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arXiv 0901.1346astro-ph.HE
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半人马座A射电叶激波处的高能粒子加速

High-energy particle acceleration at the radio-lobe shock of Centaurus A

J. H. Croston, R. P. Kraft, M. J. Hardcastle, M. Birkinshaw, D. M. Worrall, P. E. J. Nulsen, R. F. Penna, G. R. Sivakoff, A. Jordan, N. J. Brassington, D. A. Ev… 展开作者

J. H. Croston, R. P. Kraft, M. J. Hardcastle, M. Birkinshaw, D. M. Worrall, P. E. J. Nulsen, R. F. Penna, G. R. Sivakoff, A. Jordan, N. J. Brassington, D. A. Evans, W. R. Forman, M. Gilfanov, J. L. Goodger, W. E. Harris, C. Jones, A. M. Juett, S. S. Murray, S. Raychaudhury, C. L. Sarazin, R. Voss, K. A. Woodley

摘要:

我们提出了新的结果,使用钱德拉甚大计划观测数据的西南射电瓣的半人马座A周围的冲击。辐射的X射线光谱在叶的西南边缘的外部周围是很好地描述了一个单一的幂律模型与银河系的吸收-热模型是非常不利的,除了在该地区最接近的核心。我们的结论是,波瓣西南部周围的X射线发射中有很大一部分是同步加速器发射的,而不是热发射的。我们推断,在冲击波最强且周围气体密度最低的区域,波瓣的膨胀正在将粒子加速到X射线同步加速器发射能量,类似于超新星遗迹,如SN 1006。这种解释解决了我们早先对这种发射的纯热解释中的一个问题,即穿过激波的密度压缩需要比理论上预期的因子4大得多。我们估计,瓣正在以约2600 km/s的速度向西南方向扩张,相对于周围介质大约为8马赫。我们讨论了激波区域内谱指数的空间变化,得出的结论是,我们的观测结果将激波最强部分加速粒子的γ_max限制为10^8,这与扩散激波加速理论的预期一致。最后,我们认为这些结果的影响,为生产超高能宇宙射线(UHECRs)和TeV发射从半人马座A,得出的结论是,冲击波前区域是不太可能是一个重要的来源的UHECRs,但TeV发射从这个地区预计在水平相当于目前的限制在TeV的能量,合理的假设磁场强度。

英文摘要:

We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations. The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption -- thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We estimate that the lobe is expanding to the southwest with a velocity of ~2600 km/s, roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain gamma_max for the accelerated particles to be 10^8 at the strongest part of the shock, consistent with expectations from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strengths.

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