New measurements of Ω_m from gamma-ray bursts

¹ Dip. di Fisica, Sapienza Universitá di Roma, Piazzale Aldo Moro 5, 00185 Rome, Italy
² ICRANet-Pescara, Piazza della Repubblica 10, 65122 Pescara, Italy
e-mail: luca.izzo@icra.it; muccino@icra.it
³ ICRANet-Rio, Centro Brasileiro de Pesquisas Fisicas, Rua Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro, Brazil
e-mail: elena.zaninoni@gmail.com
⁴ INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, via Gobetti 101, 40129 Bologna, Italy
⁵ INAF–Napoli, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy

Received: 4 May 2015
Accepted: 24 August 2015

Abstract

Context. Data from cosmic microwave background radiation (CMB), baryon acoustic oscillations (BAO), and supernovae Ia (SNe-Ia) support a constant dark energy equation of state with w₀ ~ −1. Measuring the evolution of w along the redshift is one of the most demanding challenges for observational cosmology.

Aims. We discuss the existence of a close relation for gamma-ray bursts (GRBs), named Combo-relation, based on characteristic parameters of GRB phenomenology such as the prompt intrinsic peak energy E_p,i, the X-ray afterglow initial luminosity L₀ and the rest-frame duration τ of the shallow phase, and the index of the late power-law decay α_X. We use it to measure Ω_m and the evolution of the dark energy equation of state. We also propose a new calibration method for the same relation, which reduces the dependence on SNe Ia systematics.

Methods. We have selected a sample of GRBs with 1) a measured redshift z; 2) a determined intrinsic prompt peak energy E_p,i; and 3) a good coverage of the observed (0.3–10) keV afterglow light curves. The fitting technique of the rest-frame (0.3–10) keV luminosity light curves represents the core of the Combo-relation. We separate the early steep decay, considered a part of the prompt emission, from the X-ray afterglow additional component. Data with the largest positive residual, identified as flares, are automatically eliminated until the p-value of the fit becomes greater than 0.3.

Results. We strongly minimize the dependency of the Combo-GRB calibration on SNe Ia. We also measure a small extra-Poissonian scatter of the Combo-relation, which allows us to infer from GRBs alone Ω_M = 0.29^+0.23_-0.15 (1σ) for the ΛCDM cosmological model, and Ω_M = 0.40^+0.22_-0.16, w₀ = −1.43^+0.78_-0.66 for the flat-Universe variable equation of state case.

Conclusions. In view of the increasing size of the GRB database, thanks to future missions, the Combo-relation is a promising tool for measuring Ω_m with an accuracy comparable to that exhibited by SNe Ia, and to investigate the dark energy contribution and evolution up to z ~ 10.