1 Introduction

It is suspected that spinning black holes reside in a variety of astrophysical sources. Frame dragging creates a special region called the ergosphere in which any material or energy must rotate in the same direction as the black hole. The energetic reactions near the black hole maybe responsible for relativistic jets in active galactic nuclei (AGN), microquasars and gamma ray bursts (Koide et al. 2002; Frail et al. 2001). In GRBs the source of the enormous energy in gamma rays maybe the cataclysmic formation of a spinning black hole involving mergers of compact objects such as neutron star (NS) binaries or NS and black holes (Piran 1999; Ruffert & Janka 1999) and also during or after the collapse of massive stars (MacFadyen & Woosley 1999; Paczynski 1998; Vietri & Stella 1998; Reeves et al. 2002). The central engine is hidden from view and only gravitational radiation and neutrinos may escape and reach the observer directly from the engine. A key feature of the internal shock model is that the observed gamma rays reflect the variability of the central engine and the GRB duration may be determined by the engine (Rees & Mészáros 1994; Piran 1999). The cumulative output in gamma rays of a burst indirectly reflects the output of the central engine via a relativistic jet. The advantage of using the cumulative light curve is that it reveals the trends by smoothing the spiky nature of the running light curve. The cumulative light curves of most bursts can be approximated by a linear function of time and GRBs may be regarded as relaxation systems that continuously accumulate energy in the reservoir and discontinuously release it (McBreen et al. 2002a). In a relatively small number of GRBs, the cumulative light curves depart from linearity in a consistent way. The selection and properties of these GRBs are presented here in Sects. 2 and 3, discussed in Sect. 4 and summarised in Sect. 5.