Radio Transients and their Environments

The interstellar medium is the principal ingredient for star formation and hence, it is necessary to study the properties of the interstellar medium. Radio sources in our Galaxy and beyond can be used as a probe of the intervening medium. In this dissertation, I present an attempt to use radio transients like pulsars and fast radio bursts and their interactions with the environment around them to study interstellar medium. We show that radio emission from pulsars is absorbed by dense ionized gas in their surroundings, causing a turnover in their flux density spectrum that can be used to reveal information about the absorbing medium. We carried out a multi-wavelength observation campaign of PSR B0611+22. The pulsar shows peculiar emission variability that is broadband in nature. Moreover, we show that the flux density spectrum of B0611+22 is unusual which can be attributed to the environment it lies in. We also present predictions of fast radio burst detections from upcoming low frequency surveys. We show that future surveys with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) will be able to detect~1 radio burst per hour even if the radio burst undergoes significant absorption and scattering. Finally, we present our results of pulsar population synthesis to understand the pulsar population in the Galactic Centre (GC) and place conservative upper limits on the GC pulsar population. We obtain an upper limit of 52 CPs and 10,000 MSPs in the GC. The dense, ionized environment of the GC gives us the opportunity to predict the probability of detection by considering scattering and absorption as the principle sources of flux mitigation. Our results suggest that the optimal frequency range for a pulsar survey in the GC is 9--14~GHz. A larger sample of absorbed spectrum pulsars and fast radio bursts will be beneficial not only for the study of emission processes but also for discerning the properties of the material permeating through space

On gigahertz spectral turnovers in pulsars

Pulsars are known to emit non-thermal radio emission that is generally a power-law function of frequency. In some cases, a turnover is seen at frequencies around 100~MHz. Kijak et al. have reported the presence of a new class of ''Gigahertz Peaked Spectrum'' (GPS) pulsars that show spectral turnovers at frequencies around 1 GHz. We apply a model based on free-free thermal absorption to explain these turnovers in terms of surrounding material such as the dense environments found in HII regions, Pulsar Wind Nebulae (PWNe), or in cold, partially ionized molecular clouds. We show that the turnover frequency depends on the electron temperature of the environment close to the pulsar, as well as the emission measure along the line of sight. We fitted this model to the radio fluxes of known GPS pulsars and show that it can replicate the GHz turnover. From the thermal absorption model, we demonstrate that normal pulsars would exhibit a GPS-like behaviour if they were in a dense environment. We discuss the application of this model in the context of determining the population of neutron stars within the central parsec of the Galaxy. We show that a non-negligible fraction of this population might exhibit high-frequency spectral turnovers, which has implications on the detectability of these sources in the Galactic centre.Comment: 7 pages, 3 figures, Accepted for publication in MNRA

Expectations for Fast Radio Bursts in Neutron Star-Massive Star binaries

Recent observations of a small sample of repeating Fast Radio Bursts (FRBs) have revealed a periodicity in their bursting activity that may be suggestive of a binary origin for the modulation. We set out to explore the scenario where a subset of repeating FRBs originates in binary systems hosting a highly energetic neutron star and a massive companion star, akin to $\gamma$-ray binaries and young High-Mass X-ray Binaries. Firstly, we specifically focus on the host galaxy properties and binary formation rates. Subsequently, we investigate the expected evolution of the rotation and dispersion measure in this scenario, the predicted birth-site offsets, and the origin of the persistent radio emission observed in a subset of these systems. The host galaxies for repeating FRBs favour the formation of neutron star-massive star binary systems but any conclusive evidence will require future discoveries and localizations of FRBs. The birth rate of high-mass X-ray binaries, used as a proxy for all considered binaries, significantly exceeds the estimated rate of FRBs, which can be explained if only a small subset of these systems produce FRBs. We show that under simple assumptions, we can reproduce the DM and RM evolution that is seen in a subset of repeating FRBs. We also discuss the possibility of detecting a persistent radio source associated with the FRB due to an intra-binary shock between companion star wind and either the pulsar wind or giant magnetar flares. The observed long-term luminosity stability of the Persistent Radio Sources is most consistent with a giant flare-powered scenario. However, this explanation is highly dependent on the magnetic field properties of the neutron star. With these explorations, we have aimed to provide a framework to discuss future FRB observations in the context of neutron star-massive star binary scenarios.Comment: 15 pages, 7 figures, 2 appendices, accepted for publication in A&A. Abstract truncated to fit the word limit on arXi

