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arXiv 0902.1755astro-ph.HE
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超新星遗迹的射电辐射:对激波后磁场放大和星系磁场的影响

Radio Emission from Supernova Remnants: Implications for Post-Shock Magnetic Field Amplification and the Magnetic Fields of Galaxies

Todd A. Thompson, Eliot Quataert, Norman Murray

摘要:

利用文献中的观测结果,我们表明,超新星遗迹(SNRs)的非热射电光度(L)是一个强大的功能的平均气体表面密度(Sigma)的星系中的遗迹居住,从正常的螺旋发光星爆。我们结合一个简单的理论,电子冷却的SNR观测到的射电光度估计剩余磁场强度(B_SNR):L和西格玛之间的相关性意味着B_SNR也增加与西格玛。我们探讨了这种相关性的两种解释:(1)B_SNR是由激波后磁场放大产生的,B_SNR ^2与Sigma成正比;(2)B_SNR是由周围ISM磁场(B_ISM)的激波压缩产生的,在密度较大的星系中,B_ISM较大。我们发现,冲击压缩是,平均而言,足以产生观测到的无线电发射从超新星爆发的SNR,放大后的冲击磁场是不需要的。相比之下,在正常的螺旋星系中,激波后的场放大(放大了几倍--10倍)与数据是一致的;我们发现了初步的证据,即阿尔芬速度和B_SNR^2与激波后压力的比值(“B_SNR^2_B”)在不同星系的信噪比中是恒定的。我们讨论的观测测试,可用于区分这两种解释的射电光度的信噪比。无论哪种假设是正确的,SNR的无线电发射都为B_ISM提供了一个上限,该上限与最小能量假设无关。对于致密的星爆,ISM的磁能密度低于流体静力平衡所需的密度;因此,在星爆的最大尺度上,磁场在动力学上并不重要,这与像我们这样的螺旋星系形成对比。这种二分法可能对银河发电机理论有影响。

英文摘要:

Using observations from the literature, we show that the non-thermal radio luminosity (L) of supernova remnants (SNRs) is a strong function of the average gas surface density (Sigma) of the galaxy in which the remnants reside, from normal spirals to luminous starbursts. We combine a simple theory for electron cooling in SNRs with the observed radio luminosities to estimate the remnant magnetic field strength (B_SNR): the correlation between L and Sigma implies that B_SNR also increases with Sigma. We explore two interpretations of this correlation: (1) B_SNR is generated by post-shock magnetic field amplification, with B_SNR^2 proportional to Sigma and (2) B_SNR results from shock-compression of the ambient ISM magnetic field (B_ISM), with B_ISM being larger in denser galaxies. We find that shock compression is, on average, sufficient to produce the observed radio emission from SNRs in the densest starbursts; amplification of post-shock magnetic fields is not required. By contrast, in normal spirals post-shock field amplification (by a factor of a few - 10) is consistent with the data; we find tentative evidence that both the Alfven speed and the ratio of B_SNR^2 to the post-shock pressure ("epsilon_B") are constant in SNRs from galaxy to galaxy. We discuss observational tests that can be used to distinguish between these two interpretations of the radio luminosities of SNRs. Regardless of which is correct, the radio emission from SNRs provides an upper limit to B_ISM that is independent of the minimum energy assumption. For the densest starbursts, the ISM magnetic energy density is below that required for hydrostatic equilibrium; thus magnetic fields are not dynamically important on the largest scales in starbursts, in contrast with spiral galaxies like our own. This dichotomy may have implications for galactic dynamo theory.

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