Faraday tomography is thought to be a powerful tool to explore cosmic magnetic field. Broadband radio polarimetric data is essential to ensure the quality of Faraday tomography, but such data is not easy to obtain because of radio frequency interferences (RFIs). In this paper, we investigate optimum frequency coverage of Faraday tomography so as to explore Faraday rotation measure (RM) due to the intergalactic magnetic field (IGMF) in filaments of galaxies. We adopt a simple model of the IGMF and estimate confidence intervals of the model parameters using the Fisher information matrix. We find that meaningful constraints for RM due to the IGMF are available with data at multiple narrowbands which are scattered over the ultra-high frequency (UHF, 300 MHz - 3000 MHz). The optimum frequency depends on the Faraday thickness of the Milky Way foreground. These results are obtained for a wide brightness range of the background source including fast radio bursts (FRBs). We discuss the relation between the polarized-intensity spectrum and the optimum frequency.
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