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arXiv 1002.3377astro-ph.HEastro-ph.CO
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Fermi/LAT伽马射线暴发射模型和喷注性质

Fermi/LAT Gamma Ray Burst emission models and jet properties

G. Ghisellini, G. Ghirlanda

摘要:

最初由EGRET在少数几个爆发中探测到的伽马射线暴的GeV发射,现在由于费米/LAT的观测,已经确定了大约10%的所有爆发的性质。GRB 090510是一个短暂的爆发,它特别有趣,因为它的好时机使得量子引力理论受到了严格的限制。从目前在0.1-30 GeV波段探测到的12个暴中,我们开始看到一些共同的性质:(i)持续时间往往比费米望远镜上的伽玛暴监测器(GBM)探测到的较软辐射的持续时间长,(ii)光谱符合F(v)~v^{-1},没有强烈的光谱演化;(iii)对于最亮的爆发,LAT检测到的通量以具有典型斜率的幂律衰减:t^{-1.5};(iv)GBM发射的峰值能量超过500 keV(静止帧)。这些性质表明,GeV通量的起源类似的过程。我们建议,这是余辉同步辐射后不久开始的提示阶段。陡峭的衰减斜率表明火球在辐射状态下发射,即所有耗散的能量都被辐射出去。GBM通量的大峰值能量表明电子-正电子对可能起着至关重要的作用。相对于GBM的GeV通量的快速开始,但有一些延迟,表明这些爆发的体洛伦兹因子伽马是1000的数量级。因此,在高能量下检测到的相对小部分的爆发可能对应于具有最大伽马的爆发的部分。如果发射发生在辐射状态,我们就可以开始理解为什么观测到的X射线和光学余辉能量比瞬发阶段发射的能量小得多。

英文摘要:

The GeV emission of Gamma Ray Bursts, first detected by EGRET in an handful of bursts, is now an established property of roughly the 10% of all bursts, thanks to the Fermi/LAT observations. GRB 090510, a short burst, is particularly interesting because the good timing allows to derive a severe limit to theories of quantum gravity. With the dozen bursts detected in the 0.1-30 GeV band so far, we start to see some common properties: (i) the duration is often longer than the duration of the softer emission detected by the Gamma Burst Monitor (GBM) onboard Fermi; (ii) the spectrum is consistent with F(v)~v^{-1} with no strong spectral evolution; (iii) for the brightest bursts, the flux detected by the LAT decays as a power law with a typical slope: t^{-1.5}; iv) the peak energy of the GBM emission exceeds 500 keV (rest frame). These properties suggest a similar process for the origin of the GeV flux. We propose that it is afterglow synchrotron emission shortly following the start of the prompt phase. The steep decay slope suggests that the fireball emits in the radiative regime, i.e. all dissipated energy is radiated away. The large peak energy of the GBM flux suggests that electron-positron pairs might play a crucial role. The rapid onset, but with some delay, of the GeV flux with respect to the GBM one suggests that the bulk Lorentz factor Gamma of these bursts is of the order of 1000. Therefore the relatively small fraction of bursts detected at high energies might correspond to the fraction of bursts having the largest Gamma. If the emission occurs in the radiative regime we can start to understand why the observed X-ray and optical afterglow energetics are much smaller than the energetics emitted during the prompt phase.

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