2 GRB profiles and durations

The wide diversity of GRB profiles has been known since their earliest observations. While several burst morphologies (e.g. smooth or spiky) are easy to identify, there are numerous gradations of these, as well as many complex forms. Figure 1 shows a group of four GRBs (>20 keV) with no apparent fine time structures, unlike many bursts observed with BATSE. Most bursts have many "spikes'' within the burst on timescales of $\sim$0.1 s and/or have numerous (sometimes dozens) of well-defined sub-pulses. The BATSE catalog (available at www.batse.msfc.nasa.gov) contains the largest dataset of GRB profiles to date. Examples of GRBs with well-defined, separated episodes of emission are also seen (Fig. 2). Attempts to quantify these structures have been largely unsuccessful. However, some of the GRB temporal features appear to be reproduced in recent models of internal shocks in GRBs (cf. Mochkovitch & Daigne 1998; Daigne & Mochkovitch 1998; Kobayashi et al. 1998; Beloborodov et al. 1998). Most models of the central engine of GRBs would not predict the large diversity and duration range of GRBs. An exception to this is the recent paper by Kluzniak & Ruderman (1998) which models the central engine as a recently-formed neutron star with a high magnetic field and extreme differential rotation. Those authors use the diversity of GRB profiles as a principal issue to be addressed by their model.

  

Figure 2: Two BATSE gamma-ray bursts that have distinct, highly separated episodes of emission. In trigger 219, the on-board system triggered on a weak event lasting only a few seconds. This initial pulse was determined to come from the same location as the main emission, which followed the trigger pulse by over 100 s

The range of the duration of gamma-ray bursts spans over five decades, from a few milliseconds to over a thousand seconds. The double peaked distribution of the duration, noted many years ago (cf. Kouveliotou et al. 1993a), is now much more evident with over two thousand observed GRBs. These two peaks in the duration distribution occur at $\sim$0.5 s and $\sim$34 s. A recent statistical analysis of this distribution (Mukherjee et al. 1998) shows an indication of a third, intermediate peak  (a separate class?)  in this distribution. The hardness-duration correlation, which had also been described previously, is now much more evident with the large number of bursts observed with BATSE, as shown in Fig. 3.

  

Figure 3: The hardness-duration distribution of BATSE GRBs, showing two distinct classes of GRBs; the shorter bursts in general tend to have a harder spectrum. The Hardness Ratio (HR) is the ratio of the two middle, broad BATSE energy bands (as indicated), integrated over the duration of the GRB. The T90 measure of the GRB duration is the standard measure used in the BATSE catalogs