TITLE: GCN CIRCULAR NUMBER: 9235 SUBJECT: GRB 090423: energetic, luminosity and jet break DATE: 09/04/24 21:57:44 GMT FROM: Giancarlo Ghirlanda at INAF/Brera L. Nava (INAF-OAB/Univ.Insubria); D. Burlon (MPE-Garching); G. Ghirlanda (INAF-OAB); G. Ghisellini (INAF-OAB); M. Nardini (SISSA) With the spectral parameters and fluence of GRB090423 as measured by Fermi (von Kienlin et al., GCN 9229) and given its redshift z=8.1 (Fernandez-Soto et al., GCN 9222), we estimate the isotropic equivalent energy Eiso = 1.03E53 and the isotropic equivalent peak luminosity Liso = 1.88E53 (the value of Eiso is consistent with that obtained with the Swift spectral results given in Palmer et al. GCN 9204). Given the rest frame peak energy Ep= 746 keV, we show ( http://www.brera.inaf.it/utenti/ghirla/GRB/090423.html ) that GRB090423 is consistent both with the Ep-Liso correlation (Yonetoku et al. 2004) and with the Ep-Eiso correlation (Amati et al. 2002, Ghirlanda et al. 2008, Nava et al. 2008; see also GCN 9227). Noteworthly, GRB090423 is remarkably similar to GRB 080913 and GRB 071020 with respect to the Amati and Yonetoku correlations. It is worth mentioning that these three GRBs are long bursts according to their observed T90 (12 s, 8 s and 4 s respectively) although, given their redshifts, T90 ~ 1 sec in the rest frame. Finally, considering the collimation corrected Ep-Egamma correlation (e.g. Ghirlanda et al. 2007), we estimate that a jet break should occur in the afterglow light curve between ~22 and 54 days (1 sigma consistency) or between ~10 and 130 days (3 sigma consistency) assuming a homogeneous circum-burst environment with standard parameters (see Nava et al. 2006, Ghirlanda et al. 2007). However, this break may not be observed in the X-ray which, given the typical steep-flat-steep decay already observed for this burst ( http://www.swift.ac.uk/xrt_curves/00350184/ ), could likely be due to another emission component (e.g. Ghisellini et al. 2009). This message may be cited.