Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data

Abstract

We identify a new, bright transient in the K2/Kepler Campaign 11 field. Its light curve rises over 7 mag in a day and then declines 3 mag over a month before quickly fading another 2 mag. The transient was still detectable at the end of the campaign. The light curve is consistent with a WZ Sge-type dwarf nova outburst. Early superhumps with a period of 82 min are seen in the first 10 days and suggest that this is the orbital period of the binary, which is typical for the WZ Sge class. Strong superhump oscillations develop 10 days after peak brightness with periods ranging between 83 and 84 min. At 25 days after the peak brightness a bump in the light curve appears to signal a subtle rebrightening phase implying that this was an unusual type-A outburst. This is the only WZ Sge-type system observed by K2/Kepler during an outburst. The early rise of this outburst is well fitted with a broken power law. In first 10 h, the system brightened linearly and then transitioned to a steep rise with a power-law index of 4.8. Looking at archival K2/Kepler data and new TESS observations, a linear rise in the first several hours at the initiation of a superoutburst appears to be common in SU Ursa Majoris stars.