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 was conducted 1 day after the onset of a TeV flare detected by the Whipple Observatory and detected an X-ray flare, with no apparent variability in the optical, UV, and EGRET GeV flux. The ASCA 2-10 keV flux peaked at 3.7 × 10^-10 ergs cm^-2 s^-1 and then decreased to 1.8 × 10^-10 ergs cm^-2 s^-1 with a doubling timescale of ∼12 hr. The shape of the X-ray spectrum varied during the observation, such that 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 those in the 2-7.5 keV band of ∼1 hr. The rapid TeV variability indicates a compact TeV-producing region, suggesting relativistic beaming with a Doppler factor δ ≥ 5. The correlation of the flux in the X-ray and the TeV bands indicates that a high-energy tail of a single electron population is responsible for both X-rays and TeV γ-rays, with radio, IR, UV and X-rays produced via the synchrotron process and GeV and TeV γ-rays produced via Comptonization. Under the assumption that the "soft lag" observed in the X-ray band is due to the synchrotron-lifetime effects, with δ = 5, we calculate the magnetic field for the X-ray-producing region to be ∼0.2 G. The Lorentz factors γ_cl of the electrons responsible for the emission in the keV and TeV bands are ∼10⁶, consistent with the values implied by the Klein-Nishina limit.