Variability of VHE $\gamma$-ray sources
  We study changes in the $\gamma$-ray intensity at very high energies observed from selected active galactic nuclei. Publicly available data collected by Cherenkov telescopes were examined by means of a simple method utilizing solely the number of source and background events. Our results point to some degree of time variability in signal observed from the investigated sources. Several measurements were found to be excessive or deficient in the number of source events when compared to the source intensity deduced from other observations. 
Stanislav Stefanik, Dalibor Nosek
astro-ph.HE