The origin of the soft X-ray excess in AGN has been a mystery ever since its discovery. We present how the time variability of this spectral component can point towards its origin. Using the powerful technique of multi-epoch fitting, we study how the soft excess in a given object depends on other parameters of the continuum and the accretion disk possibly hinting at its nature. As an example, we present results from this technique applied to the Seyfert galaxy Mrk 841, the source in which the soft excess was originally discovered. We study all (3) XMM and some of the Suzaku pointings available and find that the source displays an impressive variability in the soft X-ray band on the timescale of years. We study several common soft excess models and their ability to physically consistently explain this spectral variability. Mrk 841 is found to show a distinct variability pattern that can be best explained by the soft excess originating mostly from a thermal Comptonization component. The variability timescale can be constrained to be on the order of a few days.
On the Evolution of the Bi-Modal Distribution of Galaxies in the Plane of Specific Star Formation Rate versus Stellar Mass
We have compared the observed distribution of galaxies in the plane of specific star formation rate versus stellar mass with the predictions of the Garching semi-analytic model at redshifts 0, 1 and 2. The goal is to test whether the implementation of radio mode AGN feedback, which is responsible for terminating the formation of stars in high mass galaxies, provides an adequate match to current high-redshift observations. The fraction of quenched galaxies as a function of stellar mass in the models is in good agreement with data at z=0 and z=1. By z=2, there are too few quenched galaxies with low stellar masses in the models. At z=2, the population of galaxies with no ongoing star formation is clearly separated from the `main sequence’ of star-forming galaxies in the data. This is not found in the models, because z=2 galaxies with stellar masses less than 10^11 solar masses are predicted to host black holes with relatively low masses (less than 10^8 solar masses). The current implementation of radio mode feedback from such black holes reduces the cooling rates from the surrounding halo, but does not generate sufficient energy to stop star formation entirely. We suggest that the models may be brought into better agreement with the data if black hole growth is triggered by disc instabilities in addition to major mergers, and if feedback mechanisms associated with the formation of galactic bulges act to quench star formation in galaxies.