标题： GRB 991216的神秘射电余辉 显示英文标题

标题： The Enigmatic Radio Afterglow of GRB 991216

摘要： 我们展示了从1.4 GHz到350 GHz的宽频段射电观测结果，这些数据是在GRB 991216余辉爆发后的1到80天内采集的。 这个爆发的光学和X射线余辉相当典型，可以用喷流火球模型来解释。 相比之下，射电光变曲线在两个方面显得异常：(a) 射电光变曲线没有显示出通常几周时间尺度上的最大流量上升，而是在第1天就已经开始下降；(b) 从射电到X射线波段的幂律指数显示出了显著的陡峭化。 我们证明标准的火球模型（其中余辉来自前向冲击）无法解释(b)，并得出结论大部分射电辐射必须来源于不同的源。 我们考虑了两种模型，现有数据都无法排除这两种模型。 在第一种（常规）模型中，早期射电辐射被归因于与GRB 990123类似的情况下的反向冲击辐射。 我们预测，该爆发的瞬时光学辐射亮度将达到或超过8等。 在第二种（奇异）模型中，射电辐射起源于一个各向同性能量的火球（10^54 erg）向前冲击波，该火球扩展进入稀薄介质（10^-4 cm^-3）。 由此产生的火球在数月内仍保持相对论性，并可能通过甚长基线干涉测量技术解析。 最后，我们注意到余辉的近红外突起与GRB 971214中看到的相似，没有任何火球模型可以解释这一突起。 显示英文摘要

摘要： We present wide-band radio observations spanning from 1.4 GHz to 350 GHz of the afterglow of GRB 991216, taken from 1 to 80 days after the burst. The optical and X-ray afterglow of this burst were fairly typical and are explained by a jet fireball. In contrast, the radio light curve is unusual in two respects: (a) the radio light curve does not show the usual rise to maximum flux on timescales of weeks and instead appears to be declining already on day 1 and (b) the power law indices show significant steepening from the radio through the X-ray bands. We show that the standard fireball model, in which the afterglow is from a forward shock, is unable to account for (b) and we conclude that the bulk of the radio emission must arise from a different source. We consider two models, neither of which can be ruled out with the existing data. In the first (conventional) model, the early radio emission is attributed to emission from the reverse shock as in the case of GRB 990123. We predict that the prompt optical emission would have been as bright (or brighter) than 8th magnitude. In the second (exotic) model, the radio emission originates from the forward shock of an isotropically energetic fireball (10^54 erg) expanding into a tenuous medium (10^-4 cm^-3). The resulting fireball would remain relativistic for months and is potentially resolvable with VLBI techniques. Finally, we note that the near-IR bump of the afterglow is similar to that seen in GRB 971214 and no fireball model can explain this bump.