NICER observation list: Target ID 466303

The modeling of neutron-star LMXB X-ray spectra often suffers from a lack of coverage at either the low or high energy end. NICER spectra offer great low-energy coverage, with the potential to obtain excellent constraints on the thermal components, including parameters such as the inner disk radius and size of the boundary layer. However, this still requires additional coverage above 10 keV. Here we request three 20 ks NICER/NuSTAR ToO observations of the ultra-compact neutron-star LMXB 4U 1820-30, a bright source with large spectral variations. The observations will be triggered to observe the source at select locations in its spectral range, with the goal of improving our understanding of the evolution of the various spectral components and the underlying changes in the accretion flow.