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arXiv 0707.0931astro-ph
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GRB分布的低光度尾部:GRB 980425的情况

The low-luminosity tail of the GRB distribution: the case of GRB 980425

F. Daigne, R. Mochkovitch

摘要:

GRB 980425与SN 1998 bw在z=0.0085处的关联意味着存在一个各向同性等效光度的GRB群体,其光度比标准宇宙学情况下的小10^4倍。我们调查两种情况来解释一个弱的伽玛暴:一个正常的(本质上明亮的)伽玛暴看到离轴或本质上弱伽玛暴看到轴上。对于这两种情况下,我们首先推导出的条件,产生一个GRB 980425类事件,然后我们讨论的事件率的后果。如果我们排除GRB 980425是在“余辉时代”的前八年偶然观察到的极其罕见的事件的可能性,第一种情况意味着:(i)标准明亮GRB的本地率比通常预期的要高得多;(ii)典型的开口角比从余辉光曲线的中断观测中得出的要窄得多。此外,我们表明,在这种情况下,GRB 980425的余辉应该是非常明亮的,很容易检测到。由于这些原因,第二种情况似乎更现实。我们表明,内部冲击模型的参数空间确实允许GRB 980425类事件,在流出的情况下,只有温和的相对论性和温和的能量。宇宙中这种微弱事件的发生率必须远远高于标准明亮GRB的发生率,才能在短短几年内发现GRB 980425。然而,它仍然与观测结果相一致,因为这些GRB 980425样爆发的内在弱点不允许在宇宙学红移处检测到。最后,我们简要地讨论了这样一个高的本地率的GRB 980425类事件的未来前景的探测非电磁辐射,特别是引力波的后果。

英文摘要:

(abridged) The association of GRB 980425 with SN 1998bw at z=0.0085 implies the existence of a population of GRBs with an isotropic-equivalent luminosity which is about 10^4 times smaller than in the standard cosmological case. We investigate two scenarios to explain a weak GRB : a normal (intrinsically bright) GRB seen off-axis or an intrinsically weak GRB seen on-axis. For each of these two scenarios, we first derive the conditions to produce a GRB 980425-like event and we then discuss the consequences for the event rate. If we exclude the possibility that GRB 980425 is an occurence of an extremely rare event observed by chance during the first eight years of the 'afterglow era', the first scenario implies that (i) the local rate of standard bright GRBs is much higher than what is usually expected; (ii) the typical opening angle is much narrower than what is derived from observations of a break in the afterglow lightcurve. In addition, we show that the afterglow of GRB 980425 in this scenario should have been very bright and easily detected. For these reasons the second scenario appears more realistic. We show that the parameter space of the internal shock model indeed allows GRB 980425-like events, in cases where the outflow is only mildly-relativistic and mildly-energetic. The rate of such weak events in the Universe has to be much higher than the rate of standard bright GRBs to allow the discovery of GRB 980425 during a short period of a few years. However it is still compatible with the observations as the intrinsic weakness of these GRB 980425-like bursts does not allow detection at cosmological redshift. We finally briefly discuss the consequences of such a high local rate of GRB 980425-like events for the future prospects of detecting non-electromagnetic radiation, especially gravitational waves.

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