“裸眼”GRB080319B的即时高分辨率光谱视图
The prompt, high resolution spectroscopic view of the "naked-eye" GRB080319B
摘要：
GRB 080319 B在爆发瞬发期间达到第5光学星等。由于VLT/UVES快速响应模式，我们在γ射线暴爆发后仅8 m：30 s就观测到了它的余辉，其震级为R ~ 12。这使我们能够获得最好的信噪比，高分辨率的伽玛射线暴余辉光谱（S/N每分辨率元素~ 50）。该光谱具有丰富的吸收特征，属于z=0.937的主系统，至少分为六个分量，总速度范围为100 km/s。VLT/UVES观测捕获了处于高度激发态的吸收气体，产生了在GRB中观测到的最强的Fe II精细结构线。几个小时后，这些谱线的光学深度减少了4-20倍，光学/紫外线通量减少了约60倍。这证明所观察到的精细结构线的激发是由于伽玛射线暴紫外光子的“泵浦”。通过比较观测到的激发态吸收光子数与Fe II基态吸收光子数的比值，发现6个吸收体距离GRB位置约2-6 kpc，其中组分I距离GRB位置的距离是组分III至VI的3倍。与所有其它组分不同，组分I的特征还在于缺乏Mg I吸收。这可能是由于更近的距离和更低的密度，这表明在这个星系复合体中存在结构化的ISM。
英文摘要：
GRB080319B reached 5th optical magnitude during the burst prompt emission. Thanks to the VLT/UVES rapid response mode, we observed its afterglow just 8m:30s after the GRB onset when the magnitude was R ~ 12. This allowed us to obtain the best signal-to-noise, high resolution spectrum of a GRB afterglow ever (S/N per resolution element ~ 50). The spectrum is rich of absorption features belonging to the main system at z=0.937, divided in at least six components spanning a total velocity range of 100 km/s. The VLT/UVES observations caught the absorbing gas in a highly excited state, producing the strongest Fe II fine structure lines ever observed in a GRB. A few hours later the optical depth of these lines was reduced by a factor of 4-20, and the optical/UV flux by a factor of ~ 60. This proves that the excitation of the observed fine structure lines is due to "pumping" by the GRB UV photons. A comparison of the observed ratio between the number of photons absorbed by the excited state and those in the Fe II ground state suggests that the six absorbers are ~ 2-6 kpc from the GRB site, with component I ~ 3 times closer to the GRB site than components III to VI. Component I is characterized also by the lack of Mg I absorption, unlike all other components. This may be due both to a closer distance and a lower density, suggesting a structured ISM in this galaxy complex.
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