Scattered light echoes provide a unique way to engage in late-time study of supernovae. Formed when light from a supernova scatters off of nearby dust, and arrives at Earth long after the supernova has initially faded from the sky, light echoes can be used to study the precursor supernova through both photometric and spectroscopic methods. The detection rate of light echoes, especially from Type II supernovae, is not well understood, and large scale searches are confounded by uncertainties in supernova ages and peak luminosities. We provide a novel spectroscopic search method for detecting light echoes, and test it with 4 hours of observations of NGC 6946 using the SITELLE Imaging Fourier Transform Spectrometer mounted on the Canada-France-Hawaii Telescope. Our procedure relies on fitting a sloped model to continuum emission, and identifying negatively-sloped continua with the downslope of the emission component of a highly-broadened P-Cygni profile in the H$\alpha$ line, characteristic of supernova ejecta. We find no clear evidence for light echoes from any of the ten known Type II supernovae in NGC 6946, and only one light echo candidate from potential historical supernovae predating 1917. We do however, present an upper limit in H$\alpha$ brightness of \SI{5e-17}{erg/s/cm^2/arcsec^2} for light echoes within this galaxy. We discuss extensions of our methodology, as well as viability of this type of study in other objects, and with other instruments. / Thesis / Master of Science (MSc)

Hundreds of years after exploding, the original light from a supernova can still be observed in the form of light echoes. This light scatters off interstellar dust and is re-directed back toward Earth; due to the extra travel time, we observe the echo after the initial outburst. At some time t after observing the outburst, the surface of equal travel paths defines an ellipsoid with Earth and the supernova at the foci. If dust intersects this ellipsoid it is possible to scatter the light and produce an echo. In this thesis, I develop a relative probability model for the detection of supernova light echoes based on the physical characteristics of interstellar dust and absorption near the Galactic plane. This model includes a dust scattering function, distribution (scale height) of dust in the Galaxy, the dilution of echo flux with distance, and absorption along the supernova-dust-Earth travel paths. I have tested the model's predictions against observations and compared it with a prior survey strategy based on IRIS (re-processed IRAS) maps. Currently the IRIS-based strategy is more effective at selecting good paintings but its detection rate is only around 5%, highlighting the elusiveness of echo appearances. This work considers six historical supernovae in the Milky Way, all of which exploded in the pre-telescopic era (with the possible exception of Cas A) and were recorded as "guest stars" in astronomy records from Asia, Europe, and the Middle East. Their light echoes could give us information on these historically significant events and an opportunity to simultaneously study a supernova in outburst and several hundred years later. Early investigations suggest that the distribution of CO in the Galaxy may anti-correlate with the best paintings for light echoes; if a CO-echo link can be established, this would be useful in future light echo surveys. / Thesis / Master of Science (MSc)

This thesis presents a new method to estimate the Apparent Proper Motion (APM) vector and its uncertainty for supernova light echoes (LEs) and tests its usefulness in practice on LEs due to two old Galactic supernovae (SNe) - Cas A and Tycho. Ten instances of two-dimensional cross-correlation (2-D CC) of images containing light echoes at diferent epochs are employed to examine how well this new method works in practice. The images selected for this work originate from KPNO 4m Mosaic 1.1 images and were originally processed by the Pan-STARRS pipeline. All the APM estimates reported in this thesis are within 1sigma of estimates based on supernova distance and age provided reasonable inclinations are assumed. It was found that several factors tend to reduce the expected precision of this method and these include: 1) the existence of more than one LE feature for each epoch, 2) longer intervals between the two epochs lead to a bias, and 3) the existence of dust filaments at more than one depth along the line of sight. The results of three LE fields which were in common with the previous studies by Rest et al. in 2008 and 2011, were compared and a good agreement was found between them in difference-images with the same time interval. Since pixel values have a significant role in the introduced method, a control region is considered to eliminate the defect of the irrelevant residuals to the LE features. Hence, the introduced method was not straightforward. In addition, this method was not thoroughly manual independent, as the benefits of the visual measurement from the previous method reported by Rest et al. (2008) and (2011) were adopted for this method. However, compared to the previous manual technique, there were much less manual measurements were taken for the whole LE features in one frame. Considering all the challenges, the CC method is favourable as the APM vector uncertainty can be determined, which has not been achievable with previous method before. / Thesis / Master of Science (MSc)

<p>A deep, wide-field survey was conducted to hunt for the light echo systems associated with SN 1054 (Crab) and SN 1181 as an initial step to acquiring spectra and the prospect of extracting lightcurves of these historical, core-collapse supernovae. Images were acquired by the Canada-France-Hawaii Telescope’s MegaCam during the 2011A and 2011B semesters for fields adjacent to SN 1054 and SN 1181, respectively. A total of 367 Sloan g’ fields for the Crab and 195 Sloan r’ fields for SN 1181 were imaged twice, with a minimum of one month separation.</p> <p>Examination of 13,880 and 11,052 difference images for the Crab and SN 1181, respectively, revealed no light echoes with surface brightnesses brighter than 24.0 mag/arcsec² (the threshold for being able to acquire useful spectra). Based on our non-detections and assuming similar dust properties to nearby (detected) supernova light echo systems (Tycho and Cas A), we conclude it is unlikely that either SN was a Type II-L outburst but cannot provide constraints on other sub-types.</p> <p>We further examined the known light echo locations for Tycho and Cas A and found a statistically-significant correlation between CO brightness temperature and the presence of scattering dust. However, the spacing of grid points in existing CO surveys is too sparse to be useful even a few degrees away from the galactic plane. We have yet to identify a search strategy based on survey data which is superior than random field placement.</p> / Master of Science (MSc)

Advances in machine learning for visual recognition and ultra-low surface brightness imaging have made it possible to detect older and fainter historic supernova light echoes (SN LEs). We are particularly interested in the historic core-collapse SN (CCSN) Crab (SN 1054), as it is the only CCSN with records of direct-light observations in the last 1000 years. We have improved the SN LE machine-learning Python package ALED (Automated Light Echo Detection), created by Bhullar et al. 2021, by adding false positive masks as an additional input. ALED is visual recognition software that identifies and locates LEs in difference images. Before the invention of ALED, LE images had to be categorized by visual inspection, which was a very time-consuming task. Additionally, we have developed a method for manufacturing and augmenting LE training sets, which has previously not been applied to LEs. We manufactured Dragonfly Telephoto Array (DTA) LEs by extracting LEs from Canada-France-Hawaii Telescope difference images and overlaying them on DTA difference images. The DTA is a promising tool for LE detection because of its ability to observe ultra-low surface brightness structures. Additionally, we augmented the only existing DTA LE image by overlaying it on other DTA images. Both of these procedures provided options for further augmentation, such as changing the LE's brightness and width. We also created a process to mask the bright star difference artifacts in DTA images. These stars are typically mislabeled as LEs, and hence masking them makes LE identification simpler. We have created an effective DTA training set for ALED, which is prepared to search for LEs around the historic CCSN Crab (SN 1054), once more DTA images in that region are procured. / Thesis / Master of Science (MSc)