The Epeak-Eiso plane of long Gamma Ray Bursts and selection effects

Abstract

We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured redshift and spectral parameters, detected up to September 2007. We confirm the existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to previous studies, no sign of evolution with redshift of the Ep-Eiso correlation (either its slope and normalisation) is found. The 76 bursts define a strong Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with redshifts evenly distributed along this correlation. We study possible instrumental selection effects in the observer frame Ep,obs-Fluence plane. In particular, we concentrate on the minimum peak flux necessary to trigger a given GRB detector (trigger threshold) and the minimum fluence a burst must have to determine the value of Ep,obs (spectral analysis threshold). We find that the latter dominates in the Ep,obs-Fluence plane over the former. Our analysis shows, however, that these instrumental selection effects do not dominate for bursts detected before the launch of the Swift satellite, while the spectral analysis threshold is the dominant truncation effect of the Swift GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence correlation defined by the pre--Swift sample could be affected by other, still not understood, selection effects. Besides we caution about the conclusions on the existence of the Ep,obs-Fluence correlation based on our Swift sample alone.

To appear in MNRAS

References in corpus (8)

• The Hubble Diagram to Redshift >6 from 69 Gamma-Ray Bursts
• A Complete Catalog of Swift GRB Spectra and Durations: Demise of a Physical Origin for Pre-Swift High-Energy Correlations
• Swift observations of GRB 060614: an anomalous burst with a well behaved afterglow
• Confirming the gamma-ray burst spectral-energy correlations in the era of multiple time breaks
• Variation of the Amati Relation with the Cosmological Redshift: a Selection Effect or an Evolution Effect?
• Spectral analysis of Swift long GRBs with known redshift
• The role of afterglow break-times as GRB jet angle indicators
• The complete catalogue of gamma-ray bursts observed by the Wide Field Cameras on board BeppoSAX

Cited by in corpus (8)

• Short versus Long Gamma-Ray Bursts: spectra, energetics, and luminosities
• A Cosmology-Independent Calibration of Gamma-Ray Burst Luminosity Relations and the Hubble Diagram
• Precursors in Swift Gamma Ray Bursts with redshift
• Peak energy of the prompt emission of long Gamma Ray Bursts vs their fluence and peak flux
• Generalized Tests for Selection Effects in GRB High-Energy Correlations
• The updated spectral catalogue of INTEGRAL Gamma-Ray Bursts
• Observations of the Prompt Gamma-Ray Emission of GRB 070125
• Factor analysis of the long gamma-ray bursts