GRB 071227: an additional case of a disguised short burst

¹ Dipartimento di Fisica and ICRA, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy e-mail: [gustavo.debarros;barbara.patricelli]@icranet.org
² ICRANet, Piazzale della Repubblica 10, 65122 Pescara, Italy e-mail: [letizia.caito;maria.bernardini;bianco;luca.izzo;ruffini]@icra.it
³ Italian National Institute for Astrophysics (INAF) – IASF Bologna, via P. Gobetti 101, 40129 Bologna, Italy e-mail: amati@iasfbo.inaf.it
⁴ Italian National Institute for Astrophysics (INAF) – Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy
⁵ ICRANet, Université de Nice Sophia Antipolis, Grand Château, BP 2135, 28 avenue de Valrose, 06103 Nice Cedex 2, France

Received: 31 March 2010
Accepted: 24 June 2010

Abstract

Context. Observations of gamma-ray bursts (GRBs) have shown an hybridization between the two classes of long and short bursts. In the context of the fireshell model, the GRB light curves are formed by two different components: the proper GRB (P-GRB) and the extended afterglow. Their relative intensity is linked to the fireshell baryon loading B. The GRBs with P-GRB predominance are the short ones, the remainders are long. A new family of disguised short bursts has been identified: long bursts with a protracted low instantaneous luminosity due to a low density CircumBurst Medium (CBM). In the 15–150 keV energy band GRB 071227 exhibits a short duration (about 1.8 s) spike-like emission followed by a very soft extended tail up to one hundred seconds after the trigger. It is a faint (E_iso = 5.8 × 10 nearby GRB (z = 0.383) that does not have an associated type Ib/c bright supernova (SN). For these reasons, GRB 071227 has been classified as a short burst not fulfilling the Amati relation holding for long burst.

Aims. We check the classification of GRB 071227 provided by the fireshell model. In particular, we test whether this burst is another example of a disguised short burst, after GRB 970228 and GRB 060614, and, for this reason, whether it fulfills the Amati relation.

Methods. We simulate GRB 071227 light curves in the Swift BAT 15–50 keV bandpass and in the XRT (0.3–10 keV) energy band within the fireshell model.

Results. We perform simulations of the tail in the 15–50 keV bandpass, as well as of the first part of the X-ray afterglow. This infers that: = 5.04 × 10⁵¹ erg, B = 2.0 × 10^-4, E_P-GRB/E_aft~0.25, and = 3.33 particles cm^-3. These values are consistent with those of “long duration” GRBs. We interpret the observed energy of the first hard emission by identifying it with the P-GRB emission. The remaining long soft tail indeed fulfills the Amati relation.

Conclusions. Previously classified as a short burst, GRB 071227 on the basis of our analysis performed in the context of the fireshell scenario represents another example of a disguised short burst, after GRB 970228 and GRB 060614. Further confirmation of this result is that the soft tail of GRB 071227 fulfills the Amati relation.