Outburst of TeV photons from Markarian 421

Abstract

Markarian 421, an active galactic nucleus (AGN) of the BL Lacertae type, is the closest EGRET-detected AGN. It has been monitored by the Whipple Observatory very high energy gamma-ray telescope since its discovery at TeV energies (Punch et al. 1992), for approximately 90 nights, totaling 130 hours of observation. Observations from 1993 December 23 to 1994 May 10 showed an average source flux only half that of its 1992 discovery level. However, observations on 1994 May 14 and 15 show an increase in flux above this quiescent level by a factor of ~10. The timescale of this increase provides the best geometric constraint on the extent of TeV photon emission regions within AGNs. The observation of the high TeV flux occurred 1 day prior to the observation by ASCA of a very high 2-10 keV X-ray flux (Takahashi et al. 1994). This strong TeV outburst is reminiscent of the behavior seen for the stronger 100 MeV-GeV EGRET sources (e.g., 3C 279; Kniffen et al. 1993) but was not anticipated in view of the fact that the six EGRET observations of Mrk 421 from 1991 June to 1993 July (Lin et al. 1994) showed no evidence for variability.