Spectral Hardness and Evolution of Swift Gamma-Ray Bursts and X-Ray Afterglows

Spectral hardness of gamma-ray bursts can be characterized by either hardness ratio or peak energy, and the inconsistency of these parameters can reflect spectral diversity and complexity. We have chosen 310 Swift bursts with well-measured X-ray afterglows to analyze the distributions of their peak energies and hardness ratios before and after absorption of neutral hydrogen. We categorize them into four types according to X-ray temporal patterns and calculate the hardness ratios of both gamma-rays and X-rays. It is found that the hardness ratios of X-rays are systematically smaller than those of gamma-rays. We also find that both gamma-ray hardness ratios and peak energies of bursts of types I, II, and III are identically distributed and are significantly lower than the corresponding measurements of type IV bursts in statistics, which hints that the evolving spectra of type IV bursts are most probably affected by the geometry of outflows. This proves that the influence of neutral hydrogen absorption on the estimation of hardness ratio is negligible even though it can reduce the brightness of gamma/X-rays somewhat. Furthermore, we verify that almost all the observed spectra of gamma-ray bursts can be described by a broken power-law function and confirm a magnetar origin for the bursts with internal X-ray plateaus.