  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 $\sim 2 -
90$ nHz band shows consistency with isotropy, with the strain amplitude in
$l>0$ spherical harmonic multipoles $\lesssim 40\%$ of the monopole value. We
expect that these more general techniques will become standard tools to probe
the angular distribution of source populations.

High Energy Astrophysical Phenomena;Cosmology and Nongalactic Astrophysics;General Relativity and Quantum Cosmology