超长暴和超软暴

Ultra-long gamma-ray bursts and ultra-soft gamma-ray bursts

超长暴是一种特殊的伽马射线暴(伽马暴)类型,其释放的总能量与普通伽马暴相当,但持续时间可高达小时量级.由于超长暴辐射流量偏低,目前所观测的超长暴样本数量较少.超长暴到底是普通长暴的延伸,抑或是一种新类型的伽马暴,目前的观测还未能给出一个确定的答案.超软暴在本文定义为能谱偏软的伽马暴,主要包括X射线闪和富X射线伽马暴等类型.本文回顾了超长暴和超软暴的观测特征和理论研究,讨论其可能的爆发机制和前身星模型.引力波暴高能电磁对应体全天监测器(GECAM)具有全天视场、高灵敏度、良好的定位精度和宽能段覆盖(6 keV–5 MeV)的综合优势,且低能区探测能力比现有探测设备明显占优,有利于超长暴和超软暴的观测.我们根据GECAM的响应矩阵和本底模拟数据,对典型的超长暴(GRB 130925A)和超软暴(XRR 050525A)进行能谱模拟,并与Fermi/GBM和Swift/BAT的能谱观测进行比较,发现GECAM对于低能段能谱的测量能力要优于Fermi/GBM和Swift/BAT.另外,我们还计算了GECAM对超长暴和超软暴的探测率,发现GECAM预期每年可以探测到~20个超长暴,~50个X射线闪以及~300个富X射线伽马暴.

Ultra-Long Gamma-Ray Bursts(ULGRBs) are special Gamma-Ray Bursts(GRBs) lasting for several hours. However,their released energy is comparable with that of typical long GRBs. The observed flux of ULGRBs is typically low,making them hard to detect, and therefore, only a small sample of ULGRBs have been collected. Observationally, it remains a question of whether ULGRBs are a new population or just a tail distribution of long GRBs. Ultra-Soft GammaRay Bursts(USGRBs) refer to another type of special GRBs with soft energy spectra. USGRBs include X-ray Flashes(XRFs) and X-ray Rich GRBs(XRRs). In this paper, we first review the observations and theoretical studies of ULGRBs and USGRBs and discuss their possible explosion mechanisms and progenitor models. We then consider the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor(GECAM), which has some advantages in observing ULGRBs and USGRBs. The GECAM has a full-sky FOV, high sensitivity, good positioning accuracy,extensive energy range coverage(6 ke V–5 Me V), and lower energy threshold, making it ideal to observe ULGRBs and USGRBs. Based on the provided GECAM response matrix and background simulation data, we simulate the energy spectrum of typical ULGRB(GRB 130925 A) and USGRB(XRR 050525 A). We then compare those spectra with those observed by Fermi/GBM and Swift/BAT. We found that the GECAM can provide a more accurate energy spectrum.Furthermore, we also discuss the detection rate of ULGRBs and USGRBs observed by GECAM. We found that GECAM should detect ~20 ULGRBs, ~50 XRFs, and ~300 XRRs per year.