Constraints on the spectral index of polarized synchrotron emission from WMAP and Faraday-corrected S-PASS data

We constrain the spectral index of polarized synchrotron emission, beta (s), by correlating the recently released 2.3 GHz S-Band Polarization All Sky Survey (S-PASS) data with the 23 GHz 9-year Wilkinson Microwave Anisotropy Probe (WMAP) sky maps. We subdivide the S-PASS field, which covers the southern ecliptic hemisphere, into 95 15 degrees x15 degrees regions and estimate the spectral index of polarized synchrotron emission within each region using a simple but robust T-T plot technique. Three different versions of the S-PASS data are considered, corresponding to: no correction for Faraday rotation; Faraday correction based on the rotation measure model presented by the S-PASS team; or Faraday correction based on a rotation measure model presented by Hutschenreuter and En ss lin. We find that the correlation between S-PASS and WMAP is strongest when applying the S-PASS model. Adopting this correction model, we find that the mean spectral index of polarized synchrotron emission gradually steepens from beta (s)approximate to -2.8 at low Galactic latitudes to beta (s)approximate to -3.3 at high Galactic latitudes, in good agreement with previously published results. The flat spectral index at the low Galactic latitudes is likely partly due to depolarization effects. Finally, we consider two special cases defined by the BICEP2 and SPIDER fields and obtain mean estimates of beta (BICEP2)=-3.22 +/- 0.06 and beta (SPIDER)=-3.21 +/- 0.03, respectively. Adopting the bandpass filtered WMAP 23 GHz sky map to only include angular scales between 2 degrees and 10 degrees as a spatial template, we constrain the root-mean-square synchrotron polarization amplitude to be less than 0.03 mu K (0.009 mu K) at 90 GHz (150 GHz) for the BICEP2 field, corresponding roughly to a tensor-to-scalar ratio of r less than or similar to 0.02 (r less than or similar to 0.005). Very similar constraints are obtained for the SPIDER field. A comparison with a similar analysis performed in the 23-33 GHz range suggests a flattening of about Delta beta (s)similar to 0.1 +/- 0.2 from low to higher frequencies, but with no statistical significance due to high uncertainties.

Automated summary

We constrained the spectral index of polarized synchrotron emission by correlating S-PASS data with WMAP sky maps, finding that the spectral index varies across different regions in the southern ecliptic hemisphere. The mean spectral index gradually steepens from low Galactic latitudes to high Galactic latitudes, with the flat index at low latitudes likely due to depolarization effects. Special cases defined by the BICEP2 and SPIDER fields yielded mean estimates for the spectral index, and constraints on synchrotron polarization amplitude were obtained using bandpass filtered WMAP 23 GHz sky maps.