16896

Ultra soft Narrow Line Seyfert US-NLS are characterized by a large spectral variability comma a very steep X-ray primary continuum Gamma> 2.5 and a strong soft excess. This remarkable spectral shape questions the origin of their broad-band UV-X-ray continuum compared to standard Seyfert galaxies. We propose to perform a monitoring composed of 5x26 ks XMM observations totaldoublePoint 130ks comma spaced by 2-3 days and coordinated with HST comma to probe the FUV comma of the US-NLS RX J1355.2+5612. The HST COS data are crucial to provide the best constraints on the accretion disc temperature which is expected to be quite hot in these objects. The proposed monitoring will then allow us to 1) disentangle the different spectral components present from the UV-FUV to X-ray energy range and 2) investigate the nature of the soft X-ray excess and discriminate between the theoretical models that have been heavily discussed in the past decade.