OGLE-BLG504.12.201843: A possible extreme dwarf nova

TL;DR

This paper analyzes OGLE-BLG504.12.201843, a candidate extreme dwarf nova with unique outburst and spectral properties, suggesting it is a U Gem-type system with rare observational features and potential for further research.

Contribution

The study provides the first detailed analysis of this object’s long-term outburst behavior, spectral features, and possible accretion disc dynamics, highlighting its extreme properties and rarity.

Findings

Earliest outburst recorded in 1910.

Gradual brightening of the system before outburst.

Emission lines during outburst suggest disc winds or low inclination.

Abstract

We present the analysis of existing optical photometry and new optical spectroscopy of the candidate cataclysmic variable star OGLE-BLG504.12.201843. As was shown previously, this object has an orbital period of 0.523419 days and exhibits year-long outbursts with a mean period of 973 days. Using digitized photographic archives, we show that the earliest recorded outburst occurred in 1910. We propose that this object is a U Gem-type dwarf nova with extreme properties. The orbital variability of the system in outburst shows clear signs of an accretion disc, from which the outburst likely originates. During quiescence, the object slowly brightens by up to $0.75$ mag in the $I$ band over 600 days before the outburst and exhibits small flares with amplitude $\lesssim 0.2$ mag in the $I$ band. We interpret the gradual brightening as an increase in the luminosity and temperature of the accretion disc, which is theoretically predicted but only rarely seen in DNe. The origin of small flares remains unexplained. The spectra shows Balmer absorption lines both in quiescence and outburst, which can be associated with a bright secondary star or a cold accretion disc. During outbursts, emission lines with FWHM of about 450 km s$^{-1}$ appear, but they lack typical double-peaked profiles. We suggest that either these lines originate in the disc winds or the orbital inclination is low, the latter being consistent with constrains obtained from the orbital variability of the system. Due to its extreme properties and peculiarities, OGLE-BLG504.12.201843 is an excellent object for further follow-up studies.