利用单基线偏振测量验证光子环的存在
Demonstrating Photon Ring Existence with Single-Baseline Polarimetry
AI总结：
本文利用MAD吸积流的旋转对称偏振特性，构造增益稳健的单基线干涉测量量，证明可在长基线上通过偏振相位变化明确探测M87*和Sgr A*的光子环，并给出具体灵敏度与频率要求。
摘要：
超大质量黑洞吸积流的图像同时包含弯曲时空和等离子体结构的特征。从图像中推断时空性质需要对等离子体性质进行建模，反之亦然。事件视界望远镜合作组织利用甚长基线干涉测量（VLBI）对梅西耶87*（M87*）和银河系中心黑洞人马座A*（Sgr A*）的近视界毫米波辐射进行了成像，并在两种情况下都发现了对磁阻滞盘（MAD）吸积模式的偏好。MAD吸积使得通过未来对光子环（由近轨道运动光子构成的图像特征）的观测来实现时空测量成为可能。MAD的有序磁场和相对较弱的法拉第旋转在中等倾角观测时会产生旋转对称的偏振。在本文中，我们利用这种对称性以及平行输运对称性，构造了一个对增益稳健的干涉测量量，该量能够探测弱透镜吸积流图像与强透镜光子环之间的过渡。我们预测了长基线上偏振相位的变化，并证明利用灵敏的长基线测量可以明确无误地探测到M87*和Sgr A*中的光子环。对于M87*，我们发现快照观测中的光子环探测需要在230 GHz及以上频率、大于15 Gλ的基线上达到约1 mJy的灵敏度，这可以通过空间VLBI或更高频率的地基VLBI实现。对于Sgr A*，我们发现星际散射在230 GHz会抑制光子环的可探测性，但在345 GHz、大于12 Gλ的基线上约10 mJy的灵敏度就足够了，这可以从地面实现。对于这两个源，这些灵敏度要求可以通过重复观测和平均来放宽。
英文摘要：
Images of supermassive black hole accretion flows contain features of both curved spacetime and plasma structure. Inferring properties of the spacetime from images requires modeling the plasma properties, and vice versa. The Event Horizon Telescope Collaboration has imaged near-horizon millimeter emission from both Messier 87* (M87*) and Sagittarius A* (Sgr A*) with very-long-baseline interferometry (VLBI) and has found a preference for magnetically arrested disk (MAD) accretion in each case. MAD accretion enables spacetime measurements through future observations of the photon ring, the image feature composed of near-orbiting photons. The ordered fields and relatively weak Faraday rotation of MADs yield rotationally symmetric polarization when viewed at modest inclination. In this letter, we utilize this symmetry along with parallel transport symmetries to construct a gain-robust interferometric quantity that detects the transition between the weakly lensed accretion flow image and the strongly lensed photon ring. We predict a shift in polarimetric phases on long baselines and demonstrate that the photon rings in M87* and Sgr A* can be unambiguously detected {with sensitive, long-baseline measurements. For M87* we find that photon ring detection in snapshot observations requires $\sim1$ mJy sensitivity on $>15$ G$λ$ baselines at 230 GHz and above, which could be achieved with space-VLBI or higher-frequency ground-based VLBI. For Sgr A*, we find that interstellar scattering inhibits photon ring detectability at 230 GHz, but $\sim10$ mJy sensitivity on $>12$ G$λ$ baselines at 345 GHz is sufficient, which is accessible from the ground. For both sources, these sensitivity requirements may be relaxed by repeated observations and averaging.
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