Time domain astronomy at low radio frequencies (< 100 MHz): Instrumentation and observations

Abstract

Exploration of the transient Universe is an exciting and fast-emerging area within radio astronomy. Radio transient sources can be defined as a class of objects that emit radio waves in the form of bursts, flares, or pulses from short durations (less than a few seconds) to long durations (greater than a few seconds). These include Pulsars, FRBs, GRBs, Flaring Stars, Sun, etc. Transient phenomena are likely locations of explosive or dynamic events, and they offer tremendous potential to uncover new physics and astrophysics. In addition, short-duration transients are powerful probes of intervening media owing to dispersion, scattering, and Faraday rotation that modify the signals. However, observations of such transients at low radio frequencies is largely uncharted territory due to various practical limitations. A pulsar is a highly magnetized, rapidly rotating neutron star emitting electromagnetic radiation beams. Weighing more than our Sun, yet only about 20 km in diameter, these incredibly dense objects produce radio beams that sweep the sky like a lighthouse. They are detectable at radio frequencies through the beam of coherent radio emission from their magnetosphere whenever they intersect our line of sight. The first pulsar was observed at a low frequency of ∼ 81.5 MHz. However, much of our present knowledge about pulsars has come primarily from radio observations at frequencies >300 MHz. The primary reasons for this are: the emission from pulsars at low frequencies is more affected by dispersion and scattering during their propagation through the interstellar medium (ISM) as compared with higher frequencies, the comparatively intense synchrotron emission from the Galactic background and the Radio Frequency Interference (RFI). Pulsar observations at frequencies ≤100 MHz are necessary for understanding the emission mechanism and characteristics of the pulse profile as a function of frequency, which are still being examined. For example, many pulsars exhibit a turnover in the spectrum close to 100 MHz. Therefore, extending their spectra toward lower radio frequencies is of great interest. Low-frequency observations can also help in studying the ISM as the propagation effects of ISM are more prominent at low radio frequencies. Fast Radio Bursts (FRBs) are still eluding astronomers about their origin. Recently, FRBs have been detected at 120 MHz and 111 MHz. This suggests that detecting them at even lower frequencies might be possible. For this, continuous monitoring of the sky at low frequencies is essential. Considering this, we have set up a radio telescope in the Gauribidanur Observatory (named as GAuribidanur Pulsar System- GAPS) for dedicated observations of pulsars and other astrophysical transients in the frequency range of 50 to 80 MHz. Its purpose is to observe and understand the characteristics of known pulsars and potentially search for FRBs at these frequencies if they should occur. This observing facility is a dedicated instrument with wide sky coverage, large bandwidth, and high time resolution, which are important for observing fast transients. The thesis involved developing the prototype system and its subsequent upgrades, testing and characterizing the system, and observing targeted Pulsars.