A multifrequency angular power spectrum analysis of the Leiden polarization surveys

The Galactic synchrotron emission is expected to be the most relevant source of astrophysical contamination in cosmic microwave background polarization measurements, at least at frequencies $\nu \la 70$ GHz and at angular scales $\theta\ga 30'$. We present a multifrequency analysis of the Leiden surveys, linear polarization surveys covering the Northern Celestial Hemisphere at five frequencies between 408 MHz and 1411 MHz. By implementing specific interpolation methods to deal with these irregularly sampled data, we produced maps of the polarized diffuse Galactic radio emission with a pixel size $\simeq$$0.92^\circ$. We derived the angular power spectrum (APS) (PI, E, and B modes) of the synchrotron dominated radio emission as function of the multipole, $\ell$. We considered the whole covered region and some patches at different Galactic latitudes. By fitting the APS in terms of power laws ($C_\ell \sim \kappa\cdot\ell^{\alpha}$), we found spectral indices that steepen with increasing frequency: from $\alpha \sim -(1{-}1.5)$ at 408 MHz to $\alpha \sim -(2{-}3)$ at 1411 MHz for $10 \la \ell \la 100$ and from $\alpha \sim -0.7$ to $\alpha \sim -1.5$ for lower multipoles (the exact values depending on the considered sky region and polarization mode). The bulk of this flattening at lower frequencies can be interpreted in terms of Faraday depolarization effects. We then considered the APS at various fixed multipoles and its frequency dependence. Using the APSs of the Leiden surveys at 820 MHz and 1411 MHz, we determined possible ranges for the rotation measure, RM, in the simple case of an interstellar medium slab model. Also taking into account the polarization degree at 1.4 GHz, it is possible to break the degeneracy between the identified RM intervals. The most reasonable of them turned out to be $RM \sim 9{-}17$ rad/m2 although, given the uncertainty on the measured polarization degree, RM values in the interval ~$53{-}59$ rad/m2 cannot be excluded