X-ray and optical spectroscopic study of a γ Cassiopeiae analog source π Aquarii
X-ray and optical spectroscopic study of a γ Cassiopeiae analog source π Aquarii
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γ Cassiopeiae 模拟源 π Aquarii 的 X 射线和光学光谱研究
DOI：
10.1093/pasj/psac099
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发表时间：
2022
影响因子：
2.3
通讯作者：
Moritani Yuki
中科院分区：
文献类型：
--
作者：
Tsujimoto Masahiro;Hayashi Takayuki;Morihana Kumiko;Moritani Yuki
γ Cas类似源是Be星的一个子集，它发出强烈的硬X射线。对于它们的X射线产生机制，有两种相互竞争的观点：（a）Be星星及其盘的磁活动;（B）Be星星对一个不明致密天体的吸积。在这些来源中，π Aqr扮演着关键的角色，因为它是在许多轨道周期中观测到的仅有的两个光谱联星之一，也是三个X射线亮度足以进行详细X射线光谱分析的来源之一。Bjorkman等人(2002，ApJ，573，812）用光谱学估计了次级质量> 2.0 M的矮星，这将反驳致密天体是白色矮星（WD）。然而，他们的动力学质量解决方案是不一致的演化解决方案和他们的径向速度测量是不一致的，后来的工作由Nazé等人。(2019，A&A，632，A23）。我们重新审视这个问题，增加了一个新的数据集与努斯塔X射线天文台和HIDES埃歇尔摄谱仪。我们发现，径向速度振幅是一致的Nazé等人。(2019，A&A，632，A23），其仅为Bjorkman等人所主张的量的一半。(2002，ApJ，573，812）。在固定主质量径向速度幅值的情况下，在主质量和倾角的假设范围内，估计次质量<1.4M Ω。在假设次级质量确实是WD的情况下，我们通过用非磁性或磁性吸积WD模型拟合X射线谱，进一步独立地约束了倾斜角和次级质量。这两个结果很吻合。因此，我们认为，对于π Aqr，不应排除次级是WD的可能性。
γ Cas analog sources are a subset of Be stars that emit intense and hard X-ray emission. Two competing ideas for their X-ray production mechanism are (a) the magnetic activities of the Be star and its disk and (b) the accretion from the Be star to an unidentified compact object. Among such sources, π Aqr plays a pivotal role as it is one of the only two spectroscopic binaries observed for many orbital cycles and one of the three sources with X-ray brightness sufficient for detailed X-ray spectroscopy. Bjorkman et al. (2002, ApJ, 573, 812) estimated the secondary mass >2.0M⊙with optical spectroscopy, which would argue against the compact object being a white dwarf (WD). However, their dynamical mass solution is inconsistent with an evolutionary solution and their radial velocity measurement is inconsistent with later work by Nazé et al. (2019, A&A, 632, A23). We revisit this issue by adding a new data set with the NuSTAR X-ray observatory and the HIDES échelle spectrograph. We found that the radial velocity amplitude is consistent with Nazé et al. (2019, A&A, 632, A23), which is only half of that claimed by Bjorkman et al. (2002, ApJ, 573, 812). Fixing the radial velocity amplitude of the primary, the secondary mass is estimated as <1.4M⊙over an assumed range of the primary mass and the inclination angle. We further constrained the inclination angle and the secondary mass independently by fitting the X-ray spectra with a non-magnetic or magnetic accreting WD model under the assumption that the secondary is indeed a WD. The two results match well. We thus argue that the possibility of the secondary being a WD should not be excluded for π Aqr.