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.
GRB 060218 and the outliers with respect to the Ep-Eiso correlation
GRB 031203和GRB 980425是长暴Ep-Eiso相关的两个异常值。最近，斯威夫特发现了一个与SN事件有关的极长GRB 060218。这类爆发的光谱特征是惊人的：一方面，它的宽带SED既有热辐射分量，也有非热辐射分量，这两种辐射分量分别可以解释为伽玛暴喷流周围的热茧辐射和伽玛暴瞬发相的标准同步辐射自吸收辐射;另一方面，它的长持续时间和它的从硬到软的光谱演化使这个暗暴与长伽玛暴的Ep-Eiso关联一致。通过比较两个主要异常值的可用光谱信息，我们认为它们可能是060218的孪生子，因此，只是关于Ep_Eiso相关性的明显异常值。这种解释也表明，它是首要的重要性，研究伽玛暴的宽带光谱，以监测其光谱演化在其整个持续时间。
Confirmation of the $\eps$ -- $\eiso$ (Amati) relation from the X-ray flash XRF 050416A observed by Swift/BAT
We report Swift Burst Alert Telescope (BAT) observations of the X-ray Flash (XRF) XRF 050416A. The fluence ratio between the 15-25 keV and 25-50 keV energy bands of this event is 1.5, thus making it the softest gamma-ray burst (GRB) observed by BAT so far. The spectrum is well fitted by the Band function with E^{ \rm obs}_{ \rm peak} of 15.0_{-2.7}^{+2.3} keV. Assuming the redshift of the host galaxy (z = 0.6535), the isotropic-equivalent radiated energy E_{\rm iso} and the peak energy at the GRB rest frame (E^{ \rm src}_{ \rm peak}) of XRF 050416A are not only consistent with the correlation …
A Unified Jet Model of X-Ray Flashes, X-Ray-Rich Gamma-Ray Bursts, and Gamma-Ray Bursts: I. Power-Law-Shaped Universal and Top-Hat-Shaped Variable Opening-Angle Jet Models
