A Be star that turned bright and blue during a major outburst

<jats:p><jats:italic>Context.</jats:italic> Classical Be stars are rapidly rotating B-type stars exhibiting Balmer emission lines originating from circumstellar Keplerian gaseous disks, which likely form through episodic mass-ejection events. The currently favored model for Be star disks is the viscous decretion disk (VDD) model. However, the mechanism behind the mass ejection process during the formation of a VDD is a mystery.</jats:p>
<jats:p><jats:italic>Aims.</jats:italic> We present peculiar behavior of the Be star KIC 9715425, which exhibited several minor outbursts (MIBs) and one major outburst (MAB) observed as brightenings in broadband photometry, as well as transient H<jats:italic>α</jats:italic> line emission. The variability may provide valuable keys to understanding the nature of Be outbursts.</jats:p>
<jats:p><jats:italic>Methods.</jats:italic> Based on <jats:italic>Kepler</jats:italic>, the All-Sky Automated Survey for Supernovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite time-series photometry, the Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, multi-wavelength observations in the near-ultraviolet (NUV) from the Galaxy Evolution Explorer (GALEX) and optical from <jats:italic>Gaia</jats:italic>, Xinglong 2.16 m telescope and Isaac Newton Telescope of KIC 9715425 covering its outbursts, we determined fundamental parameters of the central star and the circumstellar disk. A frequency analysis and spectral modeling were carried out to characterize the events.</jats:p>
<jats:p><jats:italic>Results.</jats:italic> During the major outburst, we find a 36% flux increase in the GALEX NUV band compared to 25% in the <jats:italic>Kepler</jats:italic> band, suggesting that whatever produced the flux increase should be hotter than the central B-type star. This is contradictory to the conventional scenario that the VDD should be cooler. In addition, such a high flux increase can only be accounted for by a luminous disk, which should produce a much stronger H<jats:italic>α</jats:italic> emission than observed. The origin of this anomalously hot component remains unexplained.</jats:p>
<jats:p><jats:italic>Conclusions.</jats:italic> Except for the bluing, the available observations of KIC 9715425 are perfectly compatible with being a normal Be star. If the bluing was physically related to the MAB, the most economical effort to identify the nature of the underlying process could be adaptive NUV and far-ultraviolet monitoring of Be stars with cyclically repeating MABs.</jats:p>