The nearby radio galaxy M87 is a very high-energy (VHE) gamma-ray emitter established by observations with ground-based gamma-ray detectors. Here we report the long-term monitoring of M87 from 2021 to 2024 with the Large High Altitude Air Shower Observatory (LHAASO). M87 has been detected by LHAASO with a statistical significance ∼ 9σ. The observed energy spectrum extends to 20 TeV, with a possible hardening at ∼20 TeV and then a clear softening at higher energies. Assuming that the intrinsic spectrum is described by a single power law up to 20 TeV, a tight upper bound on the extragalactic background light intensity is obtained. A strong VHE flare lasting 8 days, with a rise time of days and decay time of days, was found in early 2022. A possible GeV flare is seen also in Fermi Large Area Telescope data during the VHE flare period. The variability time as short as 1 day seen in the LHAASO data suggests a compact emission region with a size of ∼3 × 1015δ cm (δ being the Doppler factor of the emitting region), corresponding to a few Schwarzschild radii of the central supermassive black hole in M87. The continuous monitoring of the source reveals a duty cycle of ∼1% for VHE flares with a flux above 10−11 erg cm−2 s−1.

中文翻译：

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使用 LHAASO 检测无线电 Galaxy M87 的极高能伽马射线发射


附近的射电星系 M87 是一个非常高能 （VHE） 的伽马射线发射器，它是通过使用地面伽马射线探测器的观测建立的。在这里，我们报告了大型高空气淋天文台 （LHAASO） 从 2021 年到 2024 年对 M87 的长期监测。M87 已被 LHAASO 检测到，具有统计学意义 ∼ 9σ。观察到的能谱延伸到 20 TeV，在 ∼20 TeV 时可能会硬化，然后在更高的能量下明显软化。假设本征光谱由高达 20 TeV 的单一幂律描述，则获得星系外背景光强度的严格上限。2022 年初发现持续 8 天的强烈 VHE 耀斑，上升时间为 天，衰减时间为 天。在 VHE 耀斑期间，在费米大面积望远镜的数据中也可以看到可能的 GeV 耀斑。在 LHAASO 数据中看到的短至 1 天的变率时间表明，一个紧凑的发射区，大小为 ∼3 × 1015δ cm（δ 是发射区的多普勒因子），对应于 M87 中中心超大质量黑洞的几个史瓦西半径。对源的连续监测显示，磁通量高于 10-11 erg cm-2 s-1 的 VHE 耀斑的占空比为 ∼1%。