The properties of prompt emission in short gamma-ray bursts with extended emission observed by Fermi/GBM

Short gamma-ray bursts (GRB) with extended emission (EL) that are composed of an initial short hard spike followed by a long-lasting LE are thought to comprise a sucategory of short GM, The narrow energy band available during the Swift era, combined with a lack of spectral information, prevented the discovery of the intrinsic properties of these events. In this paper, we perform a systematic search of short GRBs with LE using all available Fermi/GBM data. The search identified 26 GBM-detected short GRBs with LE that are similar to GRB 060614 observed by Swift/BAT We focus on investigating the spectral and temporal properties of both the hard spike and the EL component of all 26 GRBs, and explore differences and possible correlations between them, We find that while the peak energy (E-p) of the hard spikes is slightly harder than that of the EE, their fluences are comparable. The harder E-p seems to correspond to a larger fluence and peak flux, with a large scatter for both the hard spike and the EE component. Moreover, the E-p of both the hard spike and the EE are compared with other short GRBs. Finally, we also compare the properties of GRB 170817A with those of short GRBs with EE and find no significant statistical differences between them. We find that GRB 170817A has the lowest E-p, probably because it is off-axis.