Abstract Details
| Name: Saurabh Singh Affiliation: Raman Research Institute Conference ID: ASI2016_610 Title : On the Detection of Global 21-cm signal from Reionization using Interferometers Authors and Co-Authors : Ravi Subrahmanyan, N. Udaya Shankar, A. Raghunathan Affiliation: Raman Research Institute Abstract Type : Oral Abstract Category : Instrumentation and Techniques Abstract : Epoch of Reionization (EoR) marks the transition of baryons in the Universe from neutral to almost completely ionized. This is the period in the cosmic timeline when first sources of radiation formed and through the combination of various physical processes, reionized the Universe. Unfortunately, the details of these processes as well as the nature of ionizing sources are poorly constrained. Thus it is right time to look for probes that enhance our understanding about this crucial epoch. Global redshifted 21-cm signal, also known as monopole component, from atomic hydrogen is potentially the richest probe to study EoR and preceding Dark Ages. It promises to better constrain nature of the first sources of radiation as well as thermal evolution of Inter-Galactic Medium. However, detection of this faint monopole component is challenging due to high precision required in instrumental calibration and modeling of substantially brighter foregrounds and instrumental systematics. In addition, ongoing experiments aiming for the detection of the signal use total power measurements. Such class of measurements suffer from receiver noise which, on an average, can be 10^4 times higher than signal of interest. In particular, modeling of receiver noise with mK accuracy and its separation remains a formidable task for such experiments. Interferometers do not respond to receiver noise; therefore, we explore here the theory of the response of interferometers to global signals. In other words, we discuss the spatial coherence in the electric field arising from the monopole component of the 21-cm signal and methods for its detection using sensor arrays. We proceed by first showing that interferometers do respond to monopole component of the signal, contrary to the general belief. We study the response to uniform sky of two-element interferometers made of unit dipole and resonant loop antennas, then extend the analysis to interferometers made of 1-D arrays and also consider 2-D aperture antennas. We discuss the configurations that are conducive to the detection of monopole component of the signal. We observe that the maximum response, with most favorable configuration, is 1/5th of the total power measurement. This requires that integration time for such configurations be 25 times more than that of total power measurement. Hence we describe methods by which the coherence might be enhanced so that the interferometer measurements yield improved sensitivity to the monopole component. We conclude that (a) it is indeed possible to measure the global 21-cm from EoR using interferometers,(b) a practically useful configuration is with omnidirectional antennas as interferometer elements, and (c) that the spatial coherence may be enhanced using, for example, a space beam splitter between the interferometer elements. |