[论文解读] Outliers to the Isotropic Energy - Peak Energy Relation in GRBs

Ehud Nakar, Tsvi Piran|Dec 9, 2004

Gamma-ray bursts and supernovae被引用 85

一句话总结

本文提出一种统计检验方法，仅利用观测到的流强和峰值能量，识别无法满足各向同性能量–峰值能量（EpEi）关系的伽马射线暴（GRBs）。将该检验应用于BATSE数据，发现约25%的长暴为异常值，表明EpEi关系可能受观测选择效应影响，而非伽马射线暴的普遍性质，且Swift可能探测到更多此类能量高、亮度暗的异常值。

ABSTRACT

The peak energy - isotropic energy (EpEi) relation is among the most intriguing recent discoveries concerning GRBs. It can have numerous implications on our understanding of the emission mechanism of the bursts and on the application of GRBs for cosmological studies. However, this relation was verified only for a small sample of bursts with measured redshifts. We propose here a test whether a burst with an unknown redshift can potentially satisfy the EpEi relation. Applying this test to a large sample of BATSE bursts we find that a significant fraction of those bursts cannot satisfy this relation. Our test is sensitive only to dim and hard bursts and therefore this relation might still hold as an inequality (i.e. there are no intrinsically bright and soft bursts). We conclude that the observed relation seen in the sample of bursts with a known redshift might be influenced by observational biases and from the inability to locate and well localize hard and weak bursts that have only a small number of photons. In particular we point out that the threshold for detection, localization and redshift measurement is essentially higher than the threshold for detection alone. We predict that Swift will detect some hard and weak bursts that would be outliers to the EpEi relation. However, we cannot quantify this prediction. We stress the importance of understanding the detection-localization-redshift threshold for the coming Swift detections.

研究动机与目标

• 测试未知红移的暴是否可能满足EpEi关系。
• 评估红移测量样本中观测到的EpEi关系是否能代表完整的GRB群体。
• 研究观测选择效应（尤其是探测、定位和红移测量阈值）是否导致观测到的EpEi关系产生偏差。
• 预测Swift将探测到更多违反EpEi关系的硬、暗暴。

提出的方法

• 该方法利用观测到的总能量流强（F）和观测到的峰值能量（Ep,obs），针对给定红移z，计算内在Ep的下限和各向同性能量（Eiso）的上限。
• 为每个暴定义无量纲距离 d2 = (Eiso / (A_k * Ep^k))，其中 A_k 和 k 由EpEi关系推导得出。
• 若d2 ≤ 1，则暴满足EpEi关系；若d2 > 1，则为异常值。
• 该检验应用于BATSE长暴（T90 > 2 s），且在四个能量通道中具有完整流强数据的暴。
• 该方法从BATSE四通道数据估算Ep,obs，当光谱未完全解析时，设定Ep,obs的下限。
• 分析比较了多个样本中的d2值：63个具有已知光谱的暴、BATSE完整星表，以及短暴（T90 < 2 s）

实验结果

研究问题

• RQ1 是否可仅利用观测流强和峰值能量，测试未知红移的暴是否与EpEi关系一致？
• RQ2 为何在红移测量样本中仅有两个异常值，而约25%的BATSE暴不满足EpEi检验？
• RQ3 探测、定位和红移测量阈值在多大程度上导致观测到的EpEi关系产生偏差？
• RQ4 Swift是否会探测到更多违反EpEi关系的硬、暗暴，且如何量化这一现象？
• RQ5 EpEi关系是GRB的普遍性质，还是由于选择效应导致的不等式（Eiso ≤ A_k * Ep^k）？”

主要发现

• 样本中约25%的长BATSE暴不满足EpEi检验，表明它们是该关系的异常值。
• 该检验表明EpEi关系并非GRB的普遍属性，因为相当大比例的暴无法满足该关系。
• 两个已知异常值GRB 980425和GRB 031203均为低亮度、高能暴，且均以d2 > 1不满足检验。
• 短BATSE暴中，超过75%（T90 < 2 s）的d2 > 10，表明它们无法满足EpEi关系。
• 结果表明EpEi关系应被替换为不等式：Eiso ≤ A_k * Ep^k。
• 该研究得出结论：观测选择效应——尤其是红移测量阈值高于探测阈值——很可能是红移测量样本中异常值稀少的原因。