X-ray spectral variability and complex absorption in the Seyfert 1 galaxies NGC 4051 and MCG -6-30-15

GINGA observations of the Seyfert 1 galaxies NGC 4051 and MCG -6-30- 15 have led to new and exciting results. (1) Large-amplitude ( > 50%) variations have been detected, down to time scales of 200 s, that confirm and extend previous EXOSAT results. (2) These intensity variations are associated with spectral changes, the spectrum being steeper when the source is brighter. Spectral variability is well described by spectral index changes that correlates closely with the 2-10 keV luminosity, increasing by ~{DELTA}α ~ 0.4-0.5 and centered on α ~ 1.7 (photon index) when the flux increases by a factor of 3-4. (3) A flat spectral component has been detected in the spectrum of MCG -6-30-15 above 10 keV. This component is probably produced by absorption and Compton scattering of the central continuum by very thick (N_H_ > 10^24^ cm^-2^) cold matter in the environment of the central source. The detection of an iron fluorescence line at 6.4 keV with an equivalent width of ~ 200 eV supports this interpretation.