Multiband variability studies and novel broadband SED modeling of Mrk 501 in 2009
Multiband variability studies and novel broadband SED modeling of Mrk 501 in 2009
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DOI：
10.1051/0004-6361/201629540
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发表时间：
2017-07-01
影响因子：
6.5
通讯作者：
Readhead, A. C. S.
中科院分区：
文献类型：
--
作者：
Ahnen, M. L.;Ansoldi, S.;Readhead, A. C. S.
目标。我们提出了一个广泛的研究BL Lac对象Mrk 501的基础上收集的数据集，在多仪器活动期间，从2009年3月15日至2009年8月1日，其中包括，除其他仪器，MAGIC，VERITAS，维普莱10米，费米-LAT覆盖的伽马射线范围从0.1 GeV到20 TeV; RXTE和Swift覆盖从紫外线到硬X射线的波长; GASP-WEBT覆盖无线电和光波长。斯图尔特天文台和彼得堡天文台为这次活动的一小部分提供了光学偏振测量。我们评估的源和带间的相关性，伽马射线耀斑活动发生在2009年5月的变化，并解释两个同步自康普顿（SSC）内的结果。在整个活动期间观察到的多波段变异性的分数变异性，并通过计算离散相关函数的每对能量带的显着性进行了评价，专用蒙特卡罗模拟研究可能的相关性。SSC模型参数的空间探测以下一个专门的网格扫描策略，允许广泛的模型进行测试，并提供了一个研究的退化模型数据协议在个别模型参数，因此提供了一个更少的偏见比“单曲线SSC模型调整”通常在文献中报道的解释。我们发现随着能量的分数变化的增加，而没有显着的带间相关性的通量变化被发现的基础上获得的数据集。SSC模型网格扫描表明，5月22日前后的耀斑活动不能用单区SSC情景（使用具有两个中断的电子能量分布）充分建模，而可以在两个（独立）区SSC情景中适当描述。在这里，一个区域负责从平均4.5个月的观测周期的静态发射，而另一个，这是空间上分离的第一个，占主导地位的耀斑发射发生在X射线和非常高的能量（> 100 GeV，VHE）的伽马射线。5月1日的耀斑活动与电矢量偏振角（EVPA）的旋转相吻合，不能通过单区或两个独立区SSC模型满意地再现，但这部分受到缺乏严格同步观测和存在亚小时时间尺度上的大通量变化（在VHE伽马射线上检测到）的影响。VHE发射的更高的可变性和与X射线发射缺乏相关性表明，至少在2009年为期4.5个月的观测活动期间，负责VHE伽马射线的最高能量（和最可变）电子不会对类似于1 keV的发射做出主要贡献。或者，除了来自SSC情景的伽马射线发射之外，可能还有一个非常可变的成分对VHE伽马射线发射有贡献。我们的专用SSC网格扫描的研究表明，有一些退化，在一个区域和两个区域的SSC的情况下探测，与模型参数的几种组合产生类似的模型到数据的协议，和一些参数比其他更好地约束。观测到的γ射线耀斑活动，与EVPA旋转与第一个γ射线耀斑一致，类似于以前报道的低频峰值耀变体，因此表明，有许多相似之处耀变体的耀斑机制与不同的喷流属性。
Aims. We present an extensive study of the BL Lac object Mrk 501 based on a data set collected during the multi-instrument campaign spanning from 2009 March 15 to 2009 August 1, which includes, among other instruments, MAGIC, VERITAS, Whipple 10 m, and Fermi-LAT to cover the gamma-ray range from 0.1 GeV to 20 TeV; RXTE and Swift to cover wavelengths from UV to hard X-rays; and GASP-WEBT, which provides coverage of radio and optical wavelengths. Optical polarization measurements were provided for a fraction of the campaign by the Steward and St. Petersburg observatories. We evaluate the variability of the source and interband correlations, the gamma-ray flaring activity occurring in May 2009, and interpret the results within two synchrotron self-Compton (SSC) scenarios.Methods. The multiband variability observed during the full campaign is addressed in terms of the fractional variability, and the possible correlations are studied by calculating the discrete correlation function for each pair of energy bands where the significance was evaluated with dedicated Monte Carlo simulations. The space of SSC model parameters is probed following a dedicated grid-scan strategy, allowing for a wide range of models to be tested and offering a study of the degeneracy of model-to-data agreement in the individual model parameters, hence providing a less biased interpretation than the "single-curve SSC model adjustment" typically reported in the literature.Results. We find an increase in the fractional variability with energy, while no significant interband correlations of flux changes are found on the basis of the acquired data set. The SSC model grid-scan shows that the flaring activity around May 22 cannot be modeled adequately with a one-zone SSC scenario (using an electron energy distribution with two breaks), while it can be suitably described within a two (independent) zone SSC scenario. Here, one zone is responsible for the quiescent emission from the averaged 4.5-month observing period, while the other one, which is spatially separated from the first, dominates the flaring emission occurring at X-rays and very-high-energy (> 100 GeV, VHE) gamma-rays. The flaring activity from May 1, which coincides with a rotation of the electric vector polarization angle (EVPA), cannot be satisfactorily reproduced by either a one-zone or a two-independent-zone SSC model, yet this is partially affected by the lack of strictly simultaneous observations and the presence of large flux changes on sub-hour timescales (detected at VHE gamma rays).Conclusions. The higher variability in the VHE emission and lack of correlation with the X-ray emission indicate that, at least during the 4.5-month observing campaign in 2009, the highest energy (and most variable) electrons that are responsible for the VHE gamma rays do not make a dominant contribution to the similar to 1 keV emission. Alternatively, there could be a very variable component contributing to the VHE gamma-ray emission in addition to that coming from the SSC scenario. The studies with our dedicated SSC grid-scan show that there is some degeneracy in both the one-zone and the two-zone SSC scenarios probed, with several combinations of model parameters yielding a similar model-to-data agreement, and some parameters better constrained than others. The observed gamma-ray flaring activity, with the EVPA rotation coincident with the first gamma-ray flare, resembles those reported previously for low frequency peaked blazars, hence suggesting that there are many similarities in the flaring mechanisms of blazars with different jet properties.