Browsing by Author "Taylor, Andrew Russell"
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- ItemOpen AccessFull Mueller imaging: direction dependent corrections in polarimetric radio imaging(2018) Jagannathan, Preshanth; Taylor, Andrew RussellMagnetic fields pervade the universe, spanning a multitude of scales from the dipolar field on Earth, to the largest gravitationally bound structures such as galaxy clusters [1]. The magnetic fields play a vital role in the evolution of these astronomical systems. In addition to the multitude of scales, magnetic fields are present in different astronomical systems of varying strengths. The strongest observed astronomical magnetic fields are in neutron stars with a field strength of ≈ 1015 G [2], far higher than any man-made fields till date. In stark contrast magnetic fields in the interstellar medium while ubiquitous are only a few µG in field strength. Many fundamental processes in astrophysics have magnetism at their heart, be it cosmic ray particle acceleration, star formation, or the launch of radio galaxy jets, pulsars, etc. One key fundamental process that allows us to detect and characterize cosmic magnetic fields with radio astronomy is the polarization of synchrotron radiation. Synchrotron radiation is intrinsically polarized broadband continuum radiation emitted by relativistic charged particles accelerated by the presence of magnetic fields. The emissivity of the synchrotron radiation is tied to the magnetic field strength B and the spectral index α (defined such that the flux density S ∝ ν −α ) such that ε ∝ B 1+α .
- ItemOpen AccessThe faint low-frequency radio universe in continuum: exploitation of the pre-SKA deepest survey(2020) Ocran, Emmanuel; Taylor, Andrew RussellThis thesis presents a thorough and significant work on the properties of radio sources as derived from deep 610-MHz GMRT data and ancillary multi-wavelength data. The faint radio sources at 610-MHz are found out to distances such that the objects are seen as they were when the universe was less than half its current age. These data provide a first look at the faint radio sky at sensitivities that will soon be achieved by key programs on the South African MeerKAT radio telescope, and thus take a first step in the exploration of the radio universe that will be made by the Square Kilometre Array. I report deep 610-MHz GMRT observations of the EN1 field, a region of 1.86 deg2 . We achieve a nominal sensitivity of 7.1µ Jy beam−1 . From our 610 MHz mosaic image, we recover 4290 sources after accounting for multiple component sources down to a 5σ flux density limit of 35.5 µ Jy. From this data, I derive the 610 MHz source counts applying corrections for completeness, resolution bias and Eddington bias. The 610- MHz source counts show a flattening at flux densities below 1 mJy. The source counts are higher than previous observations at this frequency below this break. However, they are generally consistent with recent models of the low-frequency source population. Using ancillary multi-wavelength data in the field, I investigate the key issue of source population classification using the deepest data at an intermediate-low frequency (higher than LOFAR and lower than JVLA), where previous work has not been sensitive enough to reach the µJy population. By cross-matching against the multi-wavelength data, I identify 72% of the radio sample having reliable redshifts, of which 19% of the redshifts are based on spectroscopy. From the classification, I obtain 1685 sources as Star-Forming Galaxies (SFGs), 281 sources Radio-Quiet (RQ) and 339 sources Radio-Loud (RL) Active Galactic Nuclei (AGN) for the sub-sample with redshifts and at least one multi-wavelength AGN diagnostic. SFGs are mostly low-power radio sources, i.e L610 MHz < 1025 W Hz−1 while RQ AGN and RL AGN have radio powers L610 MHz > 1025 W Hz−1 . From cross-matching my sample with other radio surveys (GMRT at 325-MHz, FIRST at 1.4-GHz and JVLA at 5-GHz), I obtain the median spectral index from 325-MHz to 610-MHz to be −0.80 ± 0.29, 610-MHz to 1.4-GHz to be −0.83 ± 0.31 and 1.4-GHz to 5-GHz to be −1.12 ± 0.15. The main result is that the median spectral index appears to steepen at the highest frequency. With the above catalogue in hand, I use the non-parametric V/Vmax test and the radio luminosity function to investigate the cosmic evolution of different source populations. I study SFGs and derive their IR-radio correlation and luminosity function as a function of redshift. By integrating the evolving SFG luminosity functions I also derive the cosmic star formation rate density out to z = 1.5. I address the long standing question about the origin of radio emission in RQ AGN. I compare the star formation rate (SFR) derived from their far-infrared luminosity, as traced by Herschel, with the SFR computed from their radio emission. I find evidence that the main contribution to the radio emission of RQ AGN is the star formation activity in their host galaxies. At high luminosities, however, both SFGs and 1 RQ AGN display a radio excess when comparing radio