TITLE: GCN CIRCULAR NUMBER: 23041 SUBJECT: NuSTAR observations of GRB 180720B DATE: 18/07/26 16:39:39 GMT FROM: Eric C Bellm at UW E. C. Bellm (UW) and S. B. Cenko (GSFC) report: We have analyzed 38 ksec of NuSTAR data for GRB 180720B (M. H. Siegel et al., GCN Circ. 22973), from 243 ksec to 318 ksec after the BAT trigger. NuSTAR detects emission from the X-ray afterglow to approximately 30 keV. The NuSTAR spectrum can be well-modeled from 3-30 keV by an absorbed power law with spectral index 1.80+/-0.06, consistent with the value reported by XRT (K. L. Page et al., GCN Circ. 22984). The 3-30 keV flux was 4.8+/-0.2 x 10^-12 erg cm^-2 s^-1. We thank Karl Forster and the NuSTAR operations team for their assistance executing these TOO observations.