[vsnet-grb-info 3009] GRB060218: Ep,i - Eiso correlation

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4846
SUBJECT: GRB060218: Ep,i - Eiso correlation
DATE:    06/03/01 16:43:33 GMT
FROM:    Cristiano Guidorzi at ARI,Liverpool JMU  <crg at astro.livjm.ac.uk>

L. Amati (INAF-IASF Bologna), F. Frontera (Ferrara University and 
INAF-IASF Bologna), C. Guidorzi (Liverpool JM University) and E. Montanari 
(Ferrara University) report:

"Based on the photon index measured by Swift/BAT (2.5+/-0.1 in 15-150 keV, 
Sakamoto et al., GCN #4822) and Swift/XRT during the prompt emission 
(1.82+/-0.01 in 0.2-10 keV, Cusumano et al., GCN #4786), it can be inferred 
that the intrinsic peak energy Ep,i of GRB060218/SN2006aj (at the suggested 
redshift z=0.03) is < 10 keV and lies probably towards the low energy bound of 
XRT (i.e. a few keV at most). This information, when combined with the Eiso 
(1-10000 keV) value of about (5 - 8)x10^(49) erg, as derived on the basis of 
the preliminary BAT and XRT (first orbit) fluxes and spectra, shows that 
GRB060218 is likely consistent with the Ep,i - Eiso (Amati) correlation. This 
behaviour, if confirmed by more refined analysis, is at odds with that of 
GRB980425/SN1998bw and of the other possibly sub-energetic event 
GRB031203/SN2003lw, which showed Ep,i and Eiso values completely inconsistent 
with the correlation (see e.g. Amati, 2006, astro-ph/0601553 for a discussion). 
Given that one of the most popular explanations of the inconsistency of 
GRB980425 and GRB031203 with the Ep,i-Eiso correlation is that they were 
'normal' GRBs seen at very high off-axis angles (e.g. Yamazaki, Yonetoku & 
Nakamura, 2003, ApJ 594 L79 , Ramirez-Ruiz et al., 2005, ApJ, 625 L91), this 
evidence shows that GRB060218 may not be an off-axis event, as also 
suggested by Nousek et al. (GGN #4805) based on the 'chromatic' behaviour of 
afterglow light curves measured by Swift."

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