Abstract
Abstract We present our analysis of the Suzaku data of SS Cygni (SS Cyg) from 2005 both in quiescence and outburst. A fluorescent iron Kα line bears significant information about the geometry of an X-ray-emitting hot plasma and a cold reflector, such as the surfaces of the white dwarf (WD) and the accretion disk (AD). Our reflection simulation has revealed that the X-ray-emitting hot plasma is located either very close to the WD surface in the boundary layer (BL), with an upper limit radial position of <1.004 times the white dwarf radius (RWD), or near the entrance of the BL where the optically thick AD is truncated at a distance of 1.14–1.27 RWD for the assumed WD mass of 1.19 M⊙ in quiescence. In the latter configuration, the plasma torus is located just above the inner edge of the AD. The result suggests that the accreting matter is heated up close to the maximum temperature immediately after the matter enters the BL. The matter probably expands precipitously at the entrance of the BL and leaves the disk plane to reach a height comparable to the radial distance of the plasma torus from the center of the WD. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface. In addition, the plasma distributes above the disk like coronae, as suggested by a previous study, and the 90% upper limit of the coronae radial position is 1.2 RWD.
Highlights
A cataclysmic variable (CV) is a semi-detached binary system consisting of a late-type star and a white dwarf (WD) whose optical brightness fluctuates on a time scale of seconds to years
The previous study reported that the optically thick accretion disk (AD) is truncated before reaching the WD surface, and the optically thin hard-X-ray-emitting plasma is formed in the boundary layer (BL) (Patterson & Raymond 1985) which is located between the WD surface and the inner edge of the AD
In subsection 3.2, we showed based on our reflection simulation that the best fit to the 6.4 keV emission line is provided by the configuration where the plasma torus is located very close to the WD surface (Rp < 1.004) while the disk truncation radius Rc–i is not constrained at all
Summary
A cataclysmic variable (CV) is a semi-detached binary system consisting of a late-type star and a white dwarf (WD) whose optical brightness fluctuates on a time scale of seconds to years. In the case of SS Cyg, as the mass accretion rate increases, the extreme ultraviolet (EUV) flux suddenly increases simultaneously with abrupt hard X-ray flux suppression (Wheatley et al 2003) This is an optically thin to thick transition of the boundary layer (BL) that is formed between the inner edge of the Keplerian disk and the surface of the WD. We have investigated the state of the X-ray-emitting hot plasma with a primary focus on the geometry of the plasma by comparing an observed iron 6.4 keV fluorescence line with our spectral simulations of reflection from the WD and AD surfaces For this investigation, we utilize the Suzaku archival data of SS Cyg taken in 2005.
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