Detection of two TeV gamma-ray outbursts from NGC 1275 by LHAASO

Abstract

The Water Cherenkov Detector Array (WCDA) is one of the components of Large High Altitude Air Shower Observatory (LHAASO) and can monitor any sources over two-thirds of the sky for up to 7 h per day with >98 per cent duty cycle. In this work, we report the detection of two outbursts of the Fanaroff–Riley I radio galaxy NGC 1275 that were detected by LHAASO-WCDA between 2022 November and 2023 January with statistical significance of 5.2σ and 8.3σ. The observed spectral energy distribution in the range from 500 GeV to 3 TeV is fitted by a power law with the best-fitting spectral index of α = −3.37 ± 0.52 and −3.35 ± 0.29, respectively. The outburst flux above 0.5 TeV was (4.55 ± 4.21) × 10⁻¹¹ cm⁻² s⁻¹ and (3.45 ± 1.78) × 10⁻¹¹ cm⁻² s⁻¹, corresponding to 60 per cent and 45 per cent of Crab Nebula flux, respectively. Variation analysis reveals the variability time-scale of days at the TeV energy band. A simple test by one-zone synchrotron self-Compton model reproduces the data in the gamma-ray band well.