LHAASO Observations of Mrk 421 during 2021 March–2024 March: A Comprehensive Very-high-energy Catalog of Multitimescale Outbursts and Its Time-averaged Behavior

The Large High Altitude Air Shower Observatory (LHAASO) monitors sources within its field of view for up to 7 hr daily, achieving a duty cycle exceeding 98% and an annual point-source sensitivity of 1.5% Crab units (CU) in the very-high-energy (VHE) band. This unbiased sky-survey mode facilitates systematic monitoring and investigation of outburst phenomena. In this paper, we present results from an unprecedented 3 yr monitoring campaign (2021 March–2024 March) of Markarian 421 (Mrk 421) using LHAASO, spanning energies from 0.4 to 20 TeV. We find that the blazar stayed in a quiescent state in 2021 and became active starting in 2022 with a total of 23 VHE outburst events identified, where the highest observed daily significance reaches 20σ with a flux equivalent to approximately 3.3 CU. LHAASO’s continuous monitoring suggests the flaring occupancy of Mrk 421 to be around 14%. During long-term monitoring, multiwavelength (MWL) variability and correlation analyses are conducted using complementary data from the Fermi Large Area Telescope, Monitor of All-sky X-ray Image Gas Slit Camera, the Neil Gehrels Swift Observatory X-Ray Telescope, and the Zwicky Transient Facility. A significant correlation (>3σ) is observed between the X-ray and VHE bands with no detectable time lag, while the correlation between the GeV and TeV bands is weaker. The energy spectrum of the TeV emission during the quiescent state is different from that in the active state, implying the existence of two modes of energy dissipation in the blazar jet. Using simultaneous MWL data, we also analyzed both the long-term and outburst-period spectral energy distributions, and discussed the possible origin of the outburst events.