and infrared star formation rates. The vast majority of our sample lie along the SFR − M? ”main sequence” at all redshifts when using infrared star formation rates. This result opens the possibility of using the radio band to estimate the SFR even in the hosts of bright AGN where the optical-to-mid-infrared emission can be dominated by the AGN. I investigate the evolution of radio AGN out to z ∼ 1.5 with continuous models of pure density and pure luminosity evolution with Φ? ∝ ( 1 + z)(2.25±0.38)−(0.63±0.35)z and L610 MHz ∝ ( 1 + z)(3.45±0.53)−(0.55±0.29)z respectively. I also constrain the evolution of RQ AGN and RL AGN separately with a continuous model of pure luminosity evolution. For the RQ and RL AGN, we find a fairly mild evolution with redshift best fitted by pure luminosity evolution with L610 MHz ∝ ( 1 + z)(2.81±0.43)−(0.57±0.30)z for RQ AGN and L610 MHz ∝ ( 1 + z)(3.58±0.54)−(0.56±0.29)z for RL AGN. The results reveal that the 610 MHz radio AGN population thus comprises two differently evolving populations whose radio emission is mostly SF-driven or AGN-driven respectively. Finally, I probe the infrared-radio correlation and radio spectral indices of the faint radio population using stacking. I stack infrared sources in the EN1 field using the MIPS 24 micron mid-infrared survey and radio surveys created at 325 MHz, 610 MHz and 1.4 GHz. The stacking experiment shows a variation in the absolute strength of the infrared-radio correlation between these three different frequencies and the MIPS 24 micron band. I find tentative evidence of a small deviation from the correlation at the faintest infrared flux densities. The stacked radio spectral index analyses reveal that the majority of the median stacked sources exhibit steep spectra, with a spectral index that steepens with frequency between α 325 610 and α 610 1400. This work is particularly useful to pave the way for upcoming radio surveys with SKA pathfinders and precursors.
- ItemOpen AccessThe MeerKAT Radio Frequency Interference Environment(2019) Sihlangu, Isaac; Bassett, Bruce; Oozeer, Nadeem; Taylor, Andrew RussellRadio signals from astronomical sources are extremely weak and easily distorted/- corrupted or overwhelmed by man-made radio signals such as cellphones, satellites, aircraft and telescope electronics. These Radio Frequency Interference (RFI) are increasingly threatening radio observatories due to our increasingly technological world. To detect and mitigate RFI, observatories need to understand their RFI environment, what contributes to it and how it is changing. While there are few dedicated RFI monitoring systems on the MeerKAT site, the most sensitive RFI detector is the MeerKAT array itself. In this thesis we use approximately 1500 hours of MeerKAT observations to create a multi-dimensional view of the RFI at the MeerKAT site. Here we investigate a probabilistic approach to characterise the RFI environment around the MeerKAT radio telescope. In order to achieve our goal, we propose the MeerKAT Historical Probability of RFI (KATHPRFI) framework. We produced the high level requirements of the KATHPRFI framework driven by the needs of the MeerKAT users. The design approach and the design decision of the framework is presented that cover both the software and hardware constraints. The KATHPRFI produces a 5-dimensional array of the RFI probability as measured by the MeerKAT telescope during the commissioning phase (May 2018 - December 2018) for each observation file. From the 5-D array, we extracted various statistics and characterised the RFI environment around MeerKAT site. We found that there is a correlation between RFI occupancy and the time of the day which is most probably related to human activities. Furthermore, we found a correlation between the time of the day and flights passing over a region of site. Our results showed that the highest probability of RFI points towards a region including nearby towns. The results obtained are consistent with the argument that the major RFI sources for MeerKAT site are the Global Positioning System (GPS) satellite, flight Distance Measurement Equipment (DME) and the Global System for Mobile Communications (GSM). Our data also showed that the RFI occupancy decreases with an increase of baseline length, this is a result of moving RFI sources with respect to the static sky. Therefore, the phase of the RFI changes rapidly on long baselines compared to short baselines. As a result when a correlation is carried out the RFI amplitude will vanish less on short baselines compared to the long baselines. Our results provide the first highly detailed view of the MeerKAT RFI environment allowing us to track the historical evolution of the RFI, both on average, and as a function of frequency, baseline and direction. With historical baselines known, one can also provide alerts about sudden changes. This could be due to new sources of RFI or stem from any outliers in the data, which could signal telescope or correlator issues. Hence the KATHPRFI framework also provides a window into the operational health of the telescope.
- ItemOpen AccessThe optical polarization variability of the blazar PKS 2155 304(2024) Peceur, Nicolette; Taylor, Andrew Russell; Kraan-Korteweg Renee ChristineBlazars are some of the most energetic and variable objects in the universe. Blazar emission has been detected across the electromagnetic spectrum and exhibits variability on a wide range of timescales, from minutes to years. The observed emission is Doppler boosted and dominated by non-thermal radiation driven by the magnetic field of a relativistic jet. PKS 2155–304 is an archetypal blazar, located in the southern hemisphere. The source is bright and highly variable, displaying both active and quiescent states. In this thesis I make use of optical polarization and multiwavelength observations across roughly 5.5 years to probe the source variability on short (intra-day to daily), intermediate (days to months) and long timescales (months to years). The polarization, as a direct observable of the jet's magnetic field, can help gain insight into the physical processes that underlie the observed emission, while contemporaneous multiwavelength observations can assist in distinguishing between dierent emission models. I investigated the short term variability of PKS 2155–304 by using optical polarization measurements recorded over a 3 day period during a period of enhanced gamma-ray activity. The observations revealed, for the first time, evidence of quasiperiodic oscillations in the optical polarization of a blazar. A periodogram analysis of the polarized flux revealed the existence of two periodic components at ≥ 13 minutes and ≥ 30 minutes. The oscillations can be explained by turbulence behind a relativistic shock traversing a jet containing quasi-helical structures in magnetic field or electron density. To study the intermediate timescale brightness variations of the source I analysed its optical polarization and BVRJ multiband light curves during a prominent optical flare in 2010. The flare evolved over roughly 4 months with a flux doubling time of ≥ 11 days. A comparison of the polarization angle and photometric flux revealed the existence of two distinct states at low and high flux. Below 18 mJy, no clear relationship is seen between the polarization angle and flux, while there is a positive correlation above 18 mJy. I performed a photopolarimetric analysis of the high flux state, which showed that it can be attributed to a variable component with a power-law radiation spectrum of index ≠1.12 and a polarization degree of 13.3%. I then applied a shock-in-jet model to the observations, which showed that the observed variability can be attributed to a nearly edge-on shock. Within the shock-in-jet model, I derived estimates for the magnetic field (0.06 G), Doppler factor (22.3) and viewing angle of the jet (2.6¶). Lastly, I performed an investigation of the long term variability of the blazar by analysing roughly 5.5 years of radio, optical, optical polarization, X≠ray and “≠ray measurements. Using a correlation analysis, I found that the optical, X≠ray and high energy light curves were consistent with zero lag, while the radio light curve lagged behind the higher energy emission by ≥ 46 days. The lag between the radio and higher energy light curves is consistent with opacity eects due to synchrotron emission. The nearly zero lag between the synchrotron (optical and X≠ray) and Inverse Compton (high energy) emission components indicates that the same electron population is responsible for the emission. When computing the temporal structure functions of the multiband light curves, I found that the multiwavelength emission can be described by a power-law spectral density function, with an index of –1.5 for the radio, –1.3 for the optical, –0.6 for X≠rays and –1.0 at high energies. A break timescale is present for the optical and X≠ray band, below which the structure function is characterised by white noise. The structure functions also indicate the existence of periodic outbursts roughly every 1.8 years. My analysis of the multiband photometric fluxes showed that the optical emission is well-described by a variable component with a power-law radiation spectrum of index ≠1.13, consistent with synchrotron emission. For a power law electron distribution, the optically thin synchrotron emission implies an energy spectrum with an index of ≠3.26 for the electrons. Finally, the long term optical polarization supports the existence of a constant polarization component with a polarization degree of 3.4% and a polarization angle of 76¶. The results of my analysis indicate that the emission from PKS 2155–304 is composed of two components, a persistent component with low polarization degree and a magnetic field that lies roughly parallel to the direction of the jet, and a variable component with a constant power-law radiation spectral index and high polarization degree. The variable emission component is consistent with a relativistic shock propagating in the jet on timescales from day to months, where intraday quasi-periodic oscillations can arise due to turbulence behind the shock. The structure function analysis of the long term behaviour indicates that there is power in the variable component on all time scales down to the break in white noise. However, the variable component undergoes major outbursts roughly every 1.8 years. The constant power-law observed for the variable emission over roughly 5.5 years means that the particles have the same energetics. The nearly zero lag across the dierent energy bands indicates that the same particle population underlies the low and high energy emission, consistent with Synchrotron Self Compton models.