OGLE-BLG504.12.201843: A possible extreme dwarf nova

An international team of astronomers has analyzed photometric and spectroscopic observations of a candidate cataclysmic variable known as OGLE-BLG504.12.201843. The results of this analysis suggest that this object is an extreme dwarf nova of the U Gem type. The findings are reported in a paper published Oct. 3 on arXiv.org.

OGLE-BLG504.12.201843 (or O-201843 for short) was first detected in 2015 as part of the Optical Gravitational Lensing Experiment (OGLE). Previous observations of this source have found that it exhibits photometric variability with a period of 0.523419 days (interpreted as an orbital period) and that it experiences nearly year-long bursts that repeat every 950 to 1020 days.

The nature of O-201843 is still uncertain, however, based on its explosion, the object was initially classified as a cataclysmic variable (CV) candidate. In general, CVs are binary star systems consisting of a primary white dwarf accreting matter from a normal companion star. They irregularly increase in brightness by a large factor, then fall back to a quiescent state.

Since in CVs, mass transfer from the companion star often occurs through an accretion disk around the white dwarf, and in some cases, thermal instability in the disk triggers an outburst known as a dwarf nova (DN ). These novae are a type of CV that experience semi-periodic outbursts. Some DNe only experience regular 2-5 mag outbursts (U Gem type) while others show additional features.

When it comes to O-201843, previous studies have suggested that its photometry is reminiscent of a DN with extreme properties. Therefore, a group of astronomers led by Camille Landri of Charles University in Prague, Czech Republic, sought to verify this hypothesis by analyzing existing optical photometry and new optical spectroscopy, mainly from OGLE and various spectrographs.

Photometric data from O-201843 show clear features of an accretion disk from which outbursts may originate. In addition, a gradual increase in brightness and small flashes during the quiescent period between bursts were identified in the data. Astronomers interpret this brightness as an increase in the luminosity and temperature of the accretion disk, while the origin of the eruptions remains to be determined.

Based on analysis of the spectroscopic data, O-201843 was found to display a Balmer absorption that may come from a bright secondary star or a cold accretion disk. However, the features of a white dwarf and the usual features of an unstable accretion disk have not been observed.