Limits on anisotropy in the nanohertz stochastic gravitational-wave background

The paucity of observed supermassive black hole binaries (SMBHBs) may imply that the gravitational wave background (GWB) from this population is anisotropic, rendering existing analyses sub-optimal. We present the first constraints on the angular distribution of a nanohertz stochastic GWB from circular, inspiral-driven SMBHBs using the $2015$ European Pulsar Timing Array data [Desvignes et al. (in prep.)]. Our analysis of the GWB in the $\sim 2 - 90$nHzbandshowsconsistencywithisotropy,withthestrainamplitudein$l>0$sphericalharmonicmultipoles$\lesssim 40%$ of the monopole value. We expect that these more general techniques will become standard tools to probe the angular distribution of source populations.