Searching for transients in the ultraviolet

In recent decades, the significant advancements in time-domain astronomy have led to the development of an extensive range of large-scale transient-searching programmes. These surveys are carried out across almost the entire electromagnetic spectrum. The ultraviolet (UV) range, however, is an exception: no facility currently operates that searches for serendipitous transients in the UV. As a result, an important part of the observational parameter space of transients remains largely unexplored. In this thesis, I present the Transient UV Objects (TUVO) project, a recently initiated programme in which we use operational UV telescopes and our own custom-built software to perform moderate-scale UV transient searches. TUVO has a strong focus on the Ultraviolet Optical Telescope (UVOT) aboard the Neil Gehrels Swift Observatory. In my thesis I first present TUVOpipe, the pipeline we built that uses image subtraction to automatically detect transients in all new UVOT data. We detect a few to a few tens of UV transients on a daily basis. Detections are made in near real-time (within a few hours to at most a day of observations), which typically allows for follow-up observations to be carried out while the transients are still active. In the subsequent chapters of my thesis, I present studies which focus on individual UV transients of high interest which we found with our method. The transients discussed originate from various types of outbursts from accreting white dwarfs, including dwarf novae, novae, and Z-And type outbursts. In each study, the source of interest is discussed in detail using photometric (and in some cases also spectroscopic) UV, optical, and X-ray data during the outburst(s) and quiescence, and questions regarding the physical mechanisms underlying the outbursts are addressed. This thesis demonstrates that serendipitous UV transient searches can be performed even without dedicated facilities, and may allow for discoveries of interesting new sources or outburst behaviour.