超紧凑X射线双星候选47 Tuc的HST谱和定时X9
HST spectrum and timing of the ultra-compact X-ray binary candidate 47 Tuc X9
摘要：
为了确认超紧凑X射线双星候选者47 Tuc X9中施主星的性质，我们用哈勃太空望远镜/太空望远镜成像光谱仪获得了光谱（3，000 $-$ 10，000）。我们发现在X9的光谱中没有强的发射或吸收特征。特别地，我们分别对H $α$和HeII $λ4686$发射线等效宽度$-$ EW $_{\mathrm{Hα}} \lesssim 14$和$-$ EW $_{\mathrm{HeII}} \lesssim 9$设置了$3σ$上限。这比在相似的X射线光度下典型的X射线双星（对于$L_{\mathrm{2-10 keV}} \approx 10^{33}-10^{34}$ erg s $^{-1}$，典型的是$-$ EW $_{\mathrm{Hα}} \sim 50$）要低得多。这支持了我们之前的建议（Bahramian等人）。一个缺乏H的捐赠者我们对档案远紫外，$V$和$I$波段数据进行时序分析，以寻找周期性。在光学波段，我们恢复了最初在X射线中发现的7天超轨道周期，但我们没有恢复轨道周期。在远紫外线中，我们发现了27.2分钟的周期（比X射线中的28.2分钟周期短）。我们发现中子星或黑洞都可以解释X9的观测特性。我们还进行了二进制演化计算，表明在球状星团生命早期形成的初始黑洞/He星二进制可以通过引力波辐射驱动的质量传递演化成当今的系统，如X9（如果该系统中的致密物体确实是黑洞）。
英文摘要：
To confirm the nature of the donor star in the ultra-compact X-ray binary candidate 47 Tuc X9, we obtained optical spectra (3,000$-$10,000 Å) with the Hubble Space Telescope / Space Telescope Imaging Spectrograph. We find no strong emission or absorption features in the spectrum of X9. In particular, we place $3σ$ upper limits on the H$α$ and HeII $λ4686$ emission line equivalent widths $-$EW$_{\mathrm{Hα}} \lesssim 14$ Å and $-$EW$_{\mathrm{HeII}} \lesssim 9$ Å, respectively. This is much lower than seen for typical X-ray binaries at a similar X-ray luminosity (which, for $L_{\mathrm{2-10 keV}} \approx 10^{33}-10^{34}$ erg s$^{-1}$ is typically $-$EW$_{\mathrm{Hα}} \sim 50$ Å). This supports our previous suggestion (by Bahramian et al.) of an H-poor donor in X9. We perform timing analysis on archival far-ultraviolet, $V$ and $I$-band data to search for periodicities. In the optical bands we recover the seven-day superorbital period initially discovered in X-rays, but we do not recover the orbital period. In the far-ultraviolet we find evidence for a 27.2 min period (shorter than the 28.2 min period seen in X-rays). We find that either a neutron star or black hole could explain the observed properties of X9. We also perform binary evolution calculations, showing that the formation of an initial black hole / He-star binary early in the life of a globular cluster could evolve into a present-day system such as X9 (should the compact object in this system indeed be a black hole) via mass-transfer driven by gravitational wave radiation.
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