Spectral distortions of the CMB dipole

Abstract: We consider the distortions of the cosmic microwave background (CMB) dipole anisotropy related to the primordial recombination radiation (PRR) and primordial $y$- and $\mu$-distortions. The signals arise due to our motion relative to the CMB restframe and appear as a frequency-dependent distortion of the CMB temperature dipole. To leading order, the expected relative distortion of the CMB dipole does not depend on the particular observation directions and reaches the level of $10^{-6}$ for the PRR- and $\mu$-distortions and $10^{-5}$ for the $y$-distortion in the frequency range 1 -- 700 GHz. The temperature differences arising from the dipole anisotropy of the relic CMB distortions depend on the observation directions. For mutually opposite directions, collinear to the CMB dipole axis, the temperature differences because of the PRR- and $\mu$-dipole anisotropy attain values $\Delta T\simeq 10\,$nK in the considered range. The temperature difference arising from the $y$-dipole anisotropy may reach values of up to $1\,\mu$K. The key features of the considered effect are as follow: (i) an observation of the effect does not require absolute calibration; (ii) patches of sky with minimal foreground contamination can be chosen. Future measurements of the CMB dipole distortion thus will provide an alternative method for direct detection of the PRR-, $y$-, and $\mu$-distortions. The $y$-distortion dipole may be detectable with PIXIE at a few standard deviations.