The lack of variability of the iron line in MCG-6-30-15: General relativistic effects

Abstract

The spectrum and variability of the Seyfert galaxy MCG-6-30-15 can be decomposed into two apparently disconnected components: a highly variable power law and an almost constant component which contains a broad and strong iron line. We explore a possible explanation of the puzzling lack of variability of the iron line, by assuming that the variations of the power-law component are due to changes in the height of the primary source in the near vicinity of a rotating black hole. As a result of the bending of light in the strong field of the central black hole, the apparent brightness of the power-law component can vary by about a factor of 4 according to its position, while the total iron line flux variability is less than 20 per cent. This behaviour is obtained if the primary source is located within 3-4 gravitational radii (rg) from the rotation axis with a variable height of between ∼3 and 8r g. These results revive the possibility that future X-ray observations of MCG-6-30-15 can map out the strong gravity regime of accreting black holes.