2025PASJ...77.1126K

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.