Very hard gamma-ray emission from a flare of Mrk 501

Understand

We investigate the peculiar properties of a large TeV gamma-ray flare from Mrk~501 detected during the 2009 multiwavelength campaign.

• We identify the counterpart of the flare in the Fermi/LAT telescope data and study its spectral and timing characteristics.
• A strong order-of-magnitude increase of the very-high-energy gamma-ray flux during the flare was not accompanied by an increase in the X-ray flux, so that the flare was one of the "orphan"-type TeV flares observed in BL Lacs.
• The flare lasted about 1 month at energies above 10 GeV.