The Epeak-Eiso plane of long Gamma Ray Bursts and selection effects

TL;DR

This study analyzes the distribution and correlations of long Gamma Ray Bursts in different planes, confirming a strong rest frame Ep-Eiso correlation without redshift evolution and examining instrumental selection effects, especially in Swift data.

Contribution

It provides an updated analysis of the Ep-Eiso and Ep,obs-Fluence correlations with a larger sample, clarifies the role of selection effects, and questions previous assumptions about their evolution.

Findings

Confirmed strong Ep-Eiso correlation with no redshift evolution.

Identified dominant instrumental selection effects in Swift data.

Highlighted potential biases in the observed Ep,obs-Fluence correlation.

Abstract

We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured redshift and spectral parameters, detected up to September 2007. We confirm the existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to previous studies, no sign of evolution with redshift of the Ep-Eiso correlation (either its slope and normalisation) is found. The 76 bursts define a strong Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with redshifts evenly distributed along this correlation. We study possible instrumental selection effects in the observer frame Ep,obs-Fluence plane. In particular, we concentrate on the minimum peak flux necessary to trigger a given GRB detector (trigger threshold) and the minimum fluence a burst must have to determine the value of Ep,obs (spectral analysis threshold). We find that the latter dominates in the Ep,obs-Fluence plane over the former. Our analysis shows, however, that these instrumental selection effects do not dominate for bursts detected before the launch of the Swift satellite, while the spectral analysis threshold is the dominant truncation effect of the Swift GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence correlation defined by the pre--Swift sample could be affected by other, still not understood, selection effects. Besides we caution about the conclusions on the existence of the Ep,obs-Fluence correlation based on our Swift sample alone.