Are Swift gamma-ray bursts consistent with the Ghirlanda relation?

¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Lc), Italy e-mail: campana@merate.mi.astro.it
² Università degli studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy

Received: 20 December 2006
Accepted: 16 March 2007

Abstract

A few tight correlations linking several properties of gamma-ray bursts (GRBs), namely the spectral peak energy, the total radiated energy, and the afterglow break time, have been discovered with pre-Swift GRBs. They were used to constrain the cosmological parameters, together with type-Ia supernovae. However, the tightness of these correlations is a challenge to GRB models. We explore the effect of adding Swift bursts to the Ghirlanda and Liang-Zhang relations. Although they are both still valid, they become somewhat weakened mostly due to the presence of significant outliers, which otherwise are apparently normal GRBs so difficult to distinguish. The increased dispersion of the relations makes them less reliable for purposes of precision cosmology.