Light passing near a black hole can follow multiple paths from an emission source to an observer due to strong gravitational lensing. Photons following different paths take different amounts of time to reach the observer, which produces an echo signature in the image. The characteristic echo delay is determined primarily by the mass of the black hole, but it is also influenced by the black hole spin and inclination to the observer. In the Kerr geometry, echo images are demagnified, rotated, and sheared copies of the direct image and lie within a restricted region of the image. Echo images have exponentially suppressed flux, and temporal correlations within the flow make it challenging to directly detect light echoes from the total light curve. In this Letter, we propose a novel method to search for light echoes by correlating the total light curve with the interferometric signal at high spatial frequencies, which is a proxy for indirect emission. We explore the viability of our method using numerical general relativistic magnetohydrodynamic simulations of a near-face-on accretion system scaled to M87-like parameters. We demonstrate that our method can be used to directly infer the echo delay period in simulated data. An echo detection would be clear evidence that we have captured photons that have circled the black hole, and a high-fidelity echo measurement would provide an independent measure of fundamental black hole parameters. Our results suggest that detecting echoes may be achievable through interferometric observations with a modest space-based very long baseline interferometry mission.

中文翻译：

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使用超长基线干涉测量法测量黑洞光回波


由于强引力透镜，经过黑洞附近的光可以沿着从发射源到观察者的多条路径。遵循不同路径的光子需要不同的时间才能到达观察者，从而在图像中产生回波特征。特征回波延迟主要由黑洞的质量决定，但也受黑洞自旋和对观察者的倾斜度的影响。在 Kerr 几何中，回波图像是直接图像的去放大、旋转和剪切副本，位于图像的受限区域内。回波图像具有指数抑制的通量，并且流中的时间相关性使得直接从总光曲线中检测光回波变得具有挑战性。在这封信中，我们提出了一种新的方法，通过将总光曲线与高空间频率下的干涉信号相关联来搜索光回波，这是间接发射的代理。我们使用缩放到类似 M87 参数的近面吸积系统的数值广义相对论磁流体动力学模拟来探索我们方法的可行性。我们证明了我们的方法可用于直接推断仿真数据中的回声延迟周期。回波检测将是我们已经捕获了环绕黑洞的光子的明确证据，而高保真回波测量将提供基本黑洞参数的独立测量。我们的结果表明，通过适度的天基、非常长的基线干涉测量任务的干涉观测，可以实现回波的检测。