We report evidence that the asymptotic low-energy power law slope alpha (below the spectral break) of BATSE gamma-ray burst photon spectra evolves with time rather than remaining constant. We find a high degree of positive correlation exists between the time-resolved spectral break energy E_pk and alpha. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of alpha was found to correlate with that of the spectral break energy E_pk at the 99.7% and 98% confidence levels respectively. We also find that in the flux rise phase of "hard-to-soft" pulses, the mean value of alpha is often positive and in some bursts the maximum value of alpha is consistent with a value > +1. BATSE burst 3B 910927, for example, has a alpha_max equal to 1.6 +/- 0.3. These findings challenge GRB spectral models in which alpha must be negative of remain constant.Comment: 12 pages (including 6 figures), accepted to Ap

We propose a physical model of gamma-ray burst spectral evolution in which impulsively accelerated nonthermal leptons cool by saturated Compton upscattering of soft photons. This model naturally explains the recently discovered exponential decay of the spectral break energy with photon fluence, the hard-to-soft spectral evolution patterns, and other spectral and temporal properties of gamma-ray bursts.

We have analyzed the evolution of the spectral hardness parameter, E pk (the maximum of the νF ν spectrum) as a function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with the trend reported by Liang & Kargatis (1996) which found that E pk decays exponentially with respect to photon fluence Φ(t). We also fit these pulses with a slight modification of this trend, where E pk decays linearly with energy fluence. In both cases, we found the set of 41 pulses to be consistent with the trend. For the latter trend, which we believe to be more physical, the distribution of the decay constant Φ 0 is roughly log-normal, where the mean of log 10 Φ 0 is 1.75 ± 0.07 and the FWHM of log 10 Φ 0 is 1.0 ± 0.1. Regarding an earlier reported invariance in Φ 0 among different pulses in a single burst, we found probabilities of 0.49 to 0.84 (depending on the test used) that such invariance would occur by coincidence, most likely due to the narrow distribution of Φ 0 values among pulses.

We find that the optical magnitudes of GRB990123 observed by ROTSE correlates with the magnitudes extrapolated from the simultaneous BATSE spectra, strongly suggesting that the optical, X-ray and gamma-ray photons originate from a single source. We then show that the broadband optical-gamma-ray spectra can be naturally fit by the saturated Compton model. We also derive the parameters of the Compton emitting shell from first principles.

High ionization star forming (SF) galaxies are easily identified with strong emission line techniques such as the BPT diagram, and form an obvious ionization sequence on such diagrams. We use a locally optimally emitting cloud model to fit emission line ratios that constrain the excitation mechanism, spectral energy distribution, abundances and physical conditions along the star-formation ionization sequence. Our analysis takes advantage of the identification of a sample of pure star-forming galaxies, to define the ionization sequence, via mean field independent component analysis. Previous work has suggested that the major parameter controlling the ionization level in SF galaxies is the metallicity. Here we show that the observed SFsequence could alternatively be interpreted primarily as a sequence in the distribution of the ionizing flux incident on gas spread throughout a galaxy. Metallicity variations remain necessary to model the SF-sequence, however, our best models indicate that galaxies with the highest and lowest observed ionization levels (outside the range -0.37 < log [O III]/Hβ < -0.09) require the variation of an additional physical parameter other than metallicity, which we determine to be the distribution of ionizing flux in the galaxy.