The lack of variability of the broad iron line seen in the Seyfert 1 galaxy MCG–6-30-15 is studied using the EPIC pn data obtained from a 2001 observation of 325 ks , during which the count rate from the source varied by a factor of 5 .
The spectrum of 80 ks of data from when the source was bright , using the the lowest 10 ks as background , is well fitted over the 3–10 keV band with a simple power-law of photon index \Gamma = 2.11 .
The source spectrum can therefore be decomposed into an approximately constant component , containing a strong iron emission line , and a variable power law component .
Assuming that the power-law continuum extends down to 0.5 keV enables us to deduce the absorption acting on the nucleus .
A simple model involving Galactic absorption and 2 photoelectric edges ( at 0.72 and 0.86 keV ) is adequate for the EPIC spectrum above 1 keV .
It still gives a fair representation of the spectrum at lower energies where absorption lines , UTAs etc are expected .
Applying the absorption model to the low flux spectrum enables us to characterize it as reflection-dominated with iron about 3 times Solar .
Fitting this model , with an additional power-law continuum , to 32 \times 10 ks spectra of the whole observation reveals that \Gamma lies mostly between 2 and 2.3 and the normalization of the reflection-dominated component varies by up to 25 per cent .
The breadth of the iron line suggests that most of the illumination originates from about 2 gravitational radii .
Gravitational light bending will be very strong there , causing the observed continuum to be a strong function of the source height and much of the continuum radiation to return to the disk .
Together these effects can explain the otherwise puzzling disconnectedness of the continuum and reflection components of MCG–6-30-15 .