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Detection of very high-energy gamma-ray emission from the radio galaxy M87 with LHAASO

Zhen Cao, F. Aharonian, Axikegu, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, A. V. Bukevich, Q. Cao, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, A. M. Chen, E. S. Chen, H. X. Chen, Liang Chen, Lin Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen, S. Chen, S. H. Chen, S. Z. Chen, T. L. Chen, Y. Chen, N. Cheng, Y. D. Cheng, M. C. Chu, M. Y. Cui, S. W. Cui, X. H. Cui, Y. D. Cui, B. Z. Dai, H. L. Dai, Z. G. Dai, Danzengluobu, X. Q. Dong, K. K. Duan, J. H. Fan, Y. Z. Fan, J. Fang, J. H. Fang, K. Fang, C. F. Feng, H. Feng, L. Feng, S. H. Feng, X. T. Feng, Y. Feng, Y. L. Feng, S. Gabici, B. Gao, C. D. Gao, Q. Gao, W. Gao, W. K. Gao, M. M. Ge, T. T. Ge, L. S. Geng, G. Giacinti, G. H. Gong, Q. B. Gou, M. H. Gu, F. L. Guo, J. Guo, X. L. Guo, Y. Q. Guo, Y. Y. Guo, Y. A. Han, O. A. Hannuksela, M. Hasan, H. H. He, H. N. He, J. Y. He, Y. He, Y. K. Hor, B. W. Hou, C. Hou, X. Hou, H. B. Hu, Q. Hu, S. C. Hu, C. Huang, D. H. Huang, T. Q. Huang, W. J. Huang, X. T. Huang, X. Y. Huang, Y. Huang, Y. Y. Huang, X. L. Ji, H. Y. Jia, K. Jia, H. B. Jiang, K. Jiang, X. W. Jiang, Z. J. Jiang, M. Jin, M. M. Kang, I. Karpikov, D. Khangulyan, D. Kuleshov, K. Kurinov, B. B. Li, C. M. Li, Cheng Li, Cong Li, D. Li, F. Li, H. B. Li, H. C. Li, Jian Li, Jie Li, K. Li, S. D. Li, W. L. Li, W. L. Li, X. R. Li, Xin Li, Y. Z. Li, Zhe Li, Zhuo Li, E. W. Liang, Y. F. Liang, S. J. Lin, B. Liu, C. Liu, D. Liu, D. B. Liu, H. Liu, H. D. Liu, J. Liu, J. L. Liu, M. Y. Liu, R. Y. Liu, S. M. Liu, W. Liu, Y. Liu, Y. N. Liu, Q. Luo, Y. Luo, H. K. Lv, B. Q. Ma, L. L. Ma, X. H. Ma, J. R. Mao, Z. Min, W. Mitthumsiri, H. J. Mu, Y. C. Nan, A. Neronov, K. C. Y. Ng, L. J. Ou, P. Pattarakijwanich, Z. Y. Pei, J. C. Qi, M. Y. Qi, B. Q. Qiao, J. J. Qin, A. Raza, D. Ruffolo, A. Sáiz, M. Saeed, D. Semikoz, L. Shao, O. Shchegolev, X. D. Sheng, F. W. Shu, H. C. Song, Yu. V. Stenkin, V. Stepanov, Y. Su, D. X. Sun, Q. N. Sun, X. N. Sun, Z. B. Sun, J. Takata, P. H. T. Tam, Q. W. Tang, R. Tang, Z. B. Tang, W. W. Tian, L. H. Wan, C. Wang, C. B. Wang, G. W. Wang, H. G. Wang, H. H. Wang, J. C. Wang, Kai Wang, Kai Wang, L. P. Wang, L. Y. Wang, P. H. Wang, R. Wang, W. Wang, X. G. Wang, X. Y. Wang, Y. Wang, Y. D. Wang, Y. J. Wang, Z. H. Wang, Z. X. Wang, Zhen Wang, Zheng Wang, D. M. Wei, J. J. Wei, Y. J. Wei, T. Wen, C. Y. Wu, H. R. Wu, Q. W. Wu, S. Wu, X. F. Wu, Y. S. Wu, S. Q. Xi, J. Xia, G. M. Xiang, D. X. Xiao, G. Xiao, Y. L. Xin, Y. Xing, D. R. Xiong, Z. Xiong, D. L. Xu, R. F. Xu, R. X. Xu, W. L. Xu, L. Xue, D. H. Yan, J. Z. Yan, T. Yan, C. W. Yang, C. Y. Yang, F. Yang, F. F. Yang, L. L. Yang, M. J. Yang, R. Z. Yang, W. X. Yang, Y. H. Yao, Z. G. Yao, L. Q. Yin, N. Yin, X. H. You, Z. Y. You, Y. H. Yu, Q. Yuan, H. Yue, H. D. Zeng, T. X. Zeng, W. Zeng, M. Zha, B. B. Zhang, F. Zhang, H. Zhang, H. M. Zhang, H. Y. Zhang, J. L. Zhang, Li Zhang, P. F. Zhang, P. P. Zhang, R. Zhang, S. B. Zhang, S. R. Zhang, S. S. Zhang, X. Zhang, X. P. Zhang, Y. F. Zhang, Yi Zhang, Yong Zhang, B. Zhao, J. Zhao, L. Zhao, L. Z. Zhao, S. P. Zhao, X. H. Zhao, F. Zheng, W. J. Zhong, B. Zhou, H. Zhou, J. N. Zhou, M. Zhou, P. Zhou, R. Zhou, X. X. Zhou, X. X. Zhou, B. Y. Zhu, C. G. Zhu, F. R. Zhu, H. Zhu, K. J. Zhu, Y. C. Zou, X. Zuo

