Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode

Abstract: The nearby elliptical galaxy M87 contains one of the only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to gamma-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high energy (VHE) gamma-rays, as well as details of the individual observations and light curves. We also conduct phenomenological modelling to investigate the basic source properties. We present the first VHE gamma-ray flare from M87 detected since 2010. The flux above 350 GeV has more than doubled within a period of about 36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE gamma-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and emphasises the need for combined image and spectral modelling.

• The paper presents extensive multi-wavelength observations of M87 during the April 2018 EHT campaign, including the first detection of a VHE gamma-ray flare since 2010.
• Analysis of the multi-wavelength spectral energy distribution revealed enhanced X-ray and gamma-ray flux while radio and millimeter core fluxes remained stable.
• Evidence suggests a steadily increasing jet position angle in M87, correlating with bright spot variations in EHT images and indicating possible structural changes in the jet.

Multi-Wavelength Observations of M87 During the EHT Campaign

This paper presents an extensive analysis of the broadband multi-wavelength observations of the M87 galaxy, focusing on data collected during the Event Horizon Telescope (EHT) campaign in April 2018. The study encompasses observations from radio to very high energy (VHE) γ-rays, offering a comprehensive look at the characteristics and variability of the supermassive black hole (SMBH) known as M87*. This campaign involves collaborative efforts across numerous observatories and scientific teams, aimed at unraveling the complex physics surrounding M87* and enhancing our understanding of high-energy particle acceleration mechanisms.

Key Findings and Observations

• γ-ray Flaring Activity: The paper reports the first detection of a VHE γ-ray flare from M87 since 2010, marking a significant enhancement in γ-ray flux above 350 GeV by more than twofold within approximately 36 hours. This observation is crucial for understanding the source regions of VHE emissions in M87, potentially challenging existing emission models such as the simple one-zone leptonic model.
• Multi-Wavelength Spectral Energy Distribution (SED): A contemporaneous SED spanning from radio to VHE γ-rays is constructed. Notably, the X-ray flux doubled compared to 2017, although radio and millimeter core fluxes remained consistent with previous observations. This enhanced X-ray and γ-ray activity could suggest variations in the inflow-jet dynamics or changes in particle acceleration processes near the SMBH.
• Jet Position Angle Variation: Evidence for a steadily increasing jet position angle is detected, which correlates with variations in the bright spot of EHT images. This finding indicates possible structural changes in the jet, which could be linked to magnetic or relativistic effects driven by the SMBH.

Implications and Model Comparisons

The implications of these observations extend to both the theoretical and practical domains of astrophysics. The detection of the VHE γ-ray flare suggests the need for more complex models beyond single-zone leptonic models. The authors employ heuristic modeling to explore the possible emission regions and processes, revealing that the compactness and high magnetic activity near the SMBH likely play pivotal roles in particle acceleration. The inferred high-energy distribution of particles indicates a stratified emission region where both synchrotron and inverse Compton processes might contribute to the observed spectrum.

Theoretical Context and Future Directions

The results emphasize the necessity for continuous multi-wavelength (MWL) monitoring to further dissect the origins of high-energy particle acceleration. While the current data do not definitively identify the precise location of such acceleration, the findings stress the potential of combining high-resolution imaging with spectral modeling to break new ground in understanding AGN emission mechanisms. Looking forward, future observational campaigns, possibly including more sensitive instruments and extended MWL arrays, are essential for verifying these models and teasing apart the contributions from different emission zones around M87*.

Conclusion

This paper provides valuable insights into the temporal and spectral characteristics of M87* during a period of heightened activity, facilitated by an unprecedented international collaboration through the EHT and associated observational facilities. The dataset and findings presented offer not only immediate analytical outcomes but also a legacy for subsequent studies, aiding in the refinement of models that describe high-energy phenomena around SMBHs. Future investigations are encouraged to address the challenges posed by these findings, advancing our understanding of the complex interplay between accretion, jet dynamics, and particle acceleration in one of the most studied SMBHs in the Universe.