Performance of SK-Gd's Upgraded Real-time Supernova Monitoring System

Among multi-messenger observations of the next galactic core-collapsesupernova, Super-Kamiokande (SK) plays a critical role in detecting the emittedsupernova neutrinos, determining the direction to the supernova (SN), andnotifying the astronomical community of these observations in advance of theoptical signal. On 2022, SK has increased the gadolinium dissolved in its watertarget (SK-Gd) and has achieved a Gd concentration of 0.033enhanced neutron detection capability, which in turn enables more accuratedetermination of the supernova direction. Accordingly, SK-Gd's real-timesupernova monitoring system (Abe te al. 2016b) has been upgraded. SK_SN Notice,a warning system that works together with this monitoring system, was releasedon December 13, 2021, and is available through GCN Notices (Barthelmy et al.2000). When the monitoring system detects an SN-like burst of events, SK_SNNotice will automatically distribute an alarm with the reconstructed directionto the supernova candidate within a few minutes. In this paper, we present asystematic study of SK-Gd's response to a simulated galactic SN. Assuming asupernova situated at 10 kpc, neutrino fluxes from six supernova models areused to characterize SK-Gd's pointing accuracy using the same tools as theonline monitoring system. The pointing accuracy is found to vary from3-7^∘ depending on the models. However, if the supernova is closer than10 kpc, SK_SN Notice can issue an alarm with three-degree accuracy, which willbenefit follow-up observations by optical telescopes with large fields of view.