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GCN Circular 9537

Subject
GRB090618, underlying supernova search
Date
2009-06-20T09:08:39Z (15 years ago)
From
Arnon Dar at Technion-Israel Inst. of Tech <arnon@physics.technion.ac.il>
S. Dado and A. Dar report:

The inferred photon spectral index Gamma=2.01+/-0.05 of the early time 
X-ray afterglow of the relatively nearby bright GRB090618 (z=1.54, GCN 
9518) from the Swift XRT observations (Evans et al. 
http://www.swift.ac.uk/xrt_spectra/00355083/) implies, in the CB model 
(e.g. ApJ 696 (2009), 994), an asymptotic achromatic power-law decline of 
the afterglow with a spectral index beta=1.01+/-0.05 and a temporal index 
alpha=beta+1/2=1.51+/-0.05. The R-band light curve of the anticipated 
underlying supernova akin to SN1998bw is expected to reach maximum 
brightness magnitude R=23.9 (Galactic extinction included) around July 10, 
2009 (approximately 22 days after GRB090618). Using the latest optical 
measurements reported in GCN 9531 and GCN 9536, the GRB afterglow is 
expected to decay by July 10, 2009 to a magnitude R=24.6+/-0.20 yielding 
R=23.4+/-0.3 for the optical transient superposed on a bright host galaxy 
with r=22.7 +/- 0.3 (GCN 9526). Like in the case of GRB 030319 (ApJ 594 
(2003), L89), the search for an underlying supernova (GCN 9526) in 
GRB060618 may be more successful using large telescopes with high 
resolution for photometric and/or spectroscopic follow up measurements of 
its optical afterglow.
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