利用 JWST 在 2.1 和 4.8 微米波长观测 Sgr A* 的持续变率：暗弱与明亮可变辐射属于不同族群的证据
Non-stop Variability of Sgr A* using JWST at 2.1 and 4.8 micron Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission
AI总结：
该研究基于 JWST/NIRCam 对 Sgr A* 的 2.1 与 4.8 微米同步连续观测，发现其持续、亚分钟尺度变率及两类辐射过程，并用年龄分层电子同步辐射模型约束磁场与截断能量。
摘要：
我们展示了 JWST 第 1 和第 2 周期使用 NIRCam 对 Sgr A* 进行观测的首批结果；这些观测于 2023 年和 2024 年在 7 个不同历元同时获得 2.1 与 4.8 微米数据，总时长约 48 小时。我们发现，在所有历元中，2.1 与 4.8 微米的变率均相关；持续存在的短时间尺度（数秒）变化以及历元间可变辐射表明，Sgr A* 具有长期（约数天至数月）变率。本分析的一个亮点是发现了 Sgr A* 亚分钟、视界尺度时间变化的证据，从而探测内吸积盘尺度。各观测历元光变曲线的功率谱也显示出长期可变辐射。凭借连续观测，JWST 数据表明 Sgr A* 的流量在不断涨落。流量密度相关性在 2.1 微米约 3 mJy 处表现出明显的斜率转折。分析表明，有两种不同过程促成 Sgr A* 的变率。较明亮辐射比较暗弱辐射趋向于更平缓的谱指数。光变曲线的互相关首次表明，4.8 微米变率相对于 2.1 微米变率存在 3–40 秒的时间延迟。随着辐射上升和下降，这一相移会在流量密度对谱指数图中形成环。模型显示，来自不断演化、按年龄分层的电子族群的同步辐射可重现观测到的光变曲线形状，并直接估计磁场强度处于 40–90 G 范围，高能截断能量 E_c 处于 420–720 MeV 之间。
英文摘要：
We present first results of JWST Cycle 1 and 2 observations of Sgr A* using NIRCam taken simultaneously at 2.1 and 4.8 micron for a total of ~48 hours over seven different epochs in 2023 and 2024. We find correlated variability at 2.1 and 4.8 micron in all epochs, continual short-time scale (a few seconds) variability and epoch-to-epoch variable emission implying long-term ( ~days to months) variability of Sgr A*. A highlight of this analysis is the evidence for sub-minute, horizon-scale time variability of Sgr A*, probing inner accretion disk size scales. The power spectra of the light curves in each observing epoch also indicate long-term variable emission. With continuous observations, JWST data suggest that the flux of Sgr A* is fluctuating constantly. The flux density correlation exhibits a distinct break in the slope at ~3 mJy at 2.1 micron. The analysis indicates two different processes contributing to the variability of Sgr A*. Brighter emission trends towards shallower spectral indices than the fainter emission. Cross correlation of the light curves indicates for the first time, a time delay of 3 - 40 sec in the 4.8 micron variability with respect to 2.1 micron. This phase shift leads to loops in plots of flux density vs spectral index as the emission rises and falls. Modeling suggests that the synchrotron emission from the evolving, age-stratified electron population reproduces the shape of the observed light curves with a direct estimate of the magnetic field strengths in the range between 40-90 G, and upper cutoff energy, E_c, between 420 and 720 MeV.
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