Confirming the γ-ray burst spectral-energy correlations in the era of multiple time breaks

¹ Osservatorio Astronomico di Brera, via Bianchi 46, 20387 Merate, Italy e-mail: giancarlo.ghirlanda@brera.inaf.it
² Univ. degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy
³ Instituto de Astronomía, Universidad National Autónoma de México, A.P. 70-264, 04510 México, D.F., México

Received: 17 January 2007
Accepted: 8 February 2007

Abstract

We test the spectral-energy correlation including the new bursts detected (mostly) by Swift with firm measurements of their redshifts and peak energy. The problem of identifying the jet breaks in the complex and multibreak/flaring X-ray light curves observed by Swift is discussed in the complex and multibreak/flaring X-ray light curves observed by Swift. We use the optical data as the most reliable source for the identification of the jet break, since the X-ray flux may be produced by a mechanism different from the external shocks between the fireball and the circumburst medium, which are responsible for the optical afterglow. We show that the presence of an underlying SN event in XRF 050416A requires a break to occur in the afterglow optical light curve at around the expected jet break time. The possible presence of a jet break in the optical light curve of GRB 050401 is also discussed. We point out that, for measuring the jet break, it is mandatory that the optical light curve extends after the epoch where the jet break is expected. The interpretation of the early optical breaks in GRB 050922C and GRB 060206 as jet breaks is controversial because they might instead correspond to the flat-to-steep decay transition common in the early X-ray light curves. All the 16 bursts coming from Swift are consistent with the  and  correlation. No outlier is found to date. Moreover, the small dispersion of the  and  correlation, confirmed also by the Swift bursts, strengthens the case of using GRBs as standard candles.