Multimessenger astronomy with a Southern-hemisphere gravitational-wave observatory

Joint observations of gravitational waves and electromagnetic counterparts will answer questions about cosmology, gamma-ray bursts, and the behavior of matter at supranuclear densities. The addition of a Southern-hemisphere gravitational-wave observatory to proposed global networks creates a longer baseline, which is beneficial for sky localization. We analyze how an observatory in Australia can enhance the multimessenger astronomy capabilities of future networks. We estimate the number of binary neutron star mergers with joint observations of gravitational waves and kilonova counterparts detectable by the Vera C. Rubin Observatory. First, we consider a network of upgrades to current observatories. Adding an Australian observatory to a three-observatory network (comprising two observatories in the USA and one in Europe) boosts the rate of joint observations from 2.5-2.0+4.5 yr-1 to 5.6-4.5+10 yr-1 (a factor of two improvement). Then, we consider a network of next-generation observatories. Adding a 20 km Australian observatory to a global network of a Cosmic Explorer 40 km in the USA and an Einstein Telescope in Europe only marginally increases the rate from 40-32+71 yr-1 to 44-35+79 yr-1 (a factor of 1.1 improvement). The addition of an Australian observatory, however, ensures that at least two observatories are online far more often. When the Cosmic Explorer 40 km is offline for a major upgrade, the Australian observatory increases the joint observation rate from 0.5-0.4+0.8 yr-1 to 38-30+68 yr-1 (a factor of 82 improvement). When the Einstein Telescope is offline, the joint observation rate increases from 0.2-0.1+0.3 yr-1 to 19-15+34 yr-1 (a factor of 113 improvement). We sketch out the broader science case for a Southern-hemisphere gravitational-wave observatory.