Addressing Synchrotron Challenges for CMB Observations: ELFS-SA Collaboration for Robust Foreground Removal

TL;DR

This work investigates how adding low-frequency measurements (6–20 GHz) via the ELFS/SA project to a southern-hemisphere CMB experiment improves modeling of Galactic synchrotron foregrounds. Using Bayesian, pixel-based component separation and a Namaster-based cosmological likelihood, the study shows that ELFS/SA data tighten the synchrotron SED (β_s and curvature c_s) and reduce foreground residuals, thereby mitigating biases in the tensor-to-scalar ratio $r$ that would arise from complex synchrotron spectra. Across multiple synchrotron scenarios, the combined data reduce large-scale residuals and enable more reliable $r$ constraints, though gains depend on the true SED complexity and analysis strategy. These results support extending CMB frequency coverage to low frequencies for robust PGW detection and offer improved insights into Galactic foregrounds and the ISM.

Abstract

Upcoming cosmic microwave background (CMB) experiments aim to detect primordial gravitational waves with unprecedented sensitivity. Effective foreground removal is essential to avoid biases in the measurement of the tensor-to-scalar ratio ($r$) in this high-precision regime. Recent analyses highlight the unexpected complexity of synchrotron emission at low frequencies, underscoring the need for more sensitive low-frequency data. To address this challenge, the European Low-Frequency Survey (ELFS) initiative and the Simons Array collaboration propose installing two European low-frequency receivers on one of the Simons Array telescopes. These receivers will enable measurements in the Southern Hemisphