The K2 Light Curves and Stunted Outbursts of AC Cnc

Abstract

We describe two observations of the nova-like cataclysmic variable AC Cnc obtained with {\it Kepler} during its revamped second mission ({\it K2}). Using the {\it K2} 1-minute cadence mode, the data were obtained during Campaigns 5 and 18. Campaign 5 (C05) lasted from ${\sim}$2015 Apr 27 to ${\sim}$2015 July 10, a total of 74.8 days, and yielded ${\sim}$106,000 measurements. Campaign 18 (C18) lasted from ${\sim}$2018 May 13 to ${\sim}$2018 July 2, a total of 50.7 days, yielding $\sim$72,000 measurements. The C05 light curve reveals two `stunted outbursts' having properties consistent with stunted bursts observed from the ground; a stunted burst was underway during the C18 observation when it ended. During a stunted outburst, the primary eclipse is found to increase in depth but the residual brightness at mid-primary-eclipse remains nearly constant. By contrast, the secondary eclipse retains the same depth but the brightness at mid-secondary eclipse increases during outburst, following the orbital variations of the out-of-eclipse light. The eclipse ephemeris is statistically consistent with historical ephemerides and shows only marginal evidence for a period change. On the basis of the {\it K2} data, we can not confirm a previously-reported non-orbital periodicity in AC~Cnc.