Peculiar optical transients in the dark energy survey

Abstract

This thesis investigates the properties of peculiar optical transients discovered by the Dark Energy Survey (DES). DES was an optical sky survey designed to study the nature of dark energy using type Ia supernovae (SNe) as cosmic standardisable candles. The DES supernova programme (DES-SN) detected transient events in 2013 – 2018 by observing ten fields of 3 deg2 each with a weekly cadence for ∼ 5 months a year. As a result ∼ 29000 optical transients were identified by DES-SN. I performed two archival searches to identify rapidly evolving transients (RETs) in DESSN data. In the first one, ∼ 1600 Single-Season Transients (SSTs) that exhibited fast light curve evolution as estimated by Gaussian fits, were inspected resulting in discovery of 92 RETs. The RETs were characterised by short rise times (trise . 10 d) and blue peak colours (g − r . 0.6) distinguishing them from other known transient classes in DES-SN, especially with interpolated Gaussian Process light curves. Thus in the second search, all SSTs with similar rise times and peak colours as the RETs were inspected, once type Ia SNe and transients exhibiting multi-season variability or long decline timescales were excluded. 14 additional RETs were discovered, resulting in the final sample of 106 including 52 with known redshifts. The RETs were found on a range of redshifts (0.05 – 1.56) and peak luminosities (−15.8 & M & −22.6) and they were characterised by light curve evolution with time above half maximum brightness, thalf . 12 d, distinguishing them from standard SNe. Their photometric data was well fitted with a blackbody model, mostly revealing hot (∼ 8000 – 30000 K) and large (∼ 1014 – 1015 cm) photospheres that were cooling and expanding in time. Furthermore, three RETs were found to exhibit receding photospheres. The host galaxy analysis revealed that the RETs appear to occur in star-forming galaxies, indicating an origin in massive stars. However, the nickel decay cannot explain the peak luminosities with the quick rises, but due to the lack of spectroscopic data the powering mechanism could not be identified. Thus the physical nature of RETs remains unknown. I also investigated two peculiar double-peaked DES-SN transients. Their light curves were nearly identical despite 2.5 mag difference in brightness, but no transients with a similar evolution were found in the literature. While the spectra revealed blueshifted calcium absorption in one event, classification could not be secured for either. The events occurred in very different host environments: one near the nucleus of a spiral galaxy and the other at the outskirts of a passive one. While the nearly identical, but highly atypical light curves could imply a similar origin, the differences in the luminosities and host environments might also indicate that they are physically different.

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