Reclassification of gamma-ray bursts

Abstract

We have applied two different automatic classifier algorithms to the BATSE Current GRB Catalog data and we obtain three different classes of GRBs. Our results confirm the existence of a third, intermediate class of GRBs, with mean duration \sim 25-50 s, as deduced from a cluster analysis and from a neural network algorithm. Our analyses imply longer durations than those found by Mukherjee et al. (1998) and Horvath (1998), whose intermediate class had durations \sim 2-10 s. From the neural network analysis no difference in hardness between the two longest classes is found, and from both methods we find that the intermediate-duration class constitutes the most homogeneous sample of GRBs in its space distribution while the longest-duration class constitutes the most inhomogeneous one with <V/V_max> \sim 0.1, being thus the deepest population of GRBs with z_max \sim 10. The trend previously found in long bursts, of spatial inhomogeneity increasing with hardness, only holds for this new longest-duration class.Comment: 8 pages, 11 figures, to appear in MNRA

On the difference between the short and long gamma-ray bursts

“…However, unless the space-time geometry has a very particular structure, the distribution of log f (z) cannot be Gaussian. This means that the Gaussian nature of the distribution of log T 90 must be dominated by the distribution 1 There is also an evidence for the existence of a third intermediate subgroup as part of the long duration group (Horváth 1998;Mukherjee et al 1998;Hakkila et al 2000a,c;Balastegui et al 2001;Horváth 2002), which shows a distinct sky angular distribution (Mészáros et al 2000a,b;Litvin et al 2001). We do not deal with this third group here.…”

Short versus long gamma-ray bursts: a comprehensive study of energetics and prompt gamma-ray correlations

“…Early hints to the existence of distinct populations of gamma-ray transients (e.g., Mazets et al 1981;Norris et al 1984), and at least two classes of short-hard (Type-I) & long-soft (Type-II) GRBs (Dezalay et al 1992) have now been extensively corroborated and confirmed by the prompt-emission data E-mail: amir@physics.utexas.edu (AS); nemiroff@mtu.edu (RJN) from independent gamma-ray detector missions (e.g., Kouveliotou et al 1993a;Gehrels, Ramirez-Ruiz & Fox 2009; or follow-up observations of the afterglows or host galaxies (e.g., Zhang et al 2009;Berger 2011Berger , 2014. Although the possibility of more than two classes of GRBs with distinct progenitors has been extensively discussed and considered (e.g., Horvath 1998;Mukherjee et al 1998;Hakkila et al 2001;Balastegui, Ruiz-Lapuente & Canal 2001;Hakkila et al 2004b;Horvath et al 2006;Gehrels et al 2006;Chattopadhyay et al 2007;Horvath et al 2008;Virgili, Liang & Zhang 2009;Gao, Lu & Zhang 2010;Horvath et al 2012;Levan et al 2014;Kbori et al 2014), it has remained a matter of debate and speculation to this date (e.g., Hakkila et al 2000cHakkila et al ,a,b, 2003Hakkila et al , 2004aShahmoradi 2013b;Zhang et al 2014;Levan et al 2014).…”

A further study of the BATSE Gamma-Ray Burst duration distribution

“…Recently Balastegui et al (2001) have applied automatic classifier algorithms and obtain three different classes of GRBs. Further, the intermediate subgroup shows a remarkable angular distribution on the sky (Balázs et al 1998;Balázs et al 1999;Mészáros et al 2000;Litvin et al 2001 the existence of this subgroup is a matter of debate.…”