GCN Circular 4846
Subject
GRB060218: Ep,i - Eiso correlation
Event
Date
2006-03-01T16:43:33Z (20 years ago)
From
Cristiano Guidorzi at ARI,Liverpool JMU <crg@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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