The effect of stars on the lensing properties of the supermassive black hole in the Galactic center is similar to the effect of planets on microlensing by a star. We show that the dense stellar cluster around Sgr A* increases by factors of a few the probability of high-magnification lensing events of a distant background source by the black hole. Conversely, the gravitational shear of the black hole changes and enhances the microlensing properties of the individual stars. The effect is largest when the source image lies near the Einstein radius of the black hole (175 ± 020 for a source at infinity). We estimate that the probability of observing at least one distant background star, which is magnified by a factor greater than 5 in any infrared snapshot of the inner ~2'' of the Galactic center, is ~1% with a K-band detection threshold of 20 mag. The largest source of uncertainty in this estimate is the luminosity function of the background stars. The gravitational shear of the black hole lengthens the duration of high-magnification events near the Einstein radius up to a few months and introduces a large variety of light-curve shapes that are different from those of isolated microlenses. Identification of such events by image subtraction can be used to probe the mass function, density, and velocity distributions of faint stars near the black hole, which are not detectable otherwise.
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机译：恒星对银河系中心超大质量黑洞的透镜性质的影响类似于行星对恒星微透镜的影响。我们显示，围绕Sgr A *的密集恒星簇增加了由黑洞引起的远距离背景源发生高倍透镜现象的几率。相反，黑洞的重力剪切改变并增强了单个恒星的微透镜特性。当源图像位于黑洞的爱因斯坦半径附近（无限远处的源为175±020）时，效果最大。我们估计，在K波段检测阈值下，观测到至少一个远方背景星的概率约为1％，该概率在银河系中心内部〜2''的任何红外快照中被放大大于5倍。的20磁。此估计中最大的不确定性来源是背景恒星的光度函数。黑洞的重力剪切将爱因斯坦半径附近的高倍放大事件的持续时间延长了几个月，并引入了与孤立微透镜不同的各种光曲线形状。通过图像减法对此类事件的识别可用于探测黑洞附近微弱恒星的质量函数，密度和速度分布，否则无法检测到。
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