Search for gamma-ray burst classes with the RHESSI satellite

¹ Charles University, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 180 00 Prague 8, Czech Republic e-mail: ripa@sirrah.troja.mff.cuni.cz; meszaros@cesnet.cz; David.HUJA@seznam.cz
² Paul Scherrer Institute, 5232 Villigen, Switzerland e-mail: wojtek.hajdas@psi.ch; claudia.wigger@kanti-wohlen.ch
³ Kantonsschule Wohlen, 5610 Wohlen, Switzerland
⁴ Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondřejov, Czech Republic e-mail: rhudec@asu.cas.cz

Received: 4 September 2008
Accepted: 2 February 2009

Abstract

Aims. We study a sample of 427 gamma-ray bursts (GRBs), measured by the RHESSI satellite, statistically with respect to duration and hardness ratio.

Methods. Standard statistical tests were used, such as , F-test, and the maximum likelihood ratio test, to compare the number of GRB groups in the RHESSI database with that of the BATSE database.

Results. Previous studies based on the BATSE Catalog claim the existence of an intermediate GRB group, besides the long and short groups. Using only the GRB duration T₉₀ as information and  or F-test, we do not find any statistically significant intermediate group in the RHESSI data. However, maximum likelihood ratio test reveals a significant intermediate group. Using the 2-dimensional hardness/T₉₀ plane, the maximum likelihood analysis also reveals a significant intermediate group. In contrast to the BATSE database, the intermediate group in the RHESSI data set is harder than the long one.

Conclusions. The existence of an intermediate group follows not only from the BATSE data set, but also from the RHESSI one.