BN4 - End of white dwarfs and type Ia Supernova |
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Speaker |
Wada, Yuuki |
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Coauthors |
Yuasa, Takayuki; Nakazawa, Kazuhiro; Makishima, Kazuo; Hayashi, Takayuki; Ishida, Manabu |
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X-ray estimates of white dwarf masses in magnetic cataclysmic variables |
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Abstract |
Mass-accreting white dwarfs (WDs) in cataclysmic variables (CVs) are candidates for progenitors of type Ia supernovae, and hence mass estimation of such WDs is of great importance in understanding the channels to type Ia supernovae. Indeed, WDs in non-magnetic and magnetic CVs are observed to have average masses of 0.82 M_sun and 0.88 M_sun, respectively (Ritter & Kolb 2003, Yuasa et al. 2010), which are higher than those of isolated WDs (0.593 M_sun on average: Kepler et al. 2007). So far, the WD masses in CVs have mostly been estimated by orbital Doppler effects measured in optical/infrared observations. Although this method is often limited by uncertainties of the inclination angle, an alternative method using X-rays can provide inclination-free mass estimates, particularly for the intermediate polar class which is a subset of CVs hosting strongly magnetized WDs. In these systems, the accreting gas forms an accretion disk down to the Alfven radius, and then follows the magnetic field lines and finally falls onto the magnetic poles of the WD to form accretion columns. Observations of X-ray emission from such an accretion column yields the maximum temperature therein. By equating this measurement with the gravitational potential gained by the accreting matter, and incorporating the theoretical mass-radius relation of WDs, the WD mass can be estimated. Our recent study has improved the X-ray method by considering the effects of accretion-disk truncation due to the strong magnetic fields. This presentation reviews the state-of-the-art X-ray methods. |
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