非均匀喷流模型及其在BL Lac天体中的应用

Inhomogeneous Jet Model and Its Application in BL Lacertae Objects

在非均匀锥形喷流模型中,电子数密度、磁场强度随着到喷流顶点的距离呈幂律分布.该模型能成功解释活动星系核喷流核心区域的平谱射电辐射,但已有的模型计算只适用于喷流运动方向与视线夹角很大的情况,所以需要建立适用于任何视角情况的非均匀锥形喷流辐射计算公式.普遍认为BL Lac天体中喷流的运动方向与视线夹角很小,推广后的非均匀喷流模型拟合了3个BL Lac天体的射电观测谱,确定了它们喷流中电子数密度、磁场强度等物理参数.研究结果表明观测辐射谱拐折频率确定出锥形喷流离黑洞最近距离,对于这3个BL Lac天体,它们的锥形喷流离黑洞最近距离约为Schwarzschild半径.

The VLBI （Very Long Baseline Interferometry） core is the compact, flat- spectrum component that lies at the end of the jet of typical blazars. Although the knots appear to separate from the core at superluminal velocities, the core seems to be stationary. At centimeter wavelengths, the cores of blazars are optically thick due to the synchrotron self-absorption. The total synchrotron spectrum of the jet at radio frequencies is quite flat, with the spectral index α 〈 0.5 （Fv ∝ v^-α）, which is caused by the superposition of the spectra of the core and the knots that lies downstream in the jet. The jet produces a power- law spectrum at high frequencies with a 〉 0, which is determined by the energy distribution of electrons. In a self-absorbed, optically thick region, the spectrum has a power-law slope of -5/2, whatever the original distribution of electrons is. If both the distributions of magnetic field strength and number density of electrons fall off in the form of a power-law function from the apex of the jet, the spectrum from each successive section peaks at a progressively lower frequency. So if we cannot resolve the individual parts of the jet, theoverall spectrum will be fiat. The ＂core-shift＂ effect τc ∝ vs^-1/km is also caused by the same scenario, where rc is the angular size of core, us is the observing frequency, and km is around unity. Theoretically, the VLBI core is the position where the optical depth τ- equals to unity. But in practice, the VLBI core ranges from the vicinity of this position with τ=1 down to the innermost of the jet. KSnigl proposed an inhomogeneous jet model, in which the jet is assumed to have a conical geometry, that is, the velocity with which the jet is expanding sideways, is constant. The bulk velocity of the jet material along the jet axis is also assumed to be constant. The magnetic field strength and number density of the relativistic electrons in the jet are as- sumed to vary with the distance from the apex of the jet, following the power-law. Konigl＇s inhomogeneous jet model can explain both the flat spectrum characteristics of some active galactic nuclei and the dependence of the core size on the observing frequency. After that, many authors have improved the inhomogeneous jet model with various assumptions and simplifications. We reconsider this jet model with assumptions and simplifications as few as possible, including more detailed considerations such as the difference of Doppler factor of each section of jet, and the small viewing angle cases in BL Lacertae objects. We then apply our model to three BL Lacertae objects, fit their spectrum energy distribution, and derive the physical parameters of jets. We also calculate the kinetic energy and magnetic energy in the jets of these three BL Lacertae objects, and find that at pc scale the kinetic power dominates over magnetic power, indicating that the effective transfer efficiency from magnetic energy to kinetic energy.