Bayesian parameter estimation for targeted anisotropic gravitational-wave background*

Extended sources of the stochastic gravitational backgrounds have been conventionally searched on the spherical harmonics bases. The analysis during the previous observing runs by the ground-based gravitational wave detectors, such as LIGO and Virgo, have yielded the constraints on the angular power spectrum $C_ell$, yet it lacks the capability of estimating other parameters such as a spectral index.

In this work, we introduce an alternative Bayesian formalism to search for such stochastic signals with a particular distribution of anisotropies on the sky.  This approach provides a Bayesian posterior of model parameters and also enables selection tests among different signal models.  While the conventional analysis fixes the highest angular scale a priori, here we show a more systematic and quantitative way to determine the cut-off scale based on a Bayes factor, which depends on the amplitude and the angular scale of observed signals.

Also, we analyze the third observing runs of LIGO and Virgo for the population of milli-second pulsars and obtain the 95 \% constraints of the signal amplitude, $epsilon < 2.7 imes 10^{-8}$.