ASCA OBSERVATION OF AN X-RAY TEV FLARE FROM THE BL LACERTAE OBJECT MARKARIAN-421/

We observed the BL Lac object Mrk 421 with the X-ray satellite ASCA in 1994 as part of a multifrequency observation. The 24 hr observation w as conducted 1 day after the onset of a TeV flare detected by the Whip ple Observatory and detected an X-ray flare, with no apparent variabil ity in the optical, UV, and EGRET GeV flux. The ASCA 2-10 keV flux pea ked at 3.7 x 10(-10) ergs cm(-2) s(-1) and then decreased to 1.8 x 10( -10) ergs cm(-2) s(-1) with a doubling timescale of similar to 12 hr. The shape of the X-ray spectrum varied during the observation, such th at the hard X-rays always led the soft X-rays, both in brightening and dimming of the source, with a lag of the 0.5-1 keV photons versus tho se in the 2-7.5 keV band of similar to 1 hr. The rapid TeV variability indicates a compact TeV-producing region, suggesting relativistic bea ming with a Doppler factor delta greater than or equal to 5. The corre lation of the flux in the X-ray and the TeV bands indicates that a hig h-energy tail of a single electron population is responsible for both X-rays and TeV gamma-rays, with radio, IR, UV and X-rays produced via the synchrotron process and GeV and TeV gamma-rays produced via Compto nization. Under the assumption that the ''soft lag'' observed in the X -ray band is due to the synchrotron-lifetime effects, with delta = 5, we calculate the magnetic field for the X-ray-producing region to be s imilar to 0.2 G. The Lorentz factors gamma(el) of the electrons respon sible for the emission in the keV and TeV bands are similar to 10(6), consistent with the values implied by the Klein-Nishina limit.