The outflow in Mrk 509 A method to calibrate XMM-Newton EPIC-pn and RGS

We analyzed three XMM-Newton observations of the Seyfert 1 galaxy Mrk 509, with the goal of detecting small variations in the ionized outflow properties. Measurements such as these are limited by the quality of the cross-calibration between RGS, the best instrument for characterizing the spectrum, and EPIC-pn, the best instrument for characterizing the variability. For all three observations we are able to improve the relative calibration of RGS and pn consistently to 4%. In all observations we detect three different outflow components and, thanks to our accurate cross-calibration, we are able to detect small differences in the ionization parameter and column density in the highest ionized component of the outflow. This constrains the location of this component of the outflow to within 0.5 pc of the central source. Our method for modeling the relative effective area is not restricted to just this source and can in principle be extended to other types of sources as well.