Spectral distortions of the CMB dipole
  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.
