Constraining the very high energy pulsed emission from Crab pulsar with LST-1

Pulsars are neutron stars formed from the collapse of massive stars after a supernova explosion. They rotate rapidly and emit pulsed radiation with periods of seconds, or even milliseconds. Their strong magnetic field creates a magnetosphere where particles are accelerated up to very-high energies. To date, about than 300 gamma-ray pulsars have been detected by Fermi-LAT in the high energy range (HE, 100 MeV < E < 100 GeV). However, only two of them, Crab and Vela, have been detected by ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs) at TeV energies. IACTs operate above tens of GeV, where Fermi-LAT sensitivity drops. Pulsed emission at TeV energies detected by IACTs challenges existing pulsar emission models, such as the polar cap and the slot gap model, which predict an exponential cutoff at 1 GeV and do not account for a very high-energy (VHE, 100 GeV < E < 100 TeV) component. Thus, new scenarios such as the wind-zone and current-sheet models have been proposed to explain the VHE emission. To enhance our understanding of gamma-ray emission from pulsars, advanced instrumentation with improved flux sensitivity is required. This calls for a new generation of IACTs, the upcoming Cherenkov Telescope Array Observatory (CTAO). It will comprise telescopes of different sizes and sensitivities. The largest ones are called Large-Sized Telescopes (LSTs) and are optimized for the lowest energies, starting from 20 GeV. The prototype is the LST-1, operational since 2018 and producing first scientific results. In this work, an analysis of VHE pulsed gamma-ray emission from the Crab pulsar using data from LST-1 of the Northern site of the CTAO is presented. The study aims to detect pulsed emission up to 1 TeV or, alternatively, to establish upper limits on its flux, with the goal of extending previous measurements. The full dataset, collected between October 2018 and July 2025, corresponds to ∼180 hours of observations. Data were processed with the standard LST-1 pipeline and a dedicated pulsar timing package, producing phaseograms and phase-resolved spectra between 20 GeV and 1 TeV. Both expected peaks were detected with high significance (11.47σ for P1, 11.96σ for P2, and 15.72σ for P1+P2 combined). The phase-resolved spectra are well described by power laws, with spectral indices Γ_P1 = 3.0 ± 0.1 and Γ_P2 = 2.80 ± 0.07. Compared with previous LST-1 and MAGIC results, this analysis provides one additional spectral point for the first peak P1 (for both MAGIC and LST-1 datasets) and for the second one P2 (only for the LST-1 dataset), extending the measurements up to 1 TeV. The analysis also confirms the compatibility with both LST-1 and MAGIC results. These findings confirm that the Crab pulsar spectrum shows no clear evidence of a cutoff up to the TeV domain. The next step will be to test whether the observed Crab pulsar emission can be consistently reproduced within the new theoretical models recently proposed for Vela, which may provide more insight into the origin of pulsar emission in the VHE regime. The outcome of this work also highlights the potential of future observations with the array of four LSTs and, eventually, the full CTAO, to detect pulsed VHE emission from the Crab and other gamma-ray pulsars.