False outliers of the E_p,i – E_iso correlation?

¹ Dipartimento di Fisica e Scienze della TerraUniversitá di Ferrara, via Saragat 1, 44122 Ferrara, Italy
e-mail: mrtrnt@unife.it
² Universitá di Napoli “Federico II”, Dipartimento di Fisica “Ettore Pancini”, C. U. Monte S. Angelo, 80126 Napoli, Italy
³ Instituto de Astrofisica de Andalucia (IAA-CSIC), Glorieta de la Astronomia s/n, 18008 Granada, Spain
⁴ INAF–Osservatorio Astronomico di Napoli, Salita Moiariello, 16, 80131 Napoli, Italy
⁵ International Center for Relativistic Astrophysics, Piazza della Repubblica 10, 65122 Pescara, Italy
⁶ INAF–IASF Bologna, via P. Gobetti, 101, 40129 Bologna, Italy
⁷ AIM–CEA/DRF/Irfu/Service d’Astrophysique, Orme des Merisiers, 91191 Gif-sur-Yvette, France

Received: 27 February 2017
Accepted: 9 August 2017

Abstract

Context. In the context of an in-depth understanding of GRBs and their possible use in cosmology, some important correlations between the parameters that describe their emission have been discovered, among which the “E_p,i – E_iso” correlation is the most studied. Because of this, it is fundamental to shed light on the peculiar behaviour of a few events, namely GRB 980425 and GRB 031203, that appear to be important outliers of the E_p,i – E_iso correlation.

Aims. In this paper we investigate if the locations of GRB 980425 and GRB 031203, the two (apparent) outliers of the correlation, may be due to an observational bias caused by the lacking detection of the soft X-ray emissions associated with these GRBs, from respectively the Burst And Transient Source Experiment (BATSE) detector on-board the Compton Gamma-Ray Observer and INTEGRAL, that were operating at the epoch at which the observations were carried out. We analyse the observed emission of other similar sub-energetic bursts (GRBs 060218, 100316D and 161219B) observed by Swift and whose integrated emissions match the E_p,i – E_iso relation. We simulate their integrated and time-resolved emissions as would have been observed by the same detectors that observed GRB 980425 and GRB 031203, aimed at reconstructing the light curve and spectra of these bursts.

Methods. We estimate the E_p,i and the E_iso parameters from the time-resolved and total integrated simulated spectra of GRBs 060218, 100316D and 161219B as observed by BeppoSAX, BATSE, INTEGRAL, and the Wide Field Monitor (WFM) proposed for the Large Observatory For X-ray Timing (LOFT) and enhanced X-ray Timing and Polarimetry (eXTP) missions.

Results. If observed by old generation instruments, GRBs 060218, 100316D, and 161219B would appear as outliers of the E_p,i–E_iso relation, while if observed with Swift or the WFM GRB 060218 would perfectly match the correlation. We also note that the instrument BAT alone (15−150 keV) did measure GRB 060218 as an outlier.

Conclusions. We suggest that if GRB 980425 and GRB 031203 would have been observed by Swift and by eXTP, they might have matched the E_p,i–E_iso relation. This provides strong support to the idea that instrumental biases can cause some events in the lower left corner of the E_p,i – E_iso plane to appear as outliers of the so-called Amati relation.