Study of the Variable Broadband Emission of Markarian 501 during the Most Extreme Swift X-ray Activity

Markarian 501 is a bright very-high-energy (VHE, E>100 GeV) BL Lac object. Thanks to its brightness and its proximity (z=0.034), it can be detected in the VHE even during low states, making it an excellent blazar for unbiased (not dependent on the emission state) multi-wavelength studies. During a multi-wavelength campaign in 2014 (including data from radio to the VHE band -MAGIC, FACT, Fermi-LAT, Swift, KVA and Metsahovi-), Mrk 501 displayed the highest X-ray activity observed by Swift-XRT since its launch. The X-ray spectra are very hard and show variability on nightly timescales in X-rays and the VHE band. A hint of a narrow spectral feature in the VHE spectra was found in coincidence with the highest flux measured by XRT. Such feature, if real, could suggest the existence of an extra component involving a narrow ultra-energetic electron energy distribution contributing to the broadband emission of Mrk 501.