How Sample Completeness Affects Gamma-Ray Burst Classification.pdf

a r X i v : a s t r o - p h / 0 2 0 9 0 7 3 v 1 4 S e p 2 0 0 2 How Sample Completeness Affects Gamma-Ray Burst Classification Jon Hakkila and Timothy W. Giblin Department of Physics and Astronomy, College of Charleston, Charleston, SC, 29424 Richard J. Roiger and David J. Haglin Department of Computer and Information Sciences, Minnesota State University, Mankato, MN 56001 William S. Paciesas Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899 and Charles A. Meegan NASA, Marshall Space Flight Center, Huntsville, AL 35899 ABSTRACT Unsupervised pattern recognition algorithms support the existence of three gamma-ray burst classes; Class I (long, large fluence bursts of intermediate spec- tral hardness), Class II (short, small fluence, hard bursts), and Class III (soft bursts of intermediate durations and fluences). The algorithms surprisingly as- sign larger membership to Class III than to either of the other two classes. A known systematic bias has been previously used to explain the existence of Class III in terms of Class I; this bias allows the fluences and durations of some bursts to be underestimated (Hakkila et al., ApJ 538, 165, 2000). We show that this bias primarily affects only the longest bursts and cannot explain the bulk of the Class III properties. We resolve the question of Class III existence by demon- strating how samples obtained using standard trigger mechanisms fail to preserve the duration characteristics of small peak flux bursts. Sample incompleteness is thus primarily responsible for the existence of Class III. In order to avoid this incompleteness, we show how a new dual timescale peak flux can be defined in terms of peak flux and fluence. The dual timescale peak flux preserves the du- ration distribution of faint bursts and correlates better with spectral hardness – 2 – (and presumably redshift) than either peak flux or fluence. The techniques pre- sented here are generic and have applicability to the studies