University Press, 2021) Choudhuri, Samir; Bull, Philip; Garsden, Hugh
    Radio interferometer arrays such as HERA consist of many close-packed dishes arranged in a regular pattern, giving rise to a large number of ‘redundant’ baselines with the same length and orientation. Since identical baselines should see an identical sky signal, this provides a way of finding a relative gain/bandpass calibration without needing an explicit sky model. In reality, there are many reasons why baselines will not be exactly identical, giving rise to a host of effects that spoil the redundancy of the array and induce spurious structure in the calibration solutions if not accounted for. In this paper, we seek to build an understanding of how differences in the primary beam response between antennas affect redundantly calibrated interferometric visibilities and their resulting frequency (delay-space) power spectra. We use simulations to study several generic types of primary beam variation, including differences in the width of the main lobe, the angular and frequency structure of the sidelobes, and the beam ellipticity and orientation.
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    Statistical recovery of 21 cm visibilities and their power spectra with gaussian-constrained realizations and gibbs sampling
    (The Astrophysical Journal Supplement Series, 2023) Bull, Philip; Kennedy, Fraser; Wilensky, Michael J; Burba, Jacob; Choudhuri, Samir
    Radio interferometers designed to probe the 21 cm signal from Cosmic Dawn and the Epoch of Reionization must contend with systematic effects that make it difficult to achieve sufficient dynamic range to separate the 21 cm signal from foreground emission and other effects. For instance, the instrument’s chromatic response modulates the otherwise spectrally smooth foregrounds, making them difficult to model, while a significant fraction of the data must be excised due to the presence of radio-frequency interference, leaving gaps in the data. Errors in modeling the (modulated and gappy) foregrounds can easily generate spurious contamination of what should otherwise be 21 cm signal-dominated modes. Various approaches have been developed to mitigate these issues by, for example, using nonparametric reconstruction of the foregrounds, in-painting the gaps, and weighting the data to reduce the level of contamination.