First-ever discovery of a superoutburst in GP Comae Berenices buried in archival data

Abstract We report on the first detection of the only recorded outburst in the AM CVn star GP Comae Berenices (GP Com) in archival historical data from 1950–1951. The detected outburst is likely a WZ Sge-type superoutburst. Both observational and theoretical studies have suggested that GP Com cannot undergo outbursts, similar to other long-period AM CVn stars. Our discovery demonstrates that outbursts can occur even given the binary parameters of GP Com. We develop a novel algorithm assuming quasi-periodic superoutbursts to estimate the probability of missing a superoutburst in the past 140 yr recorded in archival survey data including the Digital Access to a Sky Century @ Harvard (DASCH). Our results indicate a low probability of missing a superoutburst, leading to the constraint that the supercycle of GP Com is >70 yr. We succeed in setting the upper limit of the mass-transfer rate $\dot{M}_{\rm tr} < 1.8 \times 10^{-12}$ M$_{\odot }$ yr$^{-1}$ by comparing the derived supercycle value with the accreted mass from the disk during a superoutburst of other AM CVn stars. Our findings can contribute significantly to our understanding of the activity of long orbital-period AM CVn stars and the unknown mechanisms dominating their quiescent accretion disks.