Spatial distribution of the X-ray-emitting plasma of U Geminorum in quiescence and outburst

Author:

Takeo Mai12,Hayashi Takayuki34,Ishida Manabu12,Nakaniwa Nozomi12,Maeda Yoshitomo2

Affiliation:

1. Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan

2. The Institute of Space and Astronautical Science/JAXA, 3-1-1 Yoshinodai, Chuo-ward, Sagamihara, Kanagawa 252-5210, Japan

3. NASA’s Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA

4. Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA

Abstract

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 (RWD) 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 RWD. 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.

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

Cited by 2 articles. 订阅此论文施引文献 订阅此论文施引文献,注册后可以免费订阅5篇论文的施引文献,订阅后可以查看论文全部施引文献

1. NEW SPECTROSCOPY OF U GEM;Revista Mexicana de Astronomía y Astrofísica;2023-10-01

2. Spatial distribution of the X-ray-emitting plasma of SS Cygni in quiescence and outburst;Publications of the Astronomical Society of Japan;2021-09-02

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