  We search for a first-order phase transition gravitational wave signal in 45
pulsars from the NANOGrav 12.5 year dataset. We find that the data can be
modeled in terms of a strong first order phase transition taking place at
temperatures below the electroweak scale. However, we do not observe any strong
preference for a phase-transition interpretation of the signal over the
standard astrophysical interpretation in terms of supermassive black holes
mergers; but we expect to gain additional discriminating power with future
datasets, improving the signal to noise ratio and extending the sensitivity
window to lower frequencies. An interesting open question is how well
gravitational wave observatories could separate such signals.
