TITLE: GCN CIRCULAR NUMBER: 23036 SUBJECT: GRB 180720B long follow up requested DATE: 18/07/25 08:52:55 GMT FROM: Arnon Dar at Technion-Israel Inst. of Tech 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.]