标题： 2006年7月PKS 2155-304的第二次异常伽马射线耀发的同时多波段观测 显示英文标题

标题： Simultaneous multiwavelength observations of the second exceptional gamma-ray flare of PKS 2155-304 in July 2006

摘要： 2006年7月29日至30日夜间，对BL Lac天体PKS 2155-304进行了同时的HESS/CHANDRA/光学观测，当时该源经历了2006年夏季的第二次主要伽马射线爆发。 这一事件发生在7月28日爆发（以其超快变异性著称）后的约44小时。 实现了前所未有的6到8小时不间断覆盖，分别测量了光谱和光变曲线，时间尺度达到7分钟和2分钟。 伽马射线通量达到了蟹状星云通量的11倍（>400 GeV），上升/下降时间尺度约为1小时，并且在衰减阶段叠加了一些幅度较小的耀发。 X射线和甚高能（VHE）波段的发射在流量和光谱上高度相关，未发现延迟现象。 VHE能谱显示出随时间变化的曲率，在更高流量时更强。 巨大的VHE变化（22倍）仅伴随小幅度的X射线和光学变化（分别为2倍和15%）。 该源首次在HBL中展示了大康普顿优势度（L_C/L_S ~10）——快速演化——以及VHE与X射线流量变化之间的三次方关系，在衰减阶段。 这些结果挑战了TeV耀变星的传统模型。 显示英文摘要

摘要： Simultaneous HESS/CHANDRA/optical observations were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent its second major gamma-ray outburst in Summer 2006. This event took place about 44 hours after the July 28 outburst, known for its ultrafast variability. An unprecedented 6 to 8 hours of uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2-minute timescales, respectively. The gamma-ray flux reached a maximum of 11x the Crab flux (>400 GeV), with rise/decay timescales of ~1 hour, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE bands is strongly correlated, both in flux and spectrum, with no evidence of lags. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (22x) are only accompanied by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in an HBL a large Compton dominance (L_C/L_S ~10) -- rapidly evolving -- and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge the common scenarios for the TeV-blazar emission.