Gamma-ray variability of BL lacs at E>300 GeV

Abstract We report on observations of Mrk 421 and Mrk 501 at E γ >300 GeV during the 1995 and 1996 seasons. A multiwavelength observing campaign on Mrk 421 in the period 1995 April 20 to May 5 indicates correlations in the γ-ray, X-ray, extreme ultraviolet, and optical wavelengths. Dramatic outbursts of TeV γ-rays were detected on two occasions in May of 1996 where the γ-ray flux reached the highest levels ever recorded (a factor of ∼20 greater than typical levels) with variability timescales ≲1hr. Together with the multiwavelength correlations these provide serious constraints on the Doppler factor ( δ ≳ 9.9), the size and location of the emission region ( R ≲ 10 −4 pc) and the mechanism responsible for the γ-ray emission.