Abstract

We present our analysis of the Suzaku data of U Geminorum (U Gem) from 2012 both in quiescence and outburst. Unlike SS Cygni (SS Cyg), the hard X-ray flux of U Gem is known to increase at times of optical outburst. A sophisticated spectral model and reliable distance estimate now reveal that this can be attributed to the fact that the mass accretion rate onto the white dwarf (WD) does not exceed the critical rate that causes the optically thin to thick transition of the boundary layer. From comparison of the X-ray and optical light curves, the X-ray outburst peak seems to be retarded by 2.1 ± 0.5 d, although there remains uncertainty in the X-ray peak identification, due to short data coverage. The larger delay than SS Cyg (0.9–1.4 d) also supports the lower accretion rate in U Gem. A fluorescent iron 6.4 keV emission line bears significant information about the geometry of the X-ray-emitting hot plasma and the accretion disk (AD) that reflects the hard X-ray emission. Our reflection simulation has shown that the optically thick AD is truncated at a distance of 1.20–1.25 times the white dwarf radius (R_WD) in quiescence, and the accreting matter in the disk turns into the optically thin hard-X-ray-emitting plasma at this radius. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface, although disk truncation can take place in the region of <1.012 R_WD. From the profile of the 6.4 keV line, we have also discovered that the accreting matter is heated up close to the maximum temperature immediately after the matter enters the boundary layer at the disk truncation radius. This is consistent with the fact that the hard X-ray spectra of dwarf novae, in general, can be well represented with the cooling flow model.

中文翻译：

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静止和突出的U Geminorum X射线发射等离子体的空间分布

摘要

我们目前对静止期和爆发期U Geminorum（U Gem）的Suzaku数据进行分析。与SS Cygni（SS Cyg）不同，已知U Gem的硬X射线通量会在光学爆发时增加。复杂的光谱模型和可靠的距离估算现在表明，这可以归因于以下事实：白矮星（WD）上的质量积聚速率未超过导致边界层从光学上薄到厚过渡的临界速率。通过比较X射线和光学曲线，尽管由于数据覆盖时间短，X射线峰值识别仍存在不确定性，但X射线爆发峰值似乎被延迟了2.1±0.5 d。比SS Cyg（0.9–1.4 d）更大的延迟还支持U Gem中较低的吸积率。荧光灯6。4 keV发射线承载有关发射X射线的热等离子体和反映硬X射线发射的吸积盘（AD）的几何形状的重要信息。我们的反射模拟结果表明，光学上较厚的AD在白矮星半径的1.20-1.25倍（R _WD）处于静止状态，并且圆盘中的积垢物在该半径处变为光学上稀薄的发射X射线的等离子体。另一方面，尽管盘截断可能发生在<1.012 R _WD区域，但我们的光谱分析却偏向于光学厚度较厚的盘到达WD表面的图像 。从6.4 keV线的剖面图中，我们还发现，在物质进入圆盘截断半径的边界层后，立即将堆积的物质加热至接近最高温度。这与通常可以用冷却流模型很好地表示矮新星的硬X射线光谱这一事实是一致的。