Kavli Affiliate: Subo Dong
| First 5 Authors: Camille Landri, Ondřej Pejcha, Michał Pawlak, Andrzej Udalski, Jose L. Prieto
| Summary:
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.
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