Description
Core-collapse supernovae are the birth sites of neutron stars and black holes, yet the properties of
the newly formed compact object remain elusive at optical wavelengths. Fast optical photometry
—with sub-second sampling —opens a new window on the rapidly evolving phases of supernovae
and on the late-time emission that may betray the presence of an active compact remnant.
We report on the first application of the SiFAP2 high-time-resolution photon counter at the TNG
(La Palma) to supernova photometry, using the nearby Type II-L SN 2023ixf in M101 as a test case.
Observations were carried out at 1 s sampling across six epochs spanning from ~2 days to ~772
days post-explosion, covering the rise, the plateau, the radioactive tail, and the late nebular phase.
We describe the methodological challenges of adapting an instrument designed for stellar oscillations
and pulsars to the calibration of a spectrally evolving transient, including the development
of an empirical time-dependent colour correction.
Our late-time photometry reveals a significant excess above the expected radioactive decay at
epochs beyond 700 days, consistent with an additional energy source such as ejecta–CSM interaction
or emission powered by the nascent compact remnant. We discuss the implications of these
findings in the context of neutron star formation and early pulsar activity, and we highlight the potential
of high-cadence optical facilities for the multi-messenger follow-up of nearby core-collapse
events in the era of gravitational wave astronomy.