Articles written in Journal of Astrophysics and Astronomy

    • Prospects for Detecting the 326.5MHz Redshifted 21-cm HI Signal with the Ooty Radio Telescope (ORT)

      Sk. Saiyad Ali Somnath Bharadwaj

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      Observations of the redshifted 21-cm HI fluctuations promise to be an important probe of the post-reionization era (𝑧 ≤ 6). In this paper we calculate the expected signal and foregrounds for the upgraded Ooty Radio Telescope (ORT) which operates at frequency 𝑣𝑜 = 326.5MHz which corresponds to redshift 𝑧 = 3.35. Assuming that the visibilities contain only the HI signal and system noise, we show that a 3𝜎 detection of the HI signal (∼ 1 mK) is possible at angular scales 11' to 3° with ≈ 1000 h of observation. Foreground removal is one of the major challenges for a statistical detection of the redshifted 21 cm HI signal. We assess the contribution of different foregrounds and find that the 326.5MHz sky is dominated by the extragalactic point sources at the angular scales of our interest. The expected total foregrounds are 104−105 times higher than the HI signal.

    • Fisher Matrix Predictions for Detecting the Cosmological 21-cm Signal with the Ooty Wide Field Array (OWFA)

      S. Bharadwaj A. K. Sarkar Sk. Saiyad Ali

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      We have used the Fisher matrix formalism to quantify the prospects of detecting the 𝑧 = 3.35 redshifted 21-cm HI power spectrum with the upcoming radio-imterferometric array OWFA. OWFA’s frequency and baseline coverage spans comoving Fourier modes in the range 1.8 × 10−2 ≤ k ≤ 2.7 Mpc−1. The OWFA HI signal, however, is predominantly from the range 𝑘 ≤ 0.2 Mpc−1. The larger modes, though abundant, do not contribute much to the HI signal. In this work, we have focused on combining the entire signal to achieve a detection. We find that a 5 - 𝜎 detection of 𝐴HI is possible with ∼ 150 h of observations, here $A^{2}_{\text{HI}}$ is the amplitude of the HI power spectrum. We have also carried out a joint analysis for 𝐴HI and 𝛽 the redshift space distortion parameter. Our study shows that OWFA is very sensitive to the amplitude of the HI power spectrum. However, the anisotropic distribution of the k modes does not make it very suitable for measuring 𝛽.

    • Line-of-Sight Anisotropies in the Cosmic Dawn and Epoch of Reionization 21-cm Power Spectrum

      Suman Majumdar Kanan K. Datta Raghunath Ghara Rajesh Mondal T. Roy Choudhury Somnath Bharadwaj Sk. Saiyad Ali Abhirup Datta

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      The line-of-sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reionization is quite unique in many ways compared to any other cosmological signal. Different unique effects, such as the evolution history of the signal, non-linear peculiar velocities of the matter etc. will imprint their signature along the line-of-sight axis of the observed signal. One of the major goals of the future SKA-LOW radio interferometer is to observe the cosmic dawn and the epoch of reionization through this 21-cm signal. It is thus important to understand how these various effects affect the signal for its actual detection and proper interpretation. For more than one and half decades, various groups in India have been actively trying to understand and quantify the different line-of-sight effects that are present in this signal through analytical models and simulations. In many ways the importance of this sub-field under 21-cm cosmology have been identified, highlighted and pushed forward by the Indian community. In this article, we briefly describe their contribution and implication of these effects in the context of the future surveys of the cosmic dawn and the epoch of reionization that will be conducted by the SKA-LOW.

    • Prospects of Measuring the Angular Power Spectrum of the Diffuse Galactic Synchrotron Emission with SKA1 Low

