Mass and Radius determination of magnetic cataclysmic variables utilizing X-ray emission generated from accreting plasma flows

2023 Fiscal Year Final Research Report

• Project/Area Number: 21K03623
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 16010:Astronomy-related
• Research Institution: Japan Aerospace EXploration Agency
• Principal Investigator: Mori Hideyuki 国立研究開発法人宇宙航空研究開発機構, 宇宙科学研究所, 招聘研究員 (20432354)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: X線天文学 / 強磁場激変星 / 白色矮星 / 重力赤方偏移 / プラズマ物理

Outline of Final Research Achievements

We detected significant redshifts from characteristic X-ray emission lines of hydrogen-like magnesium, silicon, and sulfur ions in the X-ray spectra of RX J1712.6-2414, a magnetic cataclysmic variable, which were obtained from the Chandra High-Energy Grating observation. These redshifts were significantly larger than the Doppler shifts of the corresponding elements contained in plasma flow accreting onto a white dwarf (WD), harbored in RX J1712.6-2414. While the line-of-sight plasma velocities are estimated to be ~ 100 km/s from the prevailing accretion-flow model, those derived from the redshifts were in a range of 200 - 500 km/s. We considered several effects to mitigate the discrepancy of the energy shifts. Finally, we concluded that a gravitational redshift is a main contributor, which enables us to directly measure the gravitational potential near the WD surface. Hence, it provides us with a new method of the mass determination; the WD mass was estimated to be > 0.9 Msun.

Free Research Field

X線天文学

Academic Significance and Societal Importance of the Research Achievements

本研究では、強磁場激変星におけるX線スペクトルの重力赤方偏移を世界で初めて検出した。X線は白色矮星表面近傍から放射されるため、星表面での重力ポテンシャルの深さを観測量として直接測定できることになる。したがって、可視光光度曲線の変動から求まる連星質量比や、X線スペクトルから衝撃波加熱されたプラズマの最大温度を推定して重力ポテンシャルを求める方法とは独立に、白色矮星の質量を推定する新たな手法を与える。また従来のプラズマ降着流モデルから計算されるドップラー効果よりも輝線エネルギーのずれへの寄与が大きく、重力赤方偏移を反映したセルフコンシステントなX線スペクトルモデルの構築が必須であることが分かった。