The quiescent state broadband X-ray spectrum and variability of Mkn 421

The BL Lac object Mkn 421 was observed three times by the X-ray observatory BeppoSAX in consecutive days during 1997 April and May. The source was in a quiescent state, with an average 2-10 keV flux of 9.0 x 10(-11) erg cm(-2 ) s(-1) Flux variation by a factor of similar or equal to 2 on timescales a s short as a few 10(4) s were more pronounced in the hard (i.e. above simil ar or equal to 3 keV) than in the soft X-rays. The broadband (0.1-40 keV) s pectrum is concave and can be most easily explained with a power-law model which steepens gradually with energy. In this framework, neither photoabsor ption edges nor resonant absorption lines are required, strengthening the c ase against the ubiquity of such features in BL Lac objects, which had been previously suggested by Einstein observations. The broadband spectrum hard ens with hard X-ray flux, mostly due to a flattening above similar or equal to 4 keV. This suggests that the relativistic highest energy electron dist ribution properties drive the X-ray spectral dynamics: either a stratificat ion of the distribution in the jet with energy or inhomogeneities in the el ectron injection mechanism could be consistent with the observed variabilit y pattern.