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arXiv 1202.2874astro-ph.HEastro-ph.CO
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超高能Cosmic Ray核在宇宙磁场中的传播及其对各向异性测量的意义

Propagation of Ultra-High-Energy Cosmic Ray Nuclei in Cosmic Magnetic Fields and Implications for Anisotropy Measurements

Hajime Takami, Susumu Inoue, Tokonatsu Yamamoto

摘要:

皮埃尔俄歇天文台(PAO)的最新结果表明,能量高于$10^{19}$ eV的超高能宇宙线(UHECR)的成分可能由重核主导。一个重要的问题是,这种UHECR核的到达方向的分布是否可以表现出可观察到的各向异性或与它们的天体物理源对象的位置相关性,尽管通过干预磁场预期的强偏转。为此,我们模拟了UHECR核的传播,包括河外磁场和银河系磁场的模型。假设只有铁核稳定地从具有相等亮度的源注入,并且根据观测到的局部宇宙中的大尺度结构在空间上分布,在PAO迄今公布的事件数量下,如果UHECR源的平均数密度为$n_s >~ 10^{-6}$ Mpc $^{-3}$,并且与$n_s >~ 10^{-5}$ Mpc $^{-3}$在95%误差内与源无交叉相关一致。另一方面,在整个天空中有1000个高于$5.5 \times 10^{19}$ eV的事件,下一代实验可以揭示即使对于$n_s <~ 10^{-3}$ Mpc $^{-3}$也具有超过99%概率的自相关,并且对于$n_s <~ 10^{-4}$ Mpc $^{-3}$具有超过99%概率的与源的交叉相关。此外,我们发现,半人马座A的贡献是需要重现目前观察到的超UHECR在半人马座地区。初级重核在传播过程中产生的次级质子在所有情况下都起着至关重要的作用,如果重核的最大能量足够高,那么在小角度尺度上产生的各向异性应该提供源位置的强烈暗示。

英文摘要:

(Abridged) Recent results from the Pierre Auger Observatory (PAO) indicate that the composition of ultra-high-energy cosmic rays (UHECRs) with energies above $10^{19}$ eV may be dominated by heavy nuclei. An important question is whether the distribution of arrival directions for such UHECR nuclei can exhibit observable anisotropy or positional correlations with their astrophysical source objects despite the expected strong deflections by intervening magnetic fields. For this purpose, we have simulated the propagation of UHECR nuclei including models for both the extragalactic magnetic field and the Galactic magnetic field. Assuming that only iron nuclei are injected steadily from sources with equal luminosity and spatially distributed according to the observed large scale structure in the local Universe, at the number of events published by the PAO so far, the arrival distribution of UHECRs would be consistent with no auto-correlation at 95% confidence if the mean number density of UHECR sources $n_s >~ 10^{-6}$ Mpc$^{-3}$, and consistent with no cross-correlation with sources within 95% errors for $n_s >~ 10^{-5}$ Mpc$^{-3}$. On the other hand, with 1000 events above $5.5 \times 10^{19}$ eV in the whole sky, next generation experiments can reveal auto-correlation with more than 99% probability even for $n_s <~ 10^{-3}$ Mpc$^{-3}$, and cross-correlation with sources with more than 99% probability for $n_s <~ 10^{-4}$ Mpc$^{-3}$. In addition, we find that the contribution of Centaurus A is required to reproduce the currently observed UHECR excess in the Centaurus region. Secondary protons generated by photodisintegration of primary heavy nuclei during propagation play a crucial role in all cases, and the resulting anisotropy at small angular scales should provide a strong hint of the source location if the maximum energies of the heavy nuclei are sufficiently high.

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