Systematics in the Gamma Ray Bursts Hubble diagram

TL;DR

This paper evaluates calibration methods for Gamma Ray Bursts as standard candles to extend the Hubble diagram, finding consistent calibration coefficients across methods and no significant evolution with redshift, supporting their use in cosmology.

Contribution

It compares three calibration techniques for GRBs, assesses their impact on the Hubble diagram, and investigates potential systematic effects, enhancing the reliability of GRBs in cosmological measurements.

Findings

Calibration coefficients are consistent across methods.

No significant evolution of calibration parameters with redshift.

Systematic effects are likely negligible with current data.

Abstract

Thanks to their enormous energy release which allows to detect them up to very high redshift, Gamma Rays Bursts (GRBs) have recently attracted a lot of interest to probe the Hubble diagram (HD) deep into the matter dominated era and hence complement Type Ia Supernoave (SNeIa). However, lacking a local GRBs sample, calibrating the scaling relations proposed as an equivalent to the Phillips law to standardize GRBs is not an easy task because of the need to estimate the GRBs luminosity distance in a model independent way. We consider here three different calibration methods based on the use of a fiducial $\Lambda$CDM model, on cosmographic parameters and on the local regression on SNeIa. We find that the calibration coefficients and the intrinsic scatter do not significantly depend on the adopted calibration procedure. We then investigate the evolution of these parameters with the redshift finding no statistically motivated improvement in the likelihood so that the no evolution assumption is actually a well founded working hypothesis. Under this assumption, we then consider possible systematics effects on the HDs introduced by the calibration method, the averaging procedure and the homogeneity of the sample arguing against any significant bias. We nevertheless stress that a larger GRBs sample with smaller uncertainties is needed to definitely conclude that the different systematics considered here have indeed a negligible impact on the HDs thus strengthening the use of GRBs as cosmological tools.