A new definition of the intermediate group of gamma-ray bursts

¹ Department of Physics, Bolyai Military University, Budapest, Box-12, 1456, Hungary e-mail: horvath.istvan@zmne.hu
² Konkoly Observatory, Budapest, Box-67, 1525, Hungary e-mail: balazs@konkoly.hu
³ Laboratory for Information Technology, Eötvös University, Budapest, Pázmány P. s. 1/A, 1117, Hungary e-mail: bagoly@ludens.elte.hu
⁴ Stockholm Observatory, AlbaNova, 106 91 Stockholm, Sweden e-mail: felix@astro.su.se
⁵ Astronomical Institute of the Charles University, V Holešovičkách 2, 180 00 Prague 8, Czech Republic e-mail: meszaros@mbox.cesnet.cz
⁶ Max Planck Institute for Astrophysics, Garching, Karl-Schwarzschild-Str. 1, Postfach 1317, 85741 Garching, Germany

Received: 21 April 2004
Accepted: 25 September 2005

Abstract

Gamma-ray bursts can be divided into three groups (“short”, “intermediate”, “long”) with respect to their durations. This classification is somewhat imprecise, since the subgroup of intermediate duration has an admixture of both short and long bursts. In this paper a physically more reasonable definition of the intermediate group is presented, using also the hardnesses of the bursts. It is shown again that the existence of the three groups is real, no further groups are needed. The intermediate group is the softest one. From this new definition it follows that  of all bursts belong to this group. An anticorrelation between the hardness and the duration is found for this subclass in contrast to the short and long groups. Despite this difference it is not clear yet whether this group represents a physically different phenomenon.