2019MNRAS.484.2944G

Multiwavelength campaigns have been carried out to study the correlation between the very high-energy (VHE) γ-ray and the X-ray emissions in blazars but no conclusive results have been achieved yet. In this paper, we add Milagro data to the existing VHE γ-ray data from HEGRA-CT1 and Whipple and test the consistency and robustness of the reported correlation between VHE γ-ray and X-ray fluxes in Mrk 421. We found that on a monthly time-scale the correlation is robust, consistent between instruments, and described as a linear function. Furthermore, most of the fluxes on shorter time-scales are consistent with the correlation within 3σ_A even, where σ_A is an estimated intrinsic scatter. However, a break-down of the correlation becomes clearly evident at high states of activity with fluxes ≳¡ 2.5×10^–10 cm^–2 s^–1 at energies above 400 GeV independently of the time-scale, observational period, or instrument; even for single flares, the X-ray and VHE γ-ray emissions lie on the correlation until the VHE γ-ray flux reaches values higher than the one mentioned above. We have interpreted our results within the one-zone synchrotron self-Compton model. We found that describing a single and unique γ-ray/X-ray correlation strongly narrows the range of possible values of the magnetic field B when a constant value of the spectral index along the correlation is assumed.