[论文解读] Keck and ESO-VLT View of the Symmetry of the Ejecta of the XRF/SN 2006aj

P. A. Mazzali, R. J. Foley+12 more|Mar 6, 2007

Gamma-ray bursts and supernovae被引用 33

一句话总结

本研究利用凯克望远镜和VLT对XRF/SN 2006aj在爆发后晚期阶段的光谱进行分析，揭示了具有低非球对称性的对称抛射物，尽管中心存在致密核心。数据证实其⁵⁶Ni质量极低（约0.20 M☉），在8000 km s⁻¹以下的总抛射质量约为2 M☉，表明其前身星初始质量约为20 M☉ ZAMS质量，且残余物为中子星，支持XRF是球对称、较弱的伽马射线暴（GRB）类比物。

ABSTRACT

Nebular-phase spectra of SN 2006aj, which was discovered in coincidence with X-ray flash 060218, were obtained with Keck in 2006 July and the Very Large Telescope in 2006 September. At the latter epoch spectropolarimetry was also attempted, yielding an upper limit of ~ 2% for the polarization. The spectra show strong emission lines of [OI] and MgI], as expected from a Type Ic supernova, but weak CaII lines. The [FeII] lines that were strong in the spectra of SN 1998bw are much weaker in SN 2006aj, consistent with the lower luminosity of this SN. The outer velocity of the line-emitting ejecta is ~ 8000 km/s in July and ~ 7400 km/s in September, consistent with the relatively low kinetic energy of expansion of SN 2006aj. All emission lines have similar width, and the profiles are symmetric, indicating that no major asymmetries are present in the ejecta at the velocities sampled by the nebular lines (v < 8000 km/s), except perhaps in the innermost part. The spectra were modelled with a non-LTE code. The mass of 56Ni required to power the emission spectrum is ~ 0.20 Msun, in excellent agreement with the results of early light curve modelling. The oxygen mass is ~ 1.5 Msun, again much less than in SN 1998bw but larger by ~ 0.7 Msun than the value derived from the early-time modelling. The total ejected mass is ~ 2 Msun below 8000 km/s. This confirms that SN 2006aj was only slightly more massive and energetic than the prototypical Type Ic SN 1994I, but also indicates the presence of a dense inner core, containing ~ 1 Msun of mostly oxygen and carbon. The presence of such a core is inferred for all broad-lined SNe Ic. This core may have the form of an equatorial oxygen-dominated region, but it is too deep to affect the early light curve and too small to affect the late polarization spectrum.

研究动机与目标

• 利用晚期光谱研究XRF/SN 2006aj抛射物的非球对称性。
• 确定中性氧、钙和⁵⁶Ni在电离态抛射物中的质量分布。
• 评估爆炸是否具有非球对称性，特别是最内层区域。
• 将早期光变曲线模型与晚期电离态光谱进行对比。
• 评估其对XRF相关超新星前身星质量与残余物类型的影响。

提出的方法

• 利用凯克望远镜（2006年7月）和VLT（2006年9月）获取SN 2006aj的电离态光谱。
• 在VLT进行光谱偏振测量以探测非球对称性，获得约2%的偏振上限。
• 使用非-LTE辐射转移代码对电离态光谱进行建模，以推导元素质量与抛射物结构。
• 将推导出的⁵⁶Ni、氧、钙质量与早期光变曲线建模结果进行比较。
• 分析谱线轮廓对称性与速度结构，以推断抛射物几何形态与密度梯度。
• 从[O i]谱线在v ≤ 2000 km s⁻¹处的流量增强，推断存在低速致密核心。

实验结果

研究问题

• RQ1 SN 2006aj的抛射物在多大程度上是非球对称的，特别是在最内层区域？
• RQ2 驱动电离态辐射所需的⁵⁶Ni质量是多少？与早期阶段的估计相比如何？
• RQ3 抛射物中氧与钙的总质量是多少？与其它Ic-BL型超新星相比如何？
• RQ4 致密内核的存在是否影响早期光变曲线或偏振？如果是，如何影响？
• RQ5 电离态光谱与偏振结果对前身星质量与残余物类型有何启示？

主要发现

• 电离态光谱显示对称的谱线轮廓，在8000 km s⁻¹以下无显著非球对称性，与低非球对称性一致。
• 驱动电离态辐射所需的⁵⁶Ni质量约为0.20 M☉，与早期光变曲线建模结果高度一致。
• 氧质量约为1.5 M☉，显著低于SN 1998bw，但比早期建模估计值高出约0.7 M☉。
• 在8000 km s⁻¹以下的总抛射质量约为2 M☉，表明爆炸能量相对较低。
• 推断存在约1 M☉的氧与碳致密内核，可能形成低速区域的盘状结构。
• 未检测到显著偏振（≤2%），表明⁵⁶Ni分布与内核并非高度非球对称，支持爆炸为球对称或近似球对称。