      Sk. Saiyad Ali Somnath Bharadwaj Samir Choudhuri Abhik Ghosh Nirupam Roy

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      The Diffuse Galactic Syncrotron Emission (DGSE) is the most important diffuse foreground component for future cosmological 21-cm observations. The DGSE is also an important probe of the cosmic ray electron and magnetic field distributions in the turbulent interstellar medium (ISM) of our galaxy. In this paper we briefly review the Tapered Gridded Estimator (TGE) which can be used to quantify the angular power spectrum $C_\ell$ of the sky signal directly from the visibilities measured in radio-interferometric observations. The salient features of the TGE are: (1) it deals with the gridded data which makes it computationally very fast, (2) it avoids a positive noise bias which normally arises from the system noise inherent to the visibility data, and (3) it allows us to taper the sky response and thereby suppresses the contribution from unsubtracted point sources in the outer parts and the side lobes of the antenna beam pattern. We also summarize earlier work where the TGE was used to measure the $C_\ell$ of the DGSE using 150 MHz GMRT data. Earlier measurements of $C_\ell$ are restricted to $\ell \le \ell _{\max } \sim 10^{3}$ for the DGSE, the signal at the larger $\ell$ values is dominated by the residual point sources after source subtraction. The higher sensitivity of the upcoming SKA1 Low will allow the point sources to be subtracted to a fainter level than possible with existing telescopes. We predict that it will be possible to measure the $C_\ell$ of the DGSE to larger values of $\ell _{\max }$ with SKA1 Low. Our results show that it should be possible to achieve $\ell _{\max }\sim 10^{4}$ and $∼10^5$ with 2 minutes and 10 hours of observations respectively.

    • Fisher Matrix-based Predictions for Measuring the z = 3.35 Binned 21-cm Power Spectrum using the Ooty Wide Field Array (OWFA)

      Anjan Kumar Sarkar Somnath Bharadwaj Sk. Saiyad Ali

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      We use the Fisher matrix formalism to predict the prospects of measuring the redshifted 21-cm power spectrum in different $k$-bins using observations with the upcoming Ooty Wide Field Array (OWFA) which will operate at $326.5 {\rm MHZ}$. This corresponds to neutral hydrogen (HI) at $z=3.35$, and a measurement of the 21-cm power spectrum provides a unique method to probe the large-scale structures at this redshift. Our analysis indicates that a $5 \sigma$ detection of the binned power spectrum is possible in the $k$ range $0.05 \leq k \leq 0.3 \, {\rm Mpc}^{-1}$ with $1000$ hours of observation. We find that the signal- to-noise ratio (${\rm SNR}$) peaks in the $k$ range $0.1- 0.2\, {\rm Mpc}^{-1}$ where a $10 \sigma$ detection is possible with $2000$ hours of observations. Our analysis also indicates that it is not very advantageous to observe beyond $1000$ h in a single field-of-view as the (${\rm SNR}$) increases rather slowly beyond this in many of the small $k$-bins. The entire analysis reported here assumes that the foregrounds have been completely removed.

    • Probing early Universe through redshifted 21-cm signal: Modeling and observational challenges


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      Cosmic dawn (CD) and the epoch of reionization (EoR) are the most important parts of cosmic history during which the first luminous structures emerged. These first objects heated and ionized the neutral atomic hydrogen in the intergalactic medium. The redshifted 21-cm radiation from the atomic hydrogen providesan excellent direct probe to study the evolution of neutral hydrogen (Hi) and thus reveal nature of the first luminous objects, their evolution and role in this last phase transition of the Universe and formation and evolution of the structures thereafter. Direct mapping of the Hi density during the CD–EoR is rather difficult with thecurrent and forthcoming instruments due to stronger foreground and other observational contamination. The first detection of this redshifted Hi signal is expected to be done through statistical estimators. Given the upmost importance of the detection and analysis of the redshifted 21-cm signal, physics of CD–EoR is considered asone of the objective of the upcoming SKA-low telescope. This paper summarizes the collective effort of Indianastronomers to understand the origin of the redshifted 21-cm signal, sources of first ionizing photons, their propagation through the IGM, various cosmological effects on the expected 21-cm signal, various statistical measures of the signal like power spectrum, bispectrum, etc. A collective effort on detection of such signalby developing e stimators of the statistical measures with rigorous assessment of their expected uncertainties, various challenges like that of the large foreground emission and calibration issues are also discussed. Various versions of the detection methods discussed here have also been used in practice with the Giant MeterwaveRadio Telescope with successful assessment of the foreground contamination and upper limits on the matter density in reionization and post-reionization era. The collective efforts compiled here has been a large part of the global effort to prepare proper observational technique, analysis procedure for the first light of the CD–EoR through the SKA-low.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

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      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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