Recent observations of the blazar PKS 2155-304 with MAGIC and LST-1 and prospects for the CTAO-South array

PKS 2155-304 is one of the best-studied high-synchrotron-peaked BL Lac objects in the southern sky and a key target for very-high-energy (VHE; E > 100 GeV) gamma-ray astronomy. The source became particularly prominent after the exceptional flare detected by the High Energy Stereoscopic System (H.E.S.S.) telescopes, in Namibia, in July 2006, during which the flux above 200 GeV exceeded ten times the quiescent level with variability on timescales as short as 200 seconds. This work presents the analysis of the most recent VHE observations of PKS 2155-304 during 2023–2024 by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes and the first Large-Sized Telescope (LST-1) of the Cherenkov Telescope Array Observatory (CTAO), both located at the Observatorio del Roque de los Muchachos on La Palma (Spain). Unlike at the H.E.S.S. site, PKS 2155-304 can be observed in the northern hemisphere only at relatively low altitude above the horizon (Zenith Angle (ZA) > 55 deg), which is an observational challenge for Cherenkov telescopes. After quality cuts, 9.4 hours of MAGIC data yield a detection significance of 19.5σ , while LST-1 provides 3.2 hours in 2023 and 13.5 hours in 2024, with significances of 9.6σ and 10.5σ respectively, demonstrating that PKS 2155-304 can be successfully studied with a single LST at large ZA. The VHE results are complemented by multi-wavelength observations from ASAS-SN (optical), Swift (UV-X-ray) and Fermi-LAT (HE; 100 MeV < E < 100 GeV). In addition, this work presents a simulation- based study of future PKS 2155-304 observations with CTAO-South in Chile, using the most recent time-dependent model of the 2006 H.E.S.S. flare as input to the CtaAgnVar software for simulation and analysis. Two array configurations are compared as limiting cases for the two LSTs foreseen in the CTA+ project: a baseline without LSTs and an enhanced configuration with four LSTs. Fitting an exponential cut-off power law in the 0.05–3 TeV range, the temporal evolution of spectral parameters is studied for two outburst scenarios. For a short outburst with 5-minute bins, the mean per-bin significances are∼148σ and∼152σ for the baseline and four-LST configurations, respectively; for a longer outburst with 40-minute bins, the values are ∼226σ and∼249σ . Overall, the study reveals no large difference in performance between the two array configurations, with the LST advantage most pronounced at the shortest timescales and lowest energies. The largest improvement from the CTA+ LSTs is expected during the early construction phase of CTAO-South, before the full MST and SST arrays are in place.