arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1206.4569astro-ph.HEastro-ph.IMstat.AP
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超高能宇宙射线产生、传播和探测的多层贝叶斯模型

Multilevel Bayesian framework for modeling the production, propagation and detection of ultra-high energy cosmic rays

Kunlaya Soiaporn, David Chernoff, Thomas Loredo, David Ruppert, Ira Wasserman

摘要:

超高能宇宙射线(UHECR)是一种原子核,其能量超过人造粒子加速器可获得的能量的1000万倍。有证据表明它们来自相对较近的河外源,但源的性质尚不清楚。我们开发了一个多层次的贝叶斯框架评估关联的UHECRs和候选源人口,和马尔可夫链蒙特卡罗算法估计模型参数和比较模型的计算,通过Chib的方法,边际似然和贝叶斯因子。我们展示了框架,通过分析皮埃尔俄歇天文台(PAO)从2004年至2009年观察到的69 UHECR的测量,使用体积完整的目录17个本地活动星系核(AGN)出15 megaparsec作为候选源。PAO使用数据的早期部分(“周期1”,有14个事件)来设置能量截止,以最大化周期1中的各向异性; 69个测量包括这个“调谐”子集,以及随后的“未调谐”事件,其能量高于相同的截止。此外,测量误差进行了近似总结。这些因素对于专业会计组织数据的独立分析是有问题的。在“标准蜡烛”源模型的上下文中(即,与一个共同的各向同性发射率),并考虑只有55个未调谐的事件,有没有显着的证据支持关联的UHECR与本地AGN与各向同性背景。概率最高的关联是与两个最近的、相邻的活动星系核,半人马座A和NGC 4945。如果采用关联模型,则可能关联的UHECR的分数可能不为零,但远低于50%。我们的框架能够估计宇宙磁场偏转宇宙射线的角度尺度;相对适度的尺度$\approx\!3^{\circ}$到$30^{\circ}$是有利的。将大部分UHECR分配给单个附近源的模型(例如,半人马座A)的情况除外,除非事先规定了非常大的偏转尺度,即使这样,它们也是不受欢迎的。然而,包括周期1的数据改变的结论显着,和模拟研究支持的想法,周期1的数据是异常的,大概是由于调整。对未来数据的准确和最佳分析可能需要更全面地披露数据。

英文摘要:

Ultra-high energy cosmic rays (UHECRs) are atomic nuclei with energies over ten million times energies accessible to human-made particle accelerators. Evidence suggests that they originate from relatively nearby extragalactic sources, but the nature of the sources is unknown. We develop a multilevel Bayesian framework for assessing association of UHECRs and candidate source populations, and Markov chain Monte Carlo algorithms for estimating model parameters and comparing models by computing, via Chib's method, marginal likelihoods and Bayes factors. We demonstrate the framework by analyzing measurements of 69 UHECRs observed by the Pierre Auger Observatory (PAO) from 2004-2009, using a volume-complete catalog of 17 local active galactic nuclei (AGN) out to 15 megaparsecs as candidate sources. An early portion of the data ("period 1," with 14 events) was used by PAO to set an energy cut maximizing the anisotropy in period 1; the 69 measurements include this "tuned" subset, and subsequent "untuned" events with energies above the same cutoff. Also, measurement errors are approximately summarized. These factors are problematic for independent analyses of PAO data. Within the context of "standard candle" source models (i.e., with a common isotropic emission rate), and considering only the 55 untuned events, there is no significant evidence favoring association of UHECRs with local AGN vs. an isotropic background. The highest-probability associations are with the two nearest, adjacent AGN, Centaurus A and NGC 4945. If the association model is adopted, the fraction of UHECRs that may be associated is likely nonzero but is well below 50%. Our framework enables estimation of the angular scale for deflection of cosmic rays by cosmic magnetic fields; relatively modest scales of $\approx\!3^{\circ}$ to $30^{\circ}$ are favored. Models that assign a large fraction of UHECRs to a single nearby source (e.g., Centaurus A) are ruled out unless very large deflection scales are specified a priori, and even then they are disfavored. However, including the period 1 data alters the conclusions significantly, and a simulation study supports the idea that the period 1 data are anomalous, presumably due to the tuning. Accurate and optimal analysis of future data will likely require more complete disclosure of the data.

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