Forecasting synchrotron spectral parameters with QUIJOTE-MFI2 in combination with Planck and WMAP

We present a parametric component separation forecast for the QUIJOTE–MFI2 instrument (10–20 GHz) that assesses its impact on constraining polarised synchrotron emission at 1° full width at half maximum and N side = 64. Using simulated sky maps based on power-law and curved synchrotron spectra, we show that adding QUIJOTE-MFI2 to existing WMAP+ Planck +MFI data yields statistically unbiased parameter estimates with substantial uncertainty reductions. The improvement factors reach ∼10 for the synchrotron spectral index ( β s ), ∼5 for the curvature parameter ( C s ), and ∼43 for polarisation amplitudes in bright regions. Deep QUIJOTE cosmological fields enable β s constraints even in intrinsically low S/N regions, where WMAP+ Planck alone remain prior-dominated. Current combined sensitivities are insufficient to detect a synchrotron curvature of C s = −0.052 on a pixel-by-pixel basis, but a 2 σ detection is achievable for | C s |≳0.14 in the brightest regions of the Galactic plane. In those deep cosmological fields, combining QUIJOTE-MFI2 with WMAP and Planck reduces the median synchrotron residual at 100 GHz by a factor of six to 0.033 μ K CMB . These results demonstrate that QUIJOTE-MFI2 will provide critical low-frequency information for modelling Galactic synchrotron emission, therefore offering valuable complementary constraints for future cosmic microwave background surveys such as LiteBIRD and the Simons Observatory.