An international team of Astronomers examined photometric and spectroscopic observations of OGLE-BLG504.12.201843, a candidate cataclysmic variable. According to the findings of this analysis, this object is an extreme dwarf nova of the U Gem type. The findings are detailed in a paper published on arXiv.org on October 3.
OGLE-BLG504.12.201843 (abbreviated O-201843) was discovered in 2015 as part of the Optical Gravitational Lensing Experiment (OGLE). Previous observations of this source revealed a photometric variability with a period of 0.523419 days (interpreted as an orbital period) and nearly year-long outbursts that occur every 950 to 1,020 days.
O-201843’s nature is still unknown, but due to its outburst, the object was initially classified as a candidate cataclysmic variable (CV). CVs are binary star systems composed of a white dwarf primary accreting matter from a normal star companion. They fluctuate in brightness by a large factor, then return to a quiescent state.
Given that mass transfer from the companion star frequently occurs via an accretion disc surrounding the white dwarf, and in some cases, thermal instability in the disc triggers an outburst known as a dwarf nova (DN). These novae are CVs that have semi-periodic outbursts. Some DNe only have regular 2-5 mag outbursts (U Gem type), while others have more features.
Previous research on O-201843 suggested that its photometry is similar to that of a DN with extreme properties. As a result, a group of astronomers led by Camille Landri of Charles University in Prague, Czech Republic, attempted to validate this hypothesis by analysing existing optical photometry and new optical spectroscopy, primarily from OGLE and various spectrographs.
The photometric data of O-201843 clearly shows the features of an accretion disc, which could be the source of the outbursts. Furthermore, during the quiescence period between outbursts, the data showed a gradual brightening and small flares. The brightening is interpreted by astronomers as an increase in the luminosity and temperature of the accretion disc, while the origin of the flares is unknown.
Based on the spectroscopic data analysis, it was discovered that O-201843 exhibits Balmer absorption, which can come from either a bright secondary star or a cold accretion disc. However, no characteristics of a white dwarf or an unstable accretion disc have been observed.
The researchers concluded that O-201843 could be a U Gem-type dwarf nova with unusual properties. “Overall, the photometry of O-201843 shows strong similarities with DNe as well as significant differences. While these peculiarities have not been observed in other DNe, it is possible that O-201843 is an extreme U Gem DN, or is at least related to DNe “the paper’s authors concluded.