Spectro-temporal analysis of a sample of bursts from FRB 121102

FRB~121102 was the first Fast Radio Burst (FRB) that was shown to repeat. Since its discovery in 2012, more than two hundred bursts have been detected from the source. These bursts exhibit a diverse range of spectral and temporal characteristics and many questions about their origin and form remain unanswered. Here, we present a sample of radio bursts from FRB 121102 detected using the Lovell telescope at Jodrell Bank Observatory. We show four examples of bursts that show peculiar spectro-temporal characteristics and compare them with properties of bursts of FRB~121102 detected at other observatories. We report on a precursor burst that is separated by just 17~ms from the main burst, the shortest reported separation between two individual bursts to date. We also provide access to data for all the detections of FRB~121102 in this campaign.Comment: 3 pages, 1 Figure, to be published in RNAA

Limits on Absorption from a 332-MHz survey for Fast Radio Bursts

Fast Radio Bursts (FRBs) are bright, extragalactic radio pulses whose origins are still unknown. Until recently, most FRBs have been detected at frequencies greater than 1 GHz with a few exceptions at 800 MHz. The recent discoveries of FRBs at 400 MHz from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope has opened up possibilities for new insights about the progenitors while many other low frequency surveys in the past have failed to find any FRBs. Here, we present results from a FRB survey recently conducted at the Jodrell Bank Observatory at 332 MHz with the 76-m Lovell telescope for a total of 58 days. We did not detect any FRBs in the survey and report a 90$\%$ upper limit of 5500 FRBs per day per sky for a Euclidean Universe above a fluence threshold of 46 Jy ms. We discuss the possibility of absorption as the main cause of non-detections in low frequency (< 800 MHz) searches and invoke different absorption models to explain the same. We find that Induced Compton Scattering alone cannot account for absorption of radio emission and that our simulations favour a combination of Induced Compton Scattering and Free-Free Absorption to explain the non-detections. For a free-free absorption scenario, our constraints on the electron density are consistent with those expected in the post-shock region of the ionized ejecta in Super-Luminous SuperNovae (SLSNe).Comment: 12 pages, 9 Figures, 2 Tables, Second revision submitted to MNRA

Setiburst: A Robotic, Commensal, Realtime Multi-Science Backend For The Arecibo Telescope

Radio astronomy has traditionally depended on observatories allocating time to observers for exclusive use of their telescopes. The disadvantage of this scheme is that the data thus collected is rarely used for other astronomy applications, and in many cases, is unsuitable. For example, properly calibrated pulsar search data can, with some reduction, be used for spectral line surveys. A backend that supports plugging in multiple applications to a telescope to perform commensal data analysis will vastly increase the science throughput of the facility. In this paper, we presen

MeerTRAP: Twelve Galactic fast transients detected in a real-time, commensal MeerKAT survey

MeerTRAP is a real-time untargeted search project using the MeerKAT telescope to find single pulses from fast radio transients and pulsars. It is performed commensally with the MeerKAT large survey projects (LSPs), using data from up to 64 of MeerKAT's 13.96~m dishes to form hundreds of coherent beams on sky, each of which is processed in real time to search for millisecond-duration pulses. We present the first twelve Galactic sources discovered by MeerTRAP, with DMs in the range of 33--381~pc~cm$^{-3}$. One source may be Galactic or extragalactic depending on the Galactic electron density model assumed. Follow-up observations performed with the MeerKAT, Lovell, and Parkes radio telescopes have detected repeat pulses from seven of the twelve sources. Pulse periods have been determined for four sources. Another four sources could be localised to the arcsecond-level using a novel implementation of the tied-array beam localisation method.Comment: 16 pages, 14 figure