  Outbursts from cataclysmic variables, such as dwarf novae (DNe), are
prevalent throughout the galaxy and are known to emit strongly in the
ultraviolet (UV). However, the UV emission of DNe has not been studied
extensively compared with the optical. Characterising in detail the physical
processes responsible for DN outburst behaviour requires further UV data. Here
we report, as part of our recently launched Transient UV Objects Project
(TUVO), the discovery of TUVO-21acq, a new transient which we detected in the
UV using data from the Ultraviolet Optical Telescope aboard Swift. We detected
two separate outbursts and used the UV data to constrain source properties,
focusing on the amplitudes and timescales of the outbursts. During the first
outburst the source increased in brightness by >4.1, >2.4, and >3.5 magnitudes
and during the second outburst by >4.4, >3.4, and >3.6 magnitudes in the UVW1,
UVM2, and UVW2 bands, respectively. The outburst durations were in the range
6-21 days and 11-46 days, and the recurrence time is <316 days. We additionally
obtained an optical spectrum during quiescence with SALT. The spectrum
exhibited hydrogen Balmer series and helium emission lines, and a flat overall
spectral shape. We confirm the nature of the source as an accreting white dwarf
which underwent DN outburst based on photometric and spectroscopic properties.
This study serves as a proof-of-concept for the TUVO project strategy,
demonstrating its capability of discovering and classifying new, interesting UV
transients. We discuss the implications of our findings for our understanding
of the physics underlying DN outbursts, in particular with respect to the UV
emission. We examine the need for simultaneous UV and optical observations
during the onset of DN outbursts in order to help answer remaining questions,
for example in the characteristics and implications of the UV delay.
