Causal link between elevated X‑ray/UV/optical fluxes and the 2018 VHE flare

Determine whether the elevated X‑ray, ultraviolet, and optical fluxes observed near the April 2018 flare in M87 are causally associated with the VHE gamma‑ray flare by establishing temporal and spectral correlations on appropriate timescales.

Background

The campaign detected increased core X‑ray flux relative to 2017 and higher optical/UV fluxes during the period surrounding the VHE flare, but sparse sampling prevents a clear association.

The authors emphasize that similar X‑ray variability can occur without VHE flares, leaving the relationship between the high‑energy flare and lower‑energy enhancements unresolved.

References

Figure 1 shows that M87's core X-ray flux can vary to a similar degree even in the absence of a flare, so the elevated X-ray/UV/optical flux near the 2018 flare cannot be definitively linked to the gamma-ray flare itself.

— Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode (2404.17623 - Algaba et al., 24 Apr 2024) in Section 3.3.2 Flux enhancement in the X-ray band