[论文解读] Insights into the broad-band emission of the TeV blazar Mrk 501 during the first X-ray polarization measurements

MAGIC Collaboration, J. Abhir+8 more|arXiv (Cornell University)|Jan 1, 2024

Astrophysics and Cosmic Phenomena被引用 1

一句话总结

本研究首次对TeV耀变体Mrk 501开展了同时多波段观测，结合IXPE的X射线偏振数据与MAGIC的VHE伽马射线（MAGIC）、NuSTAR和Swift的X射线，以及射电至光学的全波段观测。研究揭示了能量分层的喷流辐射特征，表现为极端X射线谱态和较低的康普顿主导度，这可通过多区室轻子模型中磁化度或辐射区域大小的变化来解释。

ABSTRACT

Aims. We present the first multiwavelength study of Mrk 501 that contains simultaneous very-high-energy (VHE) γ-ray observations and X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE).Methods. We used radio-to-VHE data from a multiwavelength campaign carried out between March 1, 2022, and July 19, 2022 (MJD 59639 to MJD 59779). The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and several other instruments that cover the optical and radio bands to complement the IXPE pointings. We characterized the dynamics of the broadband emission around the X-ray polarization measurements through its multiband fractional variability and correlations, and compared changes observed in the polarization degree to changes seen in the broadband emission using a multi-zone leptonic scenario.Results. During the IXPE pointings, the VHE state is close to the average behavior, with a 0.2–1 TeV flux of 20%–50% of the emission of the Crab Nebula. Additionally, it shows low variability and a hint of correlation between VHE γ-rays and X-rays. Despite the average VHE activity, an extreme X-ray behavior is measured for the first two IXPE pointings, taken in March 2022 (MJD 59646 to 59648 and MJD 59665 to 59667), with a synchrotron peak frequency > 1 keV. For the third IXPE pointing, in July 2022 (MJD 59769 to 59772), the synchrotron peak shifts toward lower energies and the optical/X-ray polarization degrees drop. All three IXPE epochs show an atypically low Compton dominance in the γ-rays. The X-ray polarization is systematically higher than at lower energies, suggesting an energy stratification of the jet. While during the IXPE epochs the polarization angles in the X-ray, optical, and radio bands align well, we find a clear discrepancy in the optical and radio polarization angles in the middle of the campaign. Such results further support the hypothesis of an energy-stratified jet. We modeled broadband spectra taken simultaneous to the IXPE pointings, assuming a compact zone that dominates in the X-rays and the VHE band, and an extended zone stretching farther downstream in the jet that dominates the emission at lower energies. NuSTAR data allow us to precisely constrain the synchrotron peak and therefore the underlying electron distribution. The change between the different states observed in the three IXPE pointings can be explained by a change in the magnetization and/or the emission region size, which directly connects the shift in the synchrotron peak to lower energies with the drop in the polarization degree.Key words: radiation mechanisms: non-thermal / galaxies: active / BL Lacertae objects: individual: Markarian 501 / gamma rays: general / X-rays: galaxies⋆ The MWL data are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/685/A117⋆⋆ Corresponding authors: L. Heckmann, A. Arbet Engels, D. Paneque; e-mail: [e-mail].

研究动机与目标

• 研究TeV耀变体Mrk 501在首次使用IXPE进行X射线偏振观测期间的全波段辐射特性。
• 理解在观测期间所见极端X射线谱态和低康普顿主导度的物理起源。
• 通过多波段变异性与偏振角相关性，检验能量分层喷流辐射的假设。
• 利用多区室轻子模型拟合全波段能谱分布，以解释观测到的谱变与偏振变化。
• 利用NuSTAR对同步辐射峰值的精确测量，约束电子能谱分布与辐射区域参数。

提出的方法

• 从MJD 59639至59779（2022年3月1日至2022年7月19日）开展覆盖射电至VHE伽马射线的多波段观测。
• 整合IXPE（X射线偏振）、MAGIC（VHE伽马射线）、Fermi-LAT（GeV能段）、NuSTAR与Swift（X射线）以及光学/射电仪器的数据。
• 分析多波段相对变异性与跨波段相关性，以追踪IXPE指向期间的辐射动力学。
• 采用双区室轻子模型：一个紧凑区室主导X射线与VHE辐射，一个扩展区室主导低能辐射。
• 在IXPE观测历元上同时拟合全波段谱，利用NuSTAR数据精确约束同步辐射峰值频率。
• 将同步辐射峰值能量与偏振度的变化与磁化度及辐射区域大小的变化联系起来。

实验结果

研究问题

• RQ1 Mrk 501在首次IXPE指向期间所见极端X射线谱态的成因是什么？
• RQ2 X射线偏振度与角度的变化如何与全波段变异性及谱演化相关联？
• RQ3 观测到的γ射线波段低康普顿主导度在多大程度上可由喷流辐射模型解释？
• RQ4 观测到的偏振能量分层是否与多区室轻子喷流模型一致？
• RQ5 支撑观测到的谱态与偏振态跃迁的物理参数（如磁化度或辐射区域大小）是什么？

主要发现

• 在IXPE指向期间，VHE流量为蟹状星云流量的20%–50%，表明处于平均活动水平，变异性较低，并显示出VHE与X射线之间存在相关性迹象。
• 前两次IXPE指向（MJD 59646–59648与59665–59667）的同步辐射峰值频率超过1 keV，表明存在极端X射线谱态。
• 第三次IXPE指向（MJD 59769–59772）显示同步辐射峰值向较低能量移动，且光学/X射线偏振度下降。
• 所有三个IXPE历元均表现出γ射线波段异常低的康普顿主导度，表明偏离标准轻子模型。
• X射线偏振度系统性高于较低能量，支持具有有序磁场的能量分层喷流结构。
• 尽管在大部分观测期间X射线、光学与射电偏振角保持一致，但在观测中期光学与射电角度出现明显偏差，进一步表明喷流动力学复杂。