Research Note

Weak gravitational lensing effects on cosmological parameters and dark energy from gamma-ray bursts

¹ Department of Astronomy, Nanjing University, 210093 Nanjing, PR China
e-mail: fayinwang@nju.edu.cn
² Key laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, 210093 Nanjing, PR China
e-mail: dzg@nju.edu.cn

Received: 19 June 2011
Accepted: 29 October 2011

Abstract

Context. Gamma-ray bursts (GRBs) are attractive objects for constraining the nature of dark energy in a way complementary to other cosmological probes, especially at high redshifts. However, the apparent magnitude of distant GRBs can be distorted by the gravitational lensing from the density fluctuations along the line of sight.

Aims. We investigate the gravitational lensing effect on the cosmological parameters and dark energy equation of state from GRBs.

Methods. We first calibrated the GRB luminosity relations without assuming any cosmological models. The luminosity distances of low-redshift GRBs were calibrated with the cosmography method using a latest type Ia supernova (SNe Ia) sample. The luminosity distances of high-redshift GRBs were derived by assuming that the luminosity relations do not evolve with redshift. Then we investigated the non-Gaussian nature of the magnification probability distribution functions and the magnification bias of the gravitational lensing.

Results. We find that the gravitational lensing has non-negligible effects on the determination of cosmological parameters and dark energy. The gravitational lensing shifts the best-fit constraints on cosmological parameters and dark energy. Because high-redshift GRBs are more likely to be reduced, the most probable value of the observed matter density Ω_M is slightly lower than its actual value. In the ΛCDM model, we find that the matter density parameter Ω_M will shift from 0.30 to 0.26 after including the gravitational lensing effect. The gravitational lensing also affects the dark energy equation of state by shifting it to a more negative value. We constrain the dark energy equation of state out to redshift z ~ 8 using GRBs for the first time, and find that the equation of state deviates from ΛCDM at the 1σ confidence level, but agrees with w =  −1 at the 2σ confidence level.