Testing cosmic anisotropy with the Ep−Eiso (‘Amati’) correlation of GRBs

Abstract

ABSTRACT We test the possible cosmic anisotropy in 118 long GRBs with the Ep−Eiso (‘Amati’) correlation by employing the dipole fitting (DF) and hemisphere comparison (HC) methods. The distribution of the GRB sample is nearly homogeneous in the sky. The dipole anisotropy is weak in the dipole-modulated $\rm \Lambda$CDM model and the Finslerian cosmological model. The dipole directions from the GRB sample are consistent with ones given by the Pantheon SNe-Ia sample, but with smaller uncertainties. We also investigate whether the GRB sample can reduce the anisotropic signal from inhomogeneous samples like the Pantheon one. The GRB sample is then combined with the Pantheon one, thus providing an SN-G sample. In the dipole-modulated $\rm \Lambda$CDM model, the dipole direction in the SN-G sample shows a considerable change from the one in the Pantheon sample. The angle between the two directions is 26${_{.}^{\circ}}$78. For the HC method, the result of maximum anisotropy level from the G-SN sample is 0.257 ± 0.060 at 68 per cent confidence level (CL) and the corresponding preferred direction is $(l,b)=(82{_{.}^{\circ}}97^{+52{_{.}^{\circ}}73}_{~-61{_{.}^{\circ}}88}, -15{_{.}^{\circ}}09^{+60{_{.}^{\circ}}09}_{~-13{_{.}^{\circ}}54})$. The statistical significance of the $\rm AL_{max}$ is 1.4σ. The angle between the preferred direction and the one from the Pantheon sample is 44${_{.}^{\circ}}$40. Although the amount of data in the GRB sample is about a tenth of that in the Pantheon sample, the GRB sample can considerably impact the results from the Pantheon sample. Our results show that GRBs have the potential to search for a convincing cosmic anisotropy.