  The X-ray variability of the narrow line Seyfert 1 galaxy Markarian 766 is
studied using 9 ROSAT PSPC datasets. The spectrum is well described by a power
law combined with a blackbody (kT~ 70 eV) soft excess. Examination of flux
ratio changes and variability amplitude in 3 X-ray bands, shows that the power
law component varies continuously on time-scales of ~5000s and is steeper when
it is brighter. In contrast, variability of the soft excess is not detected.
Spectral modeling of 31 spectra from different observations and at a range of
count rates is also consistent with a picture in which the power law is steeper
when it is brighter, and in which the soft excess component does not vary. The
power law variability can be explained if the power law is produced by variable
thermal or non-thermal Comptonization of soft photons. This behaviour is
similar to that of Galactic black hole candidates in the low state. The X-ray
and multiwavelength properties of Markarian 766 are shown to be very similar to
those of other narrow line Seyfert 1s. This may mean that the rapid X-ray
variability seen in other narrow line Seyfert 1s may also not originate in
their strong soft excess components.
