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arXiv 1801.06186astro-ph.HE
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大质量恒星中GRB喷流光度阈值的存在性

On the existence of a luminosity threshold of GRB jets in massive stars

Miguel-Ángel Aloy, Carlos F. Cuesta-Martínez, M. Obergaulinger

摘要:

受$γ$射线暴与高能超新星爆炸的关联性的启发,我们研究了相对论性喷流在母星内部的传播,在喷流开始传播之前,可能会短暂地发射出一个SN激波。基于分析的考虑和验证了一套广泛的二维轴对称相对论流体动力学模拟,我们已经估计了一个阈值的内在射流光度,$L_{\rm j}^{\rm thr}$,成功地发射喷气。这个阈值取决于祖先的结构,因此,它对它的质量和金属性是敏感的。对于宇宙学长伽玛暴的原型宿主,一颗35 $\,M_{\odot}$的低金属丰度恒星,它是$L_{\rm j}^{\rm thr} \simeq 1.35\times 10^{49}\,$ erg $\,$ s $^{-1}$。观测到的等效各向同性$γ$射线光度$L_{\rm γ,iso,BO} \simeq 4 ε_γL_{\rm j} θ_{\rm BO}^{-2}$主要取决于爆发后的喷流张开角$θ_{\rm BO}$和将本征喷流光度转换成$γ$辐射的效率$ε_γ$ .高能喷流可以产生低光度事件,如果它们的爆发后的张开角是大的,这是在我们的模型中发现的,或者如果动能和内能到辐射的转换效率足够低。除了这个理论分析,我们展示了如何存在的SN冲击波可能会降低这个亮度阈值的数值模拟。我们预见到,在亮度阈值附近产生的喷流释放的高能瞬变将更类似于11GRB或XRF而不是GRB。

英文摘要:

Motivated by the many associations of $γ$-ray bursts (GRBs) with energetic supernova (SN) explosions, we study the propagation of relativistic jets within the progenitor star in which a SN shock wave may be launched briefly before the jets start to propagate. Based on analytic considerations and verified with an extensive set of 2D axisymmetric relativistic hydrodynamic simulations, we have estimated a threshold intrinsic jet luminosity, $L_{\rm j}^{\rm thr}$, for successfully launching a jet. This threshold depends on the structure of the progenitor and, thus, it is sensible to its mass and to its metallicity. For a prototype host of cosmological long GRBs, a low-metallicity star of 35$\,M_{\odot}$, it is $L_{\rm j}^{\rm thr} \simeq 1.35\times 10^{49}\,$erg$\,$s$^{-1}$. The observed equivalent isotropic $γ$-ray luminosity, $L_{\rm γ,iso,BO} \simeq 4 ε_γL_{\rm j} θ_{\rm BO}^{-2}$, crucially depends on the jet opening angle after breakout, $θ_{\rm BO}$, and on the efficiency for converting the intrinsic jet luminosity into $γ$-radiation, $ε_γ$. Highly energetic jets can produce low-luminosity events if either their opening angle after the breakout is large, which is found in our models, or if the conversion efficiency of kinetic and internal energy into radiation is low enough. Beyond this theoretical analysis, we show how the presence of a SN shock wave may reduce this luminosity threshold by means of numerical simulations. We foresee that the high-energy transients released by jets produced near the luminosity threshold will be more similar to llGRBs or XRFs than to GRBs.

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