TL;DR

This study presents a three-year monitoring campaign of the radio galaxy M87 with LHAASO, delivering a robust VHE detection up to about $20~\mathrm{TeV}$ and identifying a single eight-day flare in early 2022, with a compact emission region near the central SMBH implied by the rapid variability. By combining LHAASO data with Fermi-LAT observations, the authors constrain the intrinsic TeV spectrum, test EBL absorption using multiple models, and place upper limits on the EBL intensity (e.g., $f_{\rm EBL}\lesssim1.6$ for Franceschini and $\lesssim1.09$ for Dominguez-upper at 95% CL). The results favor near-core or very compact jet emission, challenge simple one-zone SSC explanations for the long-term TeV emission, and motivate multi-zone or magnetospheric acceleration scenarios, while providing a data-driven benchmark for the duty cycle and energetics of VHE flares in M87. The continuous, wide-field capabilities of LHAASO enable unprecedented, unbiased monitoring of a nearby AGN in the TeV band, with implications for jet physics and high-energy photon propagation through the extragalactic medium.

Abstract

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 Large High Altitude Air Shower Observatory (LHAASO). M87 has been detected by LHAASO with a statistical significance $\sim 9σ$. The observed energy spectrum extends to 20 TeV, with a possible hardening at $\sim 20$ 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 (EBL) intensity is obtained. A strong VHE flare lasting eight days, with the rise time of $τ_{r}^{\rm rise} = 1.05\pm0.49$~days and decay time of $τ_{d}^{\rm decay} = 2.17\pm0.58$~days, was found in early 2022. A possible GeV flare is seen also in the Fermi-LAT data during the VHE flare period. The variability time as short as one day seen in the LHAASO data suggests a compact emission region with a size of $\sim 3\times 10^{15}δ\, {\rm cm}$ ($δ$ 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 $\sim 1\%$ for VHE flares with a flux above $ 10^{-11}{\rm~erg~cm^{-2}~s^{-1}}$.

