Analysis of Polarized Dust Emission from the First Flight of the SPIDER Balloon-Borne Telescope

Using data from the first flight of SPIDER and from Planck HFI, we probe the properties of polarized emission from interstellar dust in the SPIDER observing region. Component separation algorithms operating in both the spatial and harmonic domains are applied to probe their consistency and to quantify modeling errors associated with their assumptions. Analyses spanning the full SPIDER region demonstrate that i) the spectral energy distribution of diffuse Galactic dust emission is broadly consistent with a modified-blackbody (MBB) model with a spectral index of \(\beta_\mathrm{d}=1.45\pm0.05\) \((1.47\pm0.06)\) for \(E\) (\(B\))-mode polarization, slightly lower than that reported by Planck for the full sky; ii) its angular power spectrum is broadly consistent with a power law; and iii) there is no significant detection of line-of-sight decorrelation of the astrophysical polarization. The size of the SPIDER region further allows for a statistically meaningful analysis of the variation in foreground properties within it. Assuming a fixed dust temperature \(T_\mathrm{d}=19.6\) K, an analysis of two independent sub-regions of that field results in inferred values of \(\beta_\mathrm{d}=1.52\pm0.06\) and \(\beta_\mathrm{d}=1.09\pm0.09\), which are inconsistent at the \(3.9\,\sigma\) level. Furthermore, a joint analysis of SPIDER and Planck 217 and 353 GHz data within a subset of the SPIDER region is inconsistent with a simple MBB at more than \(3\,\sigma\), assuming a common morphology of polarized dust emission over the full range of frequencies. These modeling uncertainties have a small--but non-negligible--impact on limits on the cosmological tensor-to-scalar ratio derived from the \spider dataset. The fidelity of the component separation approaches of future CMB polarization experiments may thus have a significant impact on their constraining power.