The C-Band All-Sky Survey (C-BASS): Simulated parametric fitting in single pixels in total intensity and polarization

Abstract

The cosmic microwave background B-mode signal is potentially weaker than the diffuse Galactic foregrounds over most of the sky at any frequency. A common method of separating the CMB from these foregrounds is via pixel-based parametric-model fitting. There are not currently enough all-sky maps to fit anything more than the most simple models of the sky. By simulating the emission in seven representative pixels, we demonstrate that the inclusion of a 5 GHz data point allows for more complex models of low-frequency foregrounds to be fitted than at present. It is shown that the inclusion of the CBASS data will significantly reduce the uncertainties in a number of key parameters in the modelling of both the galactic foregrounds and the CMB. The extra data allow estimates of the synchrotron spectral index to be constrained much more strongly than is presently possible, with corresponding improvements in the accuracy of the recovery of the CMB amplitude. However, we show that to place good limits on models of the synchrotron spectral curvature will require additional low-frequency data.

Hierarchical Bayesian CMB component separation with the No-U-Turn Sampler

“…In the case of the CP(L) set, we struggle to place accurate constraints on the synchrotron spectral index. This is to be expected, given the lack of low-frequency channels below 40 GHz, and is consistent with expectations from the single-pixel component separation analysis presented in Jew et al (2019). For the CP(LC) validation set, we are able to place improved constraints on the synchrotron spectral index, with the variations in the synchrotron spectral index from region-to-region tracing the variations in the input spectral index map shown in Fig.…”

“…Given the simulated data sets considered here, fitting foreground spectral parameters with any parametric method will ultimately be limited by the number of frequency channels available with sufficient sensitivity. For example, in Jew et al (2019), it was found that to constrain models including the synchrotron curvature would require additional low-frequency channels between 5 and 30 GHz. Given a maximum observed frequency of ∼400 GHz, dust spectral parameters in the modified blackbody (MBB) model are also very poorly constrained Jew et al 2019).…”

“…For example, in Jew et al (2019), it was found that to constrain models including the synchrotron curvature would require additional low-frequency channels between 5 and 30 GHz. Given a maximum observed frequency of ∼400 GHz, dust spectral parameters in the modified blackbody (MBB) model are also very poorly constrained Jew et al 2019). This problem could be alleviated to some extent by considering model reparametrizations that yield more tractable posterior geometries.…”

“…Analysis in Jew et al (2019) showed that experiments such as LiteBIRD struggle to constrain synchrotron spectral parameters, with additional low-frequency data being necessary to begin to constrain the synchrotron spectral index. To constrain more complex synchrotron curvature models would require additional lowfrequency channels.…”

“…These modelling problems are very closely linked with the available data. If it becomes apparent that more complex modelling is required, more comprehensive data covering a wide range of frequencies will be required to constrain the additional model parameters (Jew et al 2019).…”