标题： ALMA发现Punctum——在核星暴星系NGC 4945中的高度偏振毫米波源 显示英文标题

标题： ALMA discovery of Punctum -- a highly polarized mm source in nuclear starburst galaxy NGC 4945

摘要： 我们报告在NGC 4945中心区域发现了一个高度极化的毫米( mm )连续谱源，该源通过ALMA Band 3观测识别出来。 这个星暴 Seyfert 2 星系包含许多紧凑的毫米波源，但只有一个——位于大约3.4"（~60 pc）处，且在~0.1"的分辨率下无法分辨——表现出异常高的偏振度50%$\pm$14%，可能起源于非热同步辐射。 该源较暗淡，但在两次相隔14天的观测时段中被清晰检测到，通量分别为0.104$\pm$0.018和0.125$\pm$0.016 mJy，以及早期的ALMA观测中，显示在任何时间尺度上都没有变化。 光谱指数在较大的不确定性范围内保持稳定，-1.8$\pm$2.5和-1.3$\pm$2.5。 由于其紧凑性，我们将该源进一步称为Punctum，但在现有的X射线或射电观测中没有发现明显的对应体。 假设与NGC 4945中心区域有关，我们估计其在3-6 keV X射线波段（来自档案Chandra数据）的光度上限为~1 $\times$ 10$^{37}$ erg s$^{-1}$，在23 GHz（来自档案ATCA数据）的光度上限为~5 $\times$ 10$^{35}$ erg s$^{-1}$。 将无线电、毫米波（包括偏振）和X射线特性与已知发射同步辐射的天体物理源（如吸积中子星、超新星遗迹和非热银河丝状体）进行比较，发现这些情景中没有明显的匹配。 这个高度偏振源的确切性质仍未确定。 显示英文摘要

摘要： We report the discovery of a highly polarized millimeter (mm) continuum source in the central region of NGC 4945, identified through ALMA Band 3 observations. This starburst Seyfert 2 galaxy contains numerous compact mm sources, yet only one - located approximately 3.4" (~60 pc) from the galactic center and unresolved with ~0.1" resolution - exhibits an unusually high polarization degree of 50% $\pm$ 14%, likely originating from non-thermal synchrotron radiation. The source is faint, yet clearly detected in two separate epochs of observation taken 14 days apart, with flux of 0.104 $\pm$ 0.018 and 0.125 $\pm$ 0.016 mJy, as well as in earlier ALMA observations, showing no variability at any timescale. The spectral index remains stable within large uncertainties, -1.8 $\pm$ 2.5 and -1.3 $\pm$ 2.5. The source, which we further refer to as Punctum due to its compactness, revealed no clear counterparts in existing X-ray or radio observations. Assuming association with the central region of NGC 4945, we estimate upper limits for its luminosity of ~1 $\times$ 10$^{37}$ erg s$^{-1}$ in the 3-6 keV X-ray band (from archival Chandra data) and ~5 $\times$ 10$^{35}$ erg s$^{-1}$ at 23 GHz (from archival ATCA data). A comparison of the radio, mm (including polarization), and X-ray properties with known astrophysical sources emitting synchrotron radiation, such as accreting neutron stars, supernova remnants, and non-thermal galactic filaments, revealed no clear match in any of these scenarios. The exact nature of this highly polarized source remains undetermined.