Journal Article

The <i>E</i><sub>peak</sub>–<i>E</i><sub>iso</sub> plane of long gamma-ray bursts and selection effects

G. Ghirlanda, L. Nava, G. Ghisellini, C. Firmani and J. I. Cabrera

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 387, issue 1, pages 319-330
Published in print June 2008 | ISSN: 0035-8711
Published online May 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13232.x
The Epeak–Eiso plane of long gamma-ray bursts and selection effects

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We study the distribution of long gamma-ray bursts in the EpeakEiso and in the Eobspeak–fluence planes through an updated sample of 76 bursts, with measured redshift and spectral parameters, detected up to 2007 September. We confirm the existence of a strong rest-frame correlationEpeakE0.54±0.01iso. Contrary to previous studies, no sign of evolution with redshift of the EpeakEiso correlation (either its slope and its normalization) is found. The 76 bursts define a strongEobspeak–fluence correlation in the observer frame (EobspeakF0.32±0.05bol) with redshifts evenly distributed along this correlation. We study possible instrumental selection effects in the observer frameEobspeak–fluence plane. In particular, we concentrate on the minimum peak flux necessary to trigger a given gamma-ray burst (GRB) detector (trigger threshold) and the minimum fluence a burst must have to determine the value ofEobspeak (spectral analysis threshold). We find that the latter dominates in the Eobspeak–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 Eobspeak–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 Eobspeak–fluence correlation based on ourSwift sample alone.

Keywords: radiation mechanisms: non-thermal; gamma rays: bursts; X-rays: general

Journal Article.  9316 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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