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GCN Circular 23036

Subject
GRB 180720B long follow up requested
Date
2018-07-25T08:52:55Z (6 years ago)
From
Arnon Dar at Technion-Israel Inst. of Tech <arnon@physics.technion.ac.il>
Long follow up of the afterglow of the extremely bright GRB
180720B (Swift-BAT detection: Siegel et al., GCN #22973, GCN
#22975; Fermi-LAT detection: Bissaldi and Racusin, GCN #22980;
Fermi-GBM detection: Roberts et al., GCN #22981; Konus Wind
detection: Frederiks et al. GCN 23011) is urged. It will provide
another critical test of GRB theories. Its  current late-time X-ray
afterglow, measured with Swift XRT, decays like a single power-law
with a temporal index alpha=1.34+/-0.01 and an unabsorbed spectral
index beta=0.82+/-0.04 (Evans et al. Swift-XRT GRB lightcurve
repository). It satisfies well the Cannonball model closure relation
alpha=beta+1/2 (e.g., Dado and Dar, PhRvD, 94, 3007 (2016)) for
the late time unabsorbed afterglows of SN-GRBs (while those of
SN-less GRBs satisfy alpha=2,  e.g., Dado and Dar arXiv:1807.08726).
An SN akin to SN1998bw may be resolved from the optical afterglows
around day ~15. An achromatic break in the late time afterglow
is expected only if the host galaxy is aligned  near face on.

[GCN OPS NOTE(26jul18): The operator has corrected the GRB name 
in the Subject-line; and corrected Frederiks reference from 230110
to 23011.]
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