On the consistency of peculiar GRBs 060218 and 060614 with the  – correlation

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Bologna, via P. Gobetti 101, 40129 Bologna, Italy e-mail: amati@iasfbo.inaf.it
² INAF – Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
³ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
⁴ International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
⁵ Dipartimento di Fisica, Università degli Studi di Milano Bicocca, piazza delle Scienze 3, 20126 Milano, Italy
⁶ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁷ ITA “Calvi”, via Digione 20, 41034 Finale Emilia (MO), Italy
⁸ INAF – Osservatorio Astronomico di Trieste, via G. Tiepolo 11, 34131 Trieste, Italy

Received: 7 July 2006
Accepted: 27 November 2006

Abstract

We analyze and discuss the position of GRB 060218 and GRB 060614 in the plane. GRB 060218 is important because of its similarity with GRB 980425, the prototypical event of the GRB–SN connection. While GRB 980425 is an outlier of the correlation, we find that GRB 060218 is fully consistent with it. This evidence, combined with the “chromatic” behavior of the afterglow light curves, is at odds with the hypothesis that GRB 060218 was a “standard” GRB seen off-axis and supports the existence of a class of truly sub-energetic GRBs. GRB 060614 is a peculiar event not accompanied by a bright supernova. Based on published spectral information, we find that also this event is consistent with the correlation. We discuss the implications of our results for the rate of sub-energetic GRBs, the GRB/SN connection and the properties of the newly discovered sub-class of long GRBs not associated with bright supernovae. We have included in our analysis other recent GRBs with clear evidence (or clear evidence of lack) of associated SNe.