Probing cosmic anisotropy with GW/FRB as upgraded standard sirens

Recently it was shown that cosmic anisotropy can be well tested using either standard siren measurement of luminosity distance d(L)(z) from gravitational-wave (GW) observation or dispersion measure (DM(z)) from fast radio burst (FRB). It was also observed that the combined measurement of d(L)(z) . DM(z) from the GW/FRB association system as suggested in some of FRB models is more effective to constrain cosmological parameters than d(L)(z) or DM(z) separately. In this paper, we show that this upgraded siren from the combined GW/FRB observations could test cosmic anisotropy with a double relative sensitivity compared to the usual standard siren from GW observations only.