BATSE OBSERVATIONS OF GAMMA-RAY BURST SPECTRA .2. PEAK ENERGY EVOLUTION IN BRIGHT, LONG BURSTS

We investigate spectral evolution in 37 bright, long gamma-ray bursts observed with the BATSE spectroscopy detectors. High-resolution spectr a are characterized by the energy of the peak of nu F(n)u, and the evo lution of this quantity is examined relative to the emission intensity . In most cases it is found that this peak energy either rises with or slightly precedes major intensity increases and softens for the remai nder of the pulse. Interpulse emission is generally harder early in th e burst. For bursts with multiple intensity pulses, later spikes tend to be softer than earlier ones, indicating that the energy of the peak of nu F(n)u is bounded by an envelope which decays with time. Evidenc e is found that bursts in which the bulk of the flux comes well after the event which triggers the instrument tend to show less peak energy variability and are not as hard as several bursts in which the emissio n occurs promptly after the trigger. Several recently proposed burst m odels are examined in light of these results and no qualitative confli cts with the observations presented here are found.