Recent observations of PKS 2155-304 with MAGIC and LST-1 in a multi-wavelength context

TL;DR

The study reports recent multi-wavelength observations of PKS 2155-304 using MAGIC and the first LST-1 telescope at large zenith angles, enhanced by contemporaneous Fermi-LAT, Swift, and ASAS-SN data. The VHE domain reveals a strong flare in 2023 with MAGIC and elevated states in 2023–2024 as seen by LST-1, while HE gamma-ray activity remains mostly quiescent and X-ray/UV/optical bands show correlated but complex variability. These results suggest a predominantly leptonic emission picture with potential multi-zone contributions, challenging simple one-zone SSC models. The work demonstrates the value of joint, cross-instrument campaigns for time-resolved spectral studies and sets the stage for deeper investigations with CTAO and future VHE facilities.

Abstract

PKS 2155-304 is a well-known high-frequency peaked BL Lac (HBL) at redshift z=0.116, which has been extensively studied across the electromagnetic spectrum due to its rapid and large-amplitude variability. Several violent outbursts in X-rays and $γ$-rays have been observed in the past, with intra-night variability in very-high-energy $γ$-rays (VHE; E > 100 GeV) detected down to the minute timescale. The alternation of quiescent and enhanced states, observed with a tentative quasi-periodicity of 1.74 $\pm$ 0.13 years in high-energy (HE; 100 MeV < E < 100 GeV) $γ$-rays, makes this source a key target also for ground-based $γ$-ray instruments and in particular for the Imaging Atmospheric Cherenkov Telescopes. Its brightness, proximity, and well-determined redshift make this $γ$-ray source a prime target for fundamental physics studies, including tests of Lorentz Invariance Violation (LIV), searches for axion-like particles (ALPs), and constraints on the distribution of the extragalactic background light (EBL). In the last two years, PKS 2155-304 has been independently monitored 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) located at the Roque de Los Muchachos Observatory (La Palma, Spain). The observations were carried out at large zenith angles (LZA; ZA > 55°), and the VHE data have been complemented with simultaneous observations in HE $γ$-rays (Fermi-LAT), X-rays (Swift-XRT) and optical wavelengths (ASAS-SN).

Figures (1)

• Figure 1: MWL Light Curves of the blazar PKS 2155-304 in the time range MJD 60000 and MJD 60700 (from February 25, 2023 to January 25, 2025). From top to bottom: MAGIC and LST-1, Fermi-LAT, Swift/XRT, Swift/UVOT, and ASAS-SN).