利用ZTF和Gaia开展的超紧凑高态AM CVn双星巡天：新发现及对银河系空间密度的观测约束
A survey of ultra-compact high-state AM CVn binaries with ZTF and Gaia: New discoveries and observational constraints on Galactic space density
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中文总结 AI 辅助
本研究结合ZTF时域测光与Gaia数据系统搜寻银河系高态AM CVn双星，发现3颗新源并约束其局域空间密度，为LISA、TianQin及LSST的探测率预估提供观测依据。
中文摘要 AI 辅助
处于稳定质量转移高态（质量转移率$\dot{M}\gtrsim10^{-9}M_\odot/\rm{yr}$）的超紧凑AM CVn双星，预计是即将建成的空间引力波天文台最明亮的持续源之一。这类系统的轨道周期通常$P_{\rm{orb}}\lesssim30$分钟。我们通过瞄准Gaia颜色-星等图中的一个区域，结合ZTF时域测光，对银河系内的高态AM CVn双星开展了系统搜寻。我们聚焦于该样本中周期为5-30分钟的变源目标。本次巡天发现了3颗新的高态AM CVn双星（ZTF J1840-1742、ZTF J2007-0527和ZTF J2111+3158），轨道周期分别为16.86、18.63和16.71分钟，同时重新识别出3个已知系统。我们获取了高速测光和相位分辨光谱以确认它们的性质，其光谱显示氦发射线，无可探测到的氢。其中2个目标的谱线呈双峰结构，多普勒断层图证实存在吸积盘。我们估算其质量吸积率约为$10^{-9}M_\odot/\rm{yr}$，3σ X射线光度上限约为$10^{33}\rm{erg/s}$。基于该样本，我们推断银河系内高态AM CVn族的局域空间密度约为$(1.0-2.7)\times10^{-8}\rm{pc}^{-3}$（对应盘标高$h_z=300-200$ pc），占银河系AM CVn总密度的2-5%。它们的诞生率约为$(2.4-4.3)\times10^{-4}\rm{yr}^{-1}$（对应$h_z=300-200$ pc），与AM CVn总诞生率一致，表明大多数进入高态阶段的系统都能存活并演化至更长的轨道周期。该局域空间密度表明，LISA和TianQin（天琴）在各自标称的4年任务期内（信噪比$S/N\ge5$）将分别探测到银河系总族数的约36%和19%。Vera C. Rubin天文台的LSST（大口径全天巡视望远镜）在其10年巡天期间（$m_r\lesssim26.9$星等）将探测到这些系统的约34-44%。
英文摘要
Ultra-compact AM CVn binaries in the high-state of stable mass transfer ($\dot{M}\gtrsim10^{-9}M_\odot/\rm{yr}$) are expected to be among the loudest persistent sources for upcoming space-based GW observatories. These systems typically have orbital periods $P_{\rm{orb}}\lesssim30$ minutes. We present a systematic search for high-state AM CVn binaries in the Milky Way by targeting a region within the Gaia color-magnitude diagram, combined with ZTF time-domain photometry. We focus on variable targets within that sample with 5-30 minute periods. Our survey discovered three new high-state AM CVn binaries (ZTF J1840-1742, ZTF J2007-0527 and ZTF J2111+3158) with orbital periods of 16.86, 18.63, and 16.71 minutes, and recovered three known systems. We obtain high-speed photometry and phase-resolved spectroscopy to confirm their nature. Their spectra show helium emission lines with no detectable hydrogen. In two targets, the lines are double-peaked, and Doppler tomograms confirm an accretion disk. We estimate mass accretion rates of $\sim10^{-9}M_\odot/\rm{yr}$, and $3σ$ X-ray luminosity upper limits of $\sim10^{33}\rm{erg/s}$. Based on this sample, we infer a local space density of high-state AM CVn population in the Milky Way of $\sim(1.0-2.7)\times10^{-8}\rm{pc}^{-3}$ (for disk scale heights $h_z=300-200$ pc), representing 2-5% of the total Galactic AM CVn density. Their birth rate, $\sim(2.4-4.3)\times10^{-4}\rm{yr}^{-1}$ (for $h_z=300-200$ pc), is consistent with the total AM CVn birth rate, implying most systems entering the high-state phase survive and evolve to longer orbital periods. This local space density suggests LISA and TianQin will detect about 36% and 19% of the total Galactic population, respectively, during their nominal 4-yr missions ($S/N\ge5$). The Vera C. Rubin Observatory's LSST will detect about 34-44% of these systems over its 10-yr survey ($m_r\lesssim26.9$ mag).
