AT 2018cow 的哈勃空间望远镜晚期观测。II. 发现后 2–4 年一个紫外明亮底层源的演化
Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV bright Underlying Source 2--4 Yr Post-discovery
AI总结：
本文分析 AT 2018cow 发现后 2–4 年的 HST 观测，发现其紫外明亮底层源后期变暗、光谱变平，并指出高倾角进动吸积盘可解释其演化。
摘要：
在这一两篇论文系列的第二篇中，我们对发现后约 2–4 年取得的三次 HST 观测进行了详细分析，考察位于 AT 2018cow 精确位置处的一个紫外明亮底层源的演化。发现后约 2–3 年的观测揭示了一个异常蓝色（L_ν∝ν^1.99）且光学亮度相对稳定的底层源，而在第 4 年观测到 NUV 变暗，表明光谱变平（至 L_ν∝ν^1.64）。所得光谱能量分布可用一个极热但很小的黑体描述，这种变暗可能是内禀的（冷却），也可能是外在的（吸收增强）。在考虑可能的情景和解释时，基于观测到的颜色和亮度，我们不支持恒星来源和尘埃形成的显著贡献。通过比较预期功率与观测光度，我们排除了与已知产生射电辐射的周星物质发生相互作用，以及 B∼10^15 G 磁星自转减速作为可能供能源的可能性；不过，我们不能排除存在更致密 CSM 成分（例如先前抛射的氢包层）或 B≲10^14 G 磁星的可能。最后，我们发现高倾角进动吸积盘可以合理解释该底层源的颜色、亮度和演化。然而，这一情景中的一个主要不确定性是中心黑洞（BH）的质量，因为恒星质量和中等质量 BH 都面临我们简单盘模型无法解释的显著挑战，需要进一步观测和理论工作来充分约束该底层源的本质。
英文摘要：
In this second of a two-paper series, we present a detailed analysis of three HST observations taken $\sim$2--4 years post-discovery, examining the evolution of a UV-bright underlying source at the precise position of AT 2018cow. While observations at $\sim$2--3 years post-discovery revealed an exceptionally blue ($L_ν\propto ν^{1.99}$) underlying source with relatively stable optical brightness, fading in the NUV was observed at year 4, indicating flattening in the spectrum (to $L_ν\propto ν^{1.64}$). The resulting spectral energy distributions can be described by an extremely hot but small blackbody, and the fading may be intrinsic (cooling) or extrinsic (increased absorption). Considering possible scenarios and explanations, we disfavor significant contributions from stellar sources and dust formation based on the observed color and brightness. By comparing the expected power and the observed luminosity, we rule out interaction with the known radio-producing circumstellar material as well as magnetar spin down with $B\sim10^{15}\,\mathrm{G}$ as possible power sources, though we cannot rule out the possible existence of a denser CSM component (e.g., previously ejected hydrogen envelope) or a magnetar with $B\lesssim10^{14}\,\mathrm{G}$. Finally, we find that a highly-inclined precessing accretion disk can reasonably explain the color, brightness, and evolution of the underlying source. However, a major uncertainty in this scenario is the mass of the central black hole (BH), as both stellar-mass and intermediate-mass BHs face notable challenges that cannot be explained by our simple disk model, and further observations and theoretical works are needed to fully constrain the nature of this underlying source.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
