[论文解读] The millimetre variability of M81* -- Multi-epoch dual frequency mm-observations of the nucleus of M81

R. Schöedel, M. Krips+3 more|arXiv (Cornell University)|Nov 28, 2006

Astrophysical Phenomena and Observations被引用 10

一句话总结

本研究首次对M81*（低光度活动星系核）进行了1 mm与3 mm波段的同时连续谱观测，揭示了强烈的毫米波段变异性，且1 mm波段的变幅更高。数据表明各波段间存在相关变异性，并将辐射区域大小限制在小于25个史瓦西半径以内，支持其起源于紧凑喷流，凸显M81*作为Sgr A*在亚爱丁顿吸积研究中的关键类比对象。

ABSTRACT

There are still many open questions as to the physical mechanisms at work in Low Luminosity AGN that accrete in the extreme sub-Eddington regime. Simultaneous multi-wavelength studies have been very successful in constraining the properties of SgrA*, the extremely sub-Eddington black hole at the centre of our Milky Way. M81*, the nucleus of the nearby spiral galaxy M81, is an ideal source to extend the insights obtained on SgrA* toward higher luminosity AGN. Here we present observations at 3 and 1 mm that were obtained within the framework of a coordinated,multi-wavelength campaign on M81*. The continuum emission from M81* was observed during three epochs with the IRAM Plateau de Bure Interferometer simultaneously at wavelengths of 3 and 1 mm. We present the first flux measurements of M81* at wavelengths around 1 mm. We find that M81* is a continuously variable source with the higher variability observed at the shorter wavelength. Also, the variability at 3 and 1 mm appears to be correlated. Like SgrA*, M81* appears to display the strongest flux density and variability in the mm-to-submm regime. There remains still some ambiguity concerning the exact location of the turnover frequency from optically thick to optically thin emission. The observed variability time scales point to an upper size limit of the emitting region of the order 25 Schwarzschild radii. The data show that M81* is indeed a system with very similar physical properties to SgrA* and an ideal bridge toward high luminosity AGN. The data obtained clearly demonstrate the usefulness and, above all, the necessity of simultaneous multi-wavelength observations of LLAGN.

研究动机与目标

• 研究M81*在毫米波段的变异性，以理解极端亚爱丁顿吸积状态下的吸积机制。
• 确定M81*是否表现出与银河系中心黑洞Sgr A*相似的变异性与谱行为。
• 利用变异性 timescales 约束辐射区域的大小与物理起源。
• 通过分析多波段变异性与谱谱转折特征，评估喷流主导辐射在低光度活动星系核（LLAGN）中的作用。
• 证明同时多波段观测对于准确建模弱吸积黑洞的必要性。

提出的方法

• 利用IRAM Plateau de Bure干涉仪，开展在3 mm与1 mm波段的多epoch、同时双频段观测。
• 对M81*核区在三个观测epoch中进行高精度通量测量。
• 分析光变曲线，以确定两个波段的变异性幅度与timescales。
• 利用变异性timescales，通过光传播时延论证，估算辐射区域大小的上限。
• 通过比较谱指数与光变曲线形状，检验喷流驱动变异性模型（如喷流中激波传播）的合理性。
• 通过比较毫米波与亚毫米波段的通量水平与变异性趋势，评估谱转折频率。

实验结果

研究问题

• RQ1 M81*在毫米波段是否呈现变异性？其变异性与Sgr A*相比如何？
• RQ2 M81*中变异性幅度与观测频率之间存在何种关系？
• RQ3 观测到的变异性timescales能否约束M81*中辐射区域的大小？
• RQ4 M81*的谱能谱分布是否与毫米波至亚毫米波段的喷流主导辐射模型一致？
• RQ5 同时多波段观测在多大程度上改善了对LLAGN中吸积与辐射机制的约束？

主要发现

• 本研究首次在1 mm波段明确检测到M81*，证实其在亚毫米波段的存在。
• M81*在1 mm与3 mm波段均表现出持续变异性，且1 mm波段的变异性幅度高出约1.5倍。
• 在3 mm波段观测到的最短变异性timescale约为5小时，暗示辐射区域大小上限约为25个史瓦西半径。
• 2005年7月的光变曲线表明，两次耀斑衰减可能具有共同起源，与喷流中激波模型一致。
• 两个观测epoch中，谱指数均为负值且绝对值相近，支持在毫米波至亚毫米波段的谱能谱分布中存在转折。
• 观测到的变异性与谱行为强烈支持M81*的喷流主导辐射模型，尽管其光度与Sgr A*相差数个数量级，但仍表现出显著相似性。