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Frequency-Dependent Constraints on Cosmic Birefringence from the LFI and HFI Planck Data Release 4

We present new constraints on the frequency dependence of the cosmic birefringence angle from the Planck data release 4 polarization maps. An axion field coupled to electromagnetism predicts a nearly frequency-independent birefringence angle, $β_ν= β$, while Faraday rotation from local magnetic fields and Lorentz violating theories predict a cosmic birefringence angle that is proportional to the frequency, $ν$, to the power of some integer $n$, $β_ν\propto ν^n$. In this work, we first sample $β_ν$ individually for each polarized HFI frequency band in addition to the 70 GHz channel from the LFI. We also constrain a power-law formula for the birefringence angle, $β_ν=β_0(ν/ν_0)^n$, with $ν_0 = 150$ GHz. For a nearly full-sky measurement, $f_{\text{sky}}=0.93$, we find $β_0 = 0.26^{\circ}\pm0.11^\circ$ $(68\% \text{ C.L.})$ and $n=-0.45^{+0.61}_{-0.82}$ when we ignore the intrinsic $EB$ correlations of the polarized foreground emission, and $β_0 = 0.33^\circ \pm 0.12^\circ$ and $n=-0.37^{+0.49}_{-0.64}$ when we use a filamentary dust model for the foreground $EB$. Next, we use all the polarized Planck maps, including the 30 and 44 GHz frequency bands. These bands have a negligible foreground contribution from polarized dust emission. We, therefore, treat them separately. Without any modeling of the intrinsic $EB$ of the foreground, we generally find that the inclusion of the 30 and 44 GHz frequency bands raises the measured values of $β_ν$ and tightens $n$. At nearly full-sky, we measure $β_0=0.29^{\circ+0.10^\circ}_{\phantom{\circ}-0.11^\circ}$ and $n=-0.35^{+0.48}_{-0.47}$. Assuming no frequency dependence, we measure $β=0.33^\circ \pm 0.10^\circ$. If our measurements have effectively mitigated the $EB$ of the foreground, our constraints are consistent with a mostly frequency-independent signal of cosmic birefringence.