• gamma-ray burst: general;
  • cosmological parameters;
  • distance scale


Because of their enormous energy release, which allows us to detect them up to a very high redshift, gamma-ray bursts (GRBs) have recently attracted a lot of interest with regards to probing the Hubble diagram (HD) deep into the matter-dominated era; thus, GRBs complement Type Ia supernovae (SNe Ia). However, with the lack of a local GRB sample, it is not easy to calibrate the scaling relations proposed as an equivalent to the Phillips law to standardize GRBs, because of the need to estimate the GRB luminosity distance in a model-independent way. We consider here three different calibration methods, based on the use of a fiducial ΛCDM model, on cosmographic parameters and on the local regression on SNe Ia. 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. We find no statistically motivated improvement in the likelihood, so 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 GRB 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.