Connecting Prompt and Afterglow GRB emission I. Investigating the impact of optical selection effects in the Epi - Eiso plane

TL;DR

This study investigates how optical selection effects influence the observed distribution of GRB properties and the Epi-Eiso relation, highlighting biases introduced by the reliance on optical afterglow brightness for redshift measurement.

Contribution

It demonstrates that optical selection effects bias the observed GRB property distributions and the Epi-Eiso relation, emphasizing the need to account for these biases in statistical analyses.

Findings

Faint afterglows are mainly located in the upper part of the Epi-Eiso plane.

Optical brightness depends mainly on intrinsic luminosity, with slight redshift dependence.

Selection effects significantly impact the interpretation of GRB rest-frame property distributions.

Abstract

Measuring GRB properties in their rest-frame is crucial to understand the physics at work in gamma-ray bursts. This can only be done for GRBs with known redshift. Since redshifts are usually measured from the optical spectrum of the afterglow, correlations between prompt and afterglow emissions may introduce biases in the distribution of rest-frame properties of the prompt emission, especially considering that we measure the redshift of only one third of Swift GRBs. In this paper we study the brightness of optical GRB afterglows and the role of optical selection effects in the distribution of various intrinsic properties of GRBs and on the Epi - Eiso relation discovered by Amati et al. (2002). Our analysis is based on a sample of 85 GRBs with good optical follow-up and well measured prompt emission. 71 of them have a measure of redshift and 14 have no redshift. We discuss the connection between the location of GRBs in the Epi-Eiso plane and their optical brightness measured two hours after the trigger in the GRB rest frame. We show that the brightness of GRBs in our sample is mainly driven by their intrinsic luminosity and depends only slightly on their redshift. We also show that GRBs with faint afterglows are preferentially located in the upper part of the Epi-Eiso plane. This optical selection effect favors the detection of GRBs with bright afterglows located below the best fit Epi - Eiso relation whose redshift is easily measurable. We conclude that the distributions of prompt GRB properties in the rest frame undergo selection effects due to the need to measure the redshift from the optical afterglow emission. These biases put significant uncertanties when interpreting the statistical studies of GRB properties in the rest frame. We show that the Epi - Eiso relation is not immune to these selection effects.