Detection of very high-energy gamma-ray emission from the radio galaxy M87 with LHAASO

TL;DR

This study presents a three-year monitoring campaign of the radio galaxy M87 with LHAASO, delivering a robust VHE detection up to about and identifying a single eight-day flare in early 2022, with a compact emission region near the central SMBH implied by the rapid variability. By combining LHAASO data with Fermi-LAT observations, the authors constrain the intrinsic TeV spectrum, test EBL absorption using multiple models, and place upper limits on the EBL intensity (e.g., for Franceschini and for Dominguez-upper at 95% CL). The results favor near-core or very compact jet emission, challenge simple one-zone SSC explanations for the long-term TeV emission, and motivate multi-zone or magnetospheric acceleration scenarios, while providing a data-driven benchmark for the duty cycle and energetics of VHE flares in M87. The continuous, wide-field capabilities of LHAASO enable unprecedented, unbiased monitoring of a nearby AGN in the TeV band, with implications for jet physics and high-energy photon propagation through the extragalactic medium.

Abstract

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 Large High Altitude Air Shower Observatory (LHAASO). M87 has been detected by LHAASO with a statistical significance . The observed energy spectrum extends to 20 TeV, with a possible hardening at 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 (EBL) intensity is obtained. A strong VHE flare lasting eight days, with the rise time of ~days and decay time of ~days, was found in early 2022. A possible GeV flare is seen also in the Fermi-LAT data during the VHE flare period. The variability time as short as one day seen in the LHAASO data suggests a compact emission region with a size of ( 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 for VHE flares with a flux above .

Paper Structure

This paper contains 15 sections, 3 equations, 8 figures, 4 tables.

Table of Contents

  1. Introduction
  2. LHAASO observations
  3. The full-time average spectrum
  4. The light curve
  5. The 2022 VHE flare
  6. The low-state emission
  7. Fermi-LAT observations
  8. Discussions
  9. VHE flare emission
  10. Long-term low-state VHE emission
  11. Constraining the upper bound of the EBL intensity with the VHE data
  12. Influence of the possible internal absorption on the spectrum
  13. Summary
  14. LHAASO Data Reduction
  15. Fermi-LAT Data Reduction

Figures (8)

  • Figure 1: Significance maps of $2^{\circ}\times2^{\circ}$ region around M87 by LHAASO-WCDA for full-time period (left panel), flare state (middle panel) and low state (right panel), respectively. M87 is marked by cyan open cross. The gray circle indicates the 95% position error of the source. The white circle at the bottom-left corner shows the size of the LHAASO PSF (68% containment).
  • Figure 2: Observed VHE spectra of M87 by LHAASO during the full-time period assuming the PLEC function and LP function, respectively. The solid lines indicate the best-fitting results and the blue shaded regions indicate the $1\sigma$ statistical error. Left: The PLEC function is adopted to fit the observational data. Right: The LP function is adopted to fit the observational data.
  • Figure 3: VHE spectra of M87 during the full-time period (left panel), the flare state (middle panel) and the low state (right panel), assuming a single power-law model for the intrinsic spectra and a EBL absorption model of 2008AA...487..837F. The gray and blue shaded regions indicate $1\sigma$ statistical error for the intrinsic spectra and PLxEBL spectra, respectively.
  • Figure 4: Light curves of LHAASO-WCDA (top panels) and Fermi-LAT (bottom panels). Left panel: long-term light curves (three-month bin) of LHAASO-WCDA and Fermi-LAT for M87. Right panel: zoom-in light curves around the flare period. The light curve of LHAASO-WCDA is obtained with two-day bin and that of Fermi-LAT is obtained with 8-day bin. $1\sigma$ upper limit is reported when $\rm TS<9$ for the LAT light curve data. The LHAASO-WCDA flare period is shown as the gray vertical shaded region. The blue line in the top panel indicates the bayesian block and the gray lines show the long-term average flux.
  • Figure 5: Fitting of the one-day bin light curve (range from MJD 59593 to MJD 59629) of the VHE flare measured by LHAASO-WCDA with a two-sided exponential function (the red line). Fitting parameters are $F_0=(5.62\pm1.26)\times10^{-12}~ \rm TeV^{-1}~cm^{-2}~s^{-1}$, $t_0= \rm MJD~59610\pm0.58$, $\tau_{d}^{\rm rise}=1.05\pm0.49$ days and $\tau_{d}^{\rm decay}=2.17\pm0.58$ days.
  • ...and 3 more figures