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arXiv 0902.2892astro-ph.HE
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X射线双星喷流辐射模型:一次加速事件的结果

A Model for Emission from Jets in X-ray Binaries: Consequences of a Single Acceleration Episode

Asaf Pe'er, Piergiorgio Casella

摘要:

有强有力的证据表明,在黑洞X射线双星(BHXRB)的低/硬状态下存在强大的喷流。在这里,我们提出了一个模型,在该模型中,电子被加速一次在基地的射流,并通过同步辐射和可能的绝热能量损失冷却。被加速的电子在低能量下呈现麦克斯韦分布,并可能具有能量幂律尾。这些假设产生了丰富的光谱,我们详细研究。我们确定的磁场的临界值,和光谱中的五个过渡频率。特别是,我们表明:(I)衰减的磁场沿射流使,宽喷流,生产平坦的无线电频谱,而不需要电子重新加速沿射流。(II)当磁场增加到~10^5 G的临界值以上时,会导致无线电频段的通量急剧下降,而较高频率下的通量则饱和到恒定值。(III)对于强磁场,磁通量在光学/紫外波段衰减为F_nu ~ nu^{-1/2},与电子的初始分布无关。(IV)在B_0 ~ 10^4 G范围内,X射线通量逐渐变陡。(V)(VI)对于窄喷流,r(x)~ x^{alpha}(α < 1/2),不能得到平坦的射电频谱。我们提供了完整的描述,在不同的情况下的频谱,并表明我们的模型是一致的BHXRB的关键观察到的属性。

英文摘要:

There are strong evidence for powerful jets in the low/hard state of black-hole X-ray binaries (BHXRBs). Here, we present a model in which electrons are accelerated once at the base of the jet, and are cooled by synchrotron emission and possible adiabatic energy losses. The accelerated electrons assume a Maxwellian distribution at low energies and possible energetic power law tail. These assumptions yield to a wealth of spectra, which we study in details. We identify critical values of the magnetic field, and five transition frequencies in the spectra. In particular, we show that: (I) the decay of the magnetic field along the jet enables, for wide jets, production of flat radio spectra without the need for electrons re-acceleration along the jet. (II) An increase of the magnetic field above a critical value of ~10^5 G leads to a sharp decrease in the flux at the radio band, while the flux at higher frequencies saturates to a constant value. (III) For strong magnetic field, the flux decays in the optical/UV band as F_nu ~ nu^{-1/2}, irrespective of the electrons initial distribution. (IV) For B_0 ~ 10^4 G, the X-ray flux gradually steepens. (V) With adiabatic energy losses, flat spectrum can be obtained only at a limited frequency range, and under certain conditions (VI) For narrow jets, r(x) ~ x^{alpha} with alpha < 1/2, flat radio spectrum cannot be obtained. We provide full description of the spectrum in the different scenarios, and show that our model is consistent with the key observed properties of BHXRBs.

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