用射电观测照亮最暗的伽马射线暴
Illuminating the Darkest Gamma-Ray Bursts with Radio Observations
摘要：
我们目前的X射线，光学，近红外线，和伽玛射线暴110709 B和111215 A的无线电观测，以及其宿主星系的光学和近红外观测。X射线探测和深光学/近红外极限的组合确定这两个爆发为“黑暗”。射电探测的亚弧秒位置导致了宿主星系的关联，光学探测表明z < 4（110709 B）和1.8 < z < 2.7（111215 A）。利用每个爆发的射电和X射线数据，我们发现GRB 110709 B需要A_V > 5.3等，GRB 111215 A需要A_V > 8.5等（z=2），这是迄今为止推断的暗爆发的最大消光值之一。这两个暴还表现出大的中性氢柱密度（NH> 10^22/cm ^2; z=2），这与暗伽玛暴的趋势一致。最后，我们发现，对于这两个爆发，余辉发射最好用准直外流来解释，准直外流的总射束校正能量为E_gamma+E_K ~ 7-9 x 10^51 erg（z=2），扩展到高密度的风介质中（在10^17 cm处n~100-350 cm^-3）。虽然能量释放是典型的长伽玛射线暴，推断的密度可能是指示较大的质量损失率的伽玛射线暴的祖先在尘土飞扬（因此富含金属）的环境。这项研究确立了射电观测在确定暗伽玛暴起源和性质方面的关键作用。JVLA和ALMA的观测将提供一个亚弧秒位置和强大的主机协会的样本，这将有助于阐明被遮蔽的恒星形成和金属丰度在GRB祖细胞中的作用。
英文摘要：
We present X-ray, optical, near-infrared, and radio observations of GRBs 110709B and 111215A, as well as optical and near-IR observations of their host galaxies. The combination of X-ray detections and deep optical/near-infrared limits establish both bursts as "dark". Sub-arcsecond positions enabled by radio detections lead to robust host galaxy associations, with optical detections that indicate z < 4 (110709B) and 1.8 < z < 2.7 (111215A). Using the radio and X-ray data for each burst we find that GRB 110709B requires A_V > 5.3 mag and GRB 111215A requires A_V > 8.5 mag (z=2), among the largest extinction values inferred for dark bursts to-date. The two bursts also exhibit large neutral hydrogen column densities (N_H > 10^22/cm^2; z=2) as inferred from their X-ray spectra, in agreement with the trend for dark GRBs. Finally, we find that for both bursts the afterglow emission is best explained by a collimated outflow with a total beaming-corrected energy of E_gamma+E_K ~ 7-9 x 10^51 erg (z=2) expanding into a wind medium with a high density (n~100-350 cm^-3 at 10^17 cm). While the energy release is typical of long GRBs, the inferred density may be indicative of larger mass loss rates for GRB progenitors in dusty (and hence metal rich) environments. This study establishes the critical role of radio observations in determining the origin and properties of dark GRBs. Observations with the JVLA and ALMA will provide a sample with sub-arcsecond positions and robust host associations that will help shed light on obscured star formation and the role of metallicity in GRB progenitors.
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