2.4. Spectral Evolution

Observations by earlier detectors as well as by BATSE have shown that the spectrum varies during the bursts. Different trends were found. Golenetskii et al. [97] examined two channel data from five bursts observed by the KONUS experiment on Venera 13 and 14 and found a correlation between the effective temperature and the luminosity, implying that the spectral hardness is related to the luminosity. Similar results were obtained Mitrofanov et. al. [98]. Norris et al. [99] investigated ten bursts seen by instruments on the SMM (Solar Maximum Mission) satellite. They found that individual intensity pulses evolve from hard-to-soft with the hardness peaking earlier than the intensity. This was supported by more recent BATSE data [100]. Ford et al. [101] analyzed 37 bright BATSE bursts and found that the spectral evolution is a mixture of those found by Golenetskii et al. [97] and by Norris et al. [99]: The peak energy either rises with or slightly proceeds major intensity increases and softens for the remainder of the pulse. For bursts with multiple peak emission, later spikes tend to be softer than earlier ones.

A related but not similar trend is shown by the observations that the bursts are narrower at higher energies with T() ^-0.4 [102]. As we show in section 8.7.3 this behavior is consistent with synchrotron emission [103].