TUVO-21acq: A new cataclysmic variable discovered through a UV outburst

Abstract

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 DN outbursts has not been studied extensively compared with the optical. Detailed characterisation of the physical processes responsible for outburst behaviour requires further UV data, because the UV probes the inner regions of the accretion disc. Here we report, as part of our recently launched Transient UV Objects (TUVO) project, the discovery of TUVO-21acq, a new transient which we detected in the UV using data from the Ultraviolet Optical Telescope (UVOT) aboard Swift. We detected two separate outbursts and used the UVOT data to constrain source properties, focusing on the amplitudes and timescales of the outbursts. We found that during the first outburst the source increased in brightness by at least 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 of 6-21 days and 11-46 days, and we determined an upper limit for the recurrence time of 316 days. To further characterise the source, we obtained an optical spectrum during quiescence with the Southern African Large Telescope (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 that it has the capability of discovering and classifying new, interesting UV transients. We also 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 as to the characteristics and implications of the UV delay, for example.

A Serendipitous Observation of the Recently Discovered Cataclysmic Variable TUVO-21acq

“…It was not detected in these bands in ObsID 0112310301, so only the upper limits were found. Looking at TUVO-21acq's photometry across the different observations in Table 1 and Figure 2, as well as in Modiano et al (2022a)ʼs Figure 3, it can be seen that TUVO-21acq is variable in quiescence. Variability in CVs can be due to several things; some potential causes are discussed below.…”

“…The parameters, fluxes, and cstat/DoF of each are in Table 3. For comparison, Modiano et al (2022a) found that the quiescent DN X-ray spectrum from stacked Swift data could be fit with a collisionally ionized plasma model (mekal), with F X (0.3-10 keV) = 1.11 × 10 −14 erg s −1 cm −2 . The quality of the fits in Table 3 were extremely similar for all models, but the power law fit was very slightly better than the others.…”

“…It is possible that these observations caught TUVO-21acq in the post-outburst phase where relatively little material was left in the disk, and it was building up matter from the donor star. No information is available about its outburst phases before 2010, but ObsID 0903540101 was taken ∼200 days after an outburst (Modiano et al 2022a). Modiano et al (2022a) found an upper limit for TUVO-21acq's recurrence time to be 316 days, so the system would still have been rebuilding the accretion disk.…”

“…However, observations have shown that many systems undergoing high accretion rates have optically thin, hard X-ray emission-which may be evidence of a truncated accretion disk, in an intriguing echo of what was seen in the UV spectra (Balman et al 2014;Modiano et al 2022b). Simultaneous observations in the UV and X-rays are needed to allow the modeling of the accretion disk and the boundary layer together, and thus to characterize accretion in these systems in a more holistic way (Modiano et al 2022a).…”

“…In a recent observing campaign of NGC 4945 with XMM-Newton (Weaver et al 2024), serendipitous detections of the newly discovered CV TUVO-21acq (α(J2000.0), δ(J2000.0) = 13 h 05 m 44 7, −49°32′58 2) (Modiano et al 2022a)) were made in both soft X-rays, with the EPIC camera, and in the UV/optical bands, with the Optical Monitor (OM). Modiano et al (2022a) identified it as part of their Transient UV Objects project (Modiano et al 2022b). Their analysis of stacked archival Swift images and an optical spectrum from the South African Large Telescope indicated that that TUVO-21acq is a non-magnetic CV that had undergone a dwarf nova (DN) outburst.…”