On the origin of the featureless soft X-ray excess emission from the Seyfert 1 galaxy ESO 198-G24

We present medium- and high-resolution X-ray spectral study of a Seyfert 1 galaxy ESO 198-G24 using a long (∼122 ks) XMM-Newton observation performed in 2006 February. The source has a prominent featureless soft X-ray excess below 2 keV. This makes the source well suited to investigate the origin of the soft excess. Two physical models - blurred reflection and optically thick thermal Comptonization in a warm plasma, describe the soft excess equally well resulting in similar fits in the 0.3-10 keV band. These models also yield similar fits to the broad-band UV (optical monitor) and X-ray data. XMM-Newton observations performed in 2000, 2001 and 2006 on this source show flux variability. From 2001 to 2006, the UV flux increased by ∼23 per cent while the 2-10 keV X-ray flux as well as the soft-excess flux decreased by ∼20 per cent. This observation can be described in the blurred reflection scenario by a truncated accretion disc whose inner-most radius had come closer to the black hole. We find that the best-fitting inner radius of the accretion disc decreases from R_in=4.93_{-1.10}^{+1.12}R_G to R_in < 2.5R_G from 2001 to 2006. This leads to an increase in the UV flux and compressing the corona, leading to reduction of the power-law flux and therefore the soft excess. The blurred reflection model seems to better describe the soft excess for this source.