Analysis of the observed and intrinsic durations of gamma-ray bursts with known redshift

Abstract

The duration distribution of 408 GRBs with measured both duration $T_{90}$ and redshift $z$ is examined. Mixtures of a number of distributions (standard normal, skew-normal, sinh-arcsinh, and alpha-skew-normal) are fitted to the observed and intrinsic durations using the maximum log-likelihood method. The best fit is chosen via the Akaike information critetion. The aim of this work is to assess the presence of the presumed intermediate GRB class, and to provide a phenomenological model more appropriate than the common mixture of standard Gaussians. While $\log T_{90}^{obs}$ are well described by a truly trimodal fit, after moving to the rest frame the statistically most significant fit is unimodal. To trace the source of this discrepancy, 334 GRBs observed only by $Swift$/BAT are examined in the same way. In the observer frame, this results in a number of statistically plausible descriptions, being uni- and bimodal, and with the number of components ranging from one to three. After moving to the rest frame, no unambiguous conclusions may be put forward. It is concluded that the size of the sample is not big enough to infer reliably GRB properties based on a univariate statistical reasoning only.

12 pages, 10 figures; accepted in Astrophysics and Space Science

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• Multivariate analysis of BATSE gamma-ray burst properties using skewed distributions
• Two Dimensional Clustering of Gamma-Ray Bursts using durations and hardness
• Graph-based clustering of gamma-ray bursts
• Does the Redshift Distribution of Swift Long GRBs Trace the Star-Formation Rate?
• Can the Cosmological Dilation Explain the Skewness in the Gamma-Ray Burst Duration Distribution?
• Distribution of gamma-ray bursts on the t90-hardness ratio plane and their classification revisited
• How does the shape of gamma-ray bursts' pulses affect the duration distribution?
• Gamma-ray burst taxonomy: looking for the third class on the spectral peak energy-duration plane in the rest frame
• Do the Amati and Yonetoku Relations Evolve with Redshift for Swift GRBs?