Detecting new fundamental fields with pulsar timing arrays

Strong evidence of the existence of the stochastic gravitational wave background (SGWB) has been reported recently by the North American Nanohertz Observatory for Gravitational Waves, the Parkes Pulsar Timing Array, the European Pulsar Timing Array, and the Chinese Pulsar Timing Array Collaborations. The Bayesian posteriors of the SGWB amplitude and spectrum are compatible with astrophysical predictions for the SGWB from a population of supermassive black hole binaries (SMBHBs). We discuss the modifications arising from the extra scalar or vector radiation to the characteristic dimensionless strain of gravitational waves from SMBHBs and explore the possibility of detecting charges carried by supermassive black holes in modified gravity. The Bayesian posteriors on the tensor amplitude and the spectral exponent of SGWB are log10 AT = -14.64 thorn 0.19 -0.27 and alpha = -0.35 thorn 0.23 -0.25 , anda rough upper limit log10 AV;S <= -15.2 is obtained for the SGWB scalar or vector amplitude. The Bayesian factor between the models with and without charge is only 0.6, so the current PTA data do not favor charged SMBHBs over neutral SMBHBs.