Correlation between VHE γ-ray flux (MAGIC, black solid circles, and VERITAS, blue empty circles) and X-ray fluxes. Top: X-ray flux at the 0.3−2 keV band measured with Swift-XRT. Middle: X-ray flux at the 2−10 keV band measured with Swift-XRT. Bottom: X-ray flux at the 2−10 keV band measured with RXTE-PCA. The lines show the fits with linear and quadratic functions. Only MAGIC data points were used for the fits to ensure VHE-X-ray simultaneity (see Appendix A).