We present the first LOFAR image of the center of M31 at a frequency of 150 MHz. We clearly detect three supernova remnants, which, along with archival VLA data at 3 GHz and other published radio and X-ray data, allows us to characterize them in detail. Our observations also allow us to obtain upper limits of the historical SN 1885A, which is undetected even at a low frequency of 150 MHz. From analytical modeling, we find that SN 1885A will stay in its free-expansion phase for at least another couple of centuries. We find an upper limit of n H ≲ 0.04 cm−3 for the interstellar medium of SN 1885A, and that the SN ejecta density is not shallower than ∝r −9 (on average). From the 2.6σ tentative detection in X-ray, our analysis shows that nonthermal emission is expected to dominate the SN 1885A emission. Comparing our results with those on G1.9+0.3, we find that it is likely that the asymmetries in G1.9+0.3 make it a more efficient radio and X-ray emitter than SN 1885A. For Braun 80, 95, and 101, the other remnants in this region, we estimate ages of 5200, 8100, and 13,100 yr, and shock speeds of 1150, 880, and 660 km s−1, respectively. Based on this, the supernova rate in the central 0.5 kpc × 0.6 kpc of M31 is at least one per ∼3000 yr. We estimate radio spectral indices of −0.66 ± 0.05, −0.37 ± 0.03, and −0.50 ± 0.03 for the remnants, respectively, which match fairly well with previous studies.

This thesis explores low-frequency radio behaviour of supernovae and supernova remnants. Supernovae are the explosive end stages of stellar evolution. The interaction of the supernova ejecta with its surrounding circumstellar material results in synchrotron emission which can be studied using radio observations. The past decade has seen a greater exploration of the low frequency radio sky. At low radio frequencies, the ionosphere causes major problems. Specialised calibration and imaging techniques, along with new technologies for data handling have paved the way for science at low frequencies. The LOw Frequency ARray (LOFAR) is based in the Netherlands and is the focus of this thesis. In operation for over a decade now, LOFAR explores frequencies between 10 and 240 MHz. The LOFAR Two-metre Sky Survey (LoTSS) has already mapped the northern sky with 4.5 million radio sources in its latest data release covering 27% of the northern sky. In addition, LOFAR has stations across Europe forming the International LOFAR Telescope (ILT). Processing data with the ILT using standardised pipelines is a major recent development in doing Very Long Baseline Interferometry (VLBI) with LOFAR, resulting in the highest resolution possible at these frequencies. This is true even when the Square Kilometre Array (SKA) comes online. The upcoming SKA will be the world's largest radio telescope and has been a major driver of low frequency radio sky exploration.

For explosive transients like supernovae, low radio frequencies remain unexplored and present exciting possibilities on understanding absorption mechanisms at play using the low-frequency radio turnover. In Paper I, we present the first subarcsecond resolution image of the nearby galaxy M51 with the ILT. We especially focus on three known supernovae in M51 making it the first detailed study of supernovae with LOFAR. One key finding is that the LoTSS flux density for SN 2011dh is about five times higher than the flux density with the ILT image, which would affect the absorption scenario, and in turn derivation of mass-loss parameters for the supernova. This is partially explained by the fact that the ILT filters out diffuse emission, and that the beam fit to the source in LoTSS is bigger than the image beam size. In Paper II, we present the first LOFAR image of the centre of M31. We report upper-limits on the historical SN 1885A which has never been detected in the radio. Using VLA data we are also able to characterise three other remnants in the field and report spectral indices, ages, shock velocities and morphology. A key takeaway from this work is that, from estimations of the flux density and radius of the forward shock of SN 1885A, we believe the remnant is likely at the limit of detection with the ILT, given the instrument's improved noise and resolution. In Paper III, we focus on three supernovae SN 2006X and SN 1979C in the nearby galaxy M100 and SN 1986J in NGC 891. Using LoTSS data of M100 that is not yet publicly available, we report upper limits for the Type Ia SN 2006X. A key finding for SN 1979C is a marked steepening of the radio spectra at late times in our data from 2019-2020. This contradicts previous findings that reported a spectral flattening which is expected in case of a central compact object. 

Such studies help us bridge the gap between our current understanding of various objects in the radio sky and what instruments such as the SKA will bring with it in the future. 

We present an International LOFAR Telescope (ILT) subarcsecond-resolution image of the nearby galaxy M51 with a beam size of 0436 × 0366 and rms of 46 μJy. We compare this image with a European VLBI Network study of M51 and discuss the supernovae in this galaxy, which have not yet been probed at these low radio frequencies. We find a flux density of 0.97 mJy for SN 2011dh in the ILT image, which is about five times smaller than the flux density reported by the LOFAR Two-metre Sky Survey (LoTSS) at 6'' resolution using the same data set without the international stations. This difference makes evident the need for LOFAR international baselines to reliably obtain flux density measurements of compact objects in nearby galaxies. Our LOFAR flux density measurement of SN 2011dh directly translates into fitting the radio light curves for the supernova and constraining the mass-loss rates of the progenitor star. We do not detect two other supernovae in the same galaxy, SN 1994I and SN 2005cs, and our observations place limits on the evolution of both supernovae at radio wavelengths. We also discuss the radio emission from the center of M51, in which we detect the active galactic nucleus and other parts of the nuclear emission in the galaxy, with a possible detection of Component N. We discuss a few other sources, including the detection of a high-mass X-ray binary not detected by LoTSS but with a flux density in the ILT image that matches well with higher-frequency catalogs.

We present Low-Frequency Array (LOFAR) telescope observations of the radio-loud gravitational lens systems MG 0751+2716 and CLASS B1600+434. These observations produce images at 300 milliarcseconds (mas) resolution at 150 MHz. In the case of MG 0751+2716, lens modelling is used to derive a size estimate of around 2 kpc for the low-frequency source, which is consistent with a previous 27.4 GHz study in the radio continuum with Karl G. Jansky Very Large Array. This consistency implies that the low-frequency radio source is cospatial with the core-jet structure that forms the radio structure at higher frequencies, and no significant lobe emission or further components associated with star formation are detected within the magnified region of the lens. CLASS B1600+434 is a two-image lens where one of the images passes through the edge-on spiral lensing galaxy, and the low radio frequency allows us to derive limits on propagation effects, namely scattering, in the lensing galaxy. The observed flux density ratio of the two lensed images is 1.19 ± 0.04 at an observed frequency of 150 MHz. The widths of the two images give an upper limit of 0.035 kpc m−20∕3 on the integrated scattering column through the galaxy at a distance approximately 1 kpc above its plane, under the assumption that image A is not affected by scattering. This is relatively small compared to limits derived through very long baseline interferometry studies of differential scattering in lens systems. These observations demonstrate that LOFAR is an excellent instrument for studying gravitational lenses. We also report on the inability to calibrate three further lens observations: two from early observations that have less well determined station calibration, and a third observation impacted by phase transfer problems.

The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 h observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5° away, while phase solution transferral works well over ~1°. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ~0.3′′ ×0.2′′ although this varies slightly over the entire 5 deg2 field of view. We find we can achieve ~80–300 μJy bm−1 image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 μJy bm−1 for the 8 h observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ~ 3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution makes this estimate a lower limit.