标题： 首次人马座A*事件视界望远镜结果。 四、变率、形态和黑洞质量 显示英文标题

标题： First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass

摘要： 本文量化了人马座A*（Sgr A*）地平尺度辐射的时间可变性及其图像形态，这些数据由2017年4月事件视界望远镜（EHT）在1.3毫米波长处观测到。 我们发现，Sgr A*的数据表现出的可变性超过了数据不确定性或星际散射效应所能解释的范围。 这种可变性的幅度可以占到相关通量密度的很大一部分，在某些基线处甚至达到$\sim$的100%。 通过简单几何源模型的探索，我们证明环状形态比具有相同复杂度的其他形态更能很好地拟合Sgr A*的数据。 我们开发了两种策略来拟合静态几何环模型到时间可变的Sgr A*数据；一种策略是对源在短时间内静止的数据片段进行模型拟合并平均独立拟合结果，而另一种策略则使用平均源结构周围的结构性变化功率谱的参数化模型，对整个数据集进行模型拟合。 几何建模和图像域特征提取技术都确定环直径为$51.8 \pm 2.3$ $\mu$ （置信区间为68%），且环厚度被限制在环直径的$\sim$30% 到50%之间。 为了将直径测量值统一到相同的物理尺度上，我们使用从广义相对论磁流体力学（GRMHD）模拟生成的合成数据进行校准。 请注意，对于 XMATHX系列代号保持不变。 这种校准限制了引力半径角直径的大小为$4.8_{-0.7}^{+1.4}$ \mathrm{\mu as} ，我们将其与来自甚长基线干涉测量视差的独立距离测量相结合，以确定人马座A*的质量为$4.0_{-0.6}^{+1.1} \times 10^6$ M$_{\odot}$。 显示英文摘要

摘要： In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching $\sim$100\% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full dataset using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be $51.8 \pm 2.3$ $\mu$as (68\% credible intervals), with the ring thickness constrained to have an FWHM between $\sim$30\% and 50\% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be $4.8_{-0.7}^{+1.4}$ \mathrm{\mu as}, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be $4.0_{-0.6}^{+1.1} \times 10^6$ M$_{\odot}$.