Properties of the Boundary Layer of SS Cygni Unveiled with the X-ray Microcalorimeter Onboard XRISM

We observed the dwarf nova SS Cyg with the X-ray microcalorimeter onboard XRISM both in quiescence and outburst. The quiescence spectrum is explained with a multi-temperature optically thin thermal plasma emission model, characterized by He alpha and Ly alpha emission lines of Fe and Ly alpha lines of Si and S. We discovered redshifts of the Fe He alpha and Ly alpha lines that are larger than Ly alpha lines from Si and S. This indicates that the boundary layer ( BL) plasma accretes on to the white dwarf ( WD) accompanied by radiative cooling. Its radial accretion velocity estimated from the Fe redshift is 210-280 km s(-1) . Since a 6.4 keV Fe emission line is much narrower (ov= 561 km s(-1)) than that expected from the inner accretion disk rotation, we believe that it is emitted from the surface of the WD. The outburst spectrum can also be explained with the multi-temperature optically thin thermal plasma emission model, but is much softer than in quiescence and is dominated by He alpha lines. The Fe He alpha and Ly alpha lines and the 6.4 keV line are much broader than in quiescence, with Gaussian sigma values of 2390 and 3040 km s(-1) , respectively. This leads us to the idea that the hot plasma is a corona generated through magnetic activity in the optically thick BL that has strong differential rotation, and is anchored to the BL with the magnetic field. This picture is supported by the facts that the plasma follows the universal correlation between the temperature and the emission measure of the stellar flares, and the relation between the X-ray luminosity and the photospheric luminosity of low-mass stars. The fluorescent 6.4 keV line is emitted via fluorescence from the optically thick BL and/or from the equatorial accretion belt formed temporarily during the outburst.