[论文解读] The Afterglow and Host Galaxy of the Energetic Short-Hard Gamma-Ray Burst 051221

A. M. Soderberg, E. Berger+9 more|arXiv (Cornell University)|Jan 20, 2006

Gamma-ray bursts and supernovae被引用 1

一句话总结

本研究对短时长短伽马射线暴GRB051221进行了多波段观测，测得红移z=0.5459，其各向同性能量释放约为2.4×10^51 erg。缺乏喷流截断特征以及晚发能量注入的证据表明中心引擎持续活动，且经过束流修正后的动能超过1×10^50 erg，这一发现挑战了标准模型，支持其起源于致密天体并合而非大质量恒星坍缩。

ABSTRACT

We present detailed optical, X-ray and radio observations of the bright afterglow of the short GRB051221 and an optical spectrum from which we measure the redshift of the burst, z=0.5459. The isotropic-equivalent prompt energy release was about 2.4 x 10^51 erg, and using the standard afterglow synchrotron model we find that the blastwave kinetic energy is similar, E_{KE,iso}~1.4 x 10^51 erg. The lack of an observed jet break to t~13 days indicates that the opening angle is > 13 deg and the total beaming-corrected energy is > 1 x 10^50 erg, at least an order of magnitude larger than for previous short GRBs. We further show that the burst experienced an episode of energy injection by a factor of 3.4 between t=1.4 and 3.4 hours, which was accompanied by reverse shock emission in the radio band. This result provides continued evidence that the central engines of short GRBs may be active significantly longer than the duration of the burst and/or produce a wide range of Lorentz factors. Finally, we show that the host galaxy is actively forming stars at a rate of about 1.5 M_solar/yr, but at the same time exhibits evidence for an appreciable population of old stars (~1 Gyr) and near solar metallicity. These properties are intermediate between those of long GRB hosts and those of previous short bursts suggesting that the progenitor lifetimes may have a large spread. The lack of bright supernova emission and the low circumburst density (n~0.001 cm^-3), however, continue to support the idea that short bursts are not related to the death of massive stars and are instead consistent with a compact object merger. If the energy release of this burst is in fact few x 10^51 erg then this suggests that the neutrino annihilation mechanism may not be sufficient and MHD processes may be required to power the burst.

研究动机与目标

• 利用多波段余晖数据测定短伽马暴GRB051221的红移与能量释放特性。
• 通过在余晖光 light curve 中搜索喷流截断，评估束流修正后的能量释放。
• 调查射电波段中是否存在晚发能量注入及反向冲击辐射的证据。
• 表征宿主星系的恒星形成率、星族年龄与金属丰度，以推断爆发前身体系统的性质。
• 评估该爆发的能量释放特性是否与中子星并合或大质量恒星坍缩模型一致。

提出的方法

• 分析光学、X射线与射电波段的光 light curve，以建模余晖衰减并识别指示喷流结构的转折点。
• 应用标准余晖同步辐射模型，推导各向同性动能与激波参数。
• 通过谱能分布（SED）分析，在能量注入阶段探测射电波段的反向冲击辐射。
• 获取宿主星系的光学光谱以测量红移并推断星族特性。
• 通过谱能分布拟合与发射线诊断，推导宿主星系的恒星形成率与金属丰度。
• 假设为标准ISM或风状环境，根据余晖衰减速率与谱指数估算周围介质密度。

实验结果

研究问题

• RQ1 GRB051221的红移与各向同性能量释放是多少？其与其它短伽马暴相比如何？
• RQ2 余晖光 light curve 中未观测到喷流截断，是否意味着喷流张角更大，且束流修正后的能量释放高于以往观测结果？
• RQ3 余晖光 light curve 中是否存在晚发能量注入的证据？这对其中心引擎活动有何启示？
• RQ4 宿主星系的恒星形成率与星族年龄是多少？与长伽马暴和短伽马暴宿主星系相比如何？
• RQ5 观测到的能量释放特性与低周围介质密度是否与致密天体并合起源一致，而非大质量恒星坍缩？

主要发现

• 通过宿主星系的光学光谱测得GRB051221的红移为z = 0.5459。
• 基于同步辐射模型，其各向同性等效爆发能量释放约为~2.4 × 10^51 erg，激波动能约为~1.4 × 10^51 erg。
• 在t ~ 13天内未观测到喷流截断，表明喷流张角大于13°，束流修正后的动能大于1 × 10^50 erg。
• 在t = 1.4至3.4小时之间发生了约3.4倍的能量注入，同时在射电波段检测到可辨识的反向冲击辐射。
• 宿主星系的恒星形成率为~1.5 M☉/yr，含有显著的约1 Gyr龄恒星群体，且金属丰度接近太阳值。
• 周围介质密度较低（n ~ 0.001 cm⁻³），且未检测到超新星辐射，支持其起源于致密天体并合，而非大质量恒星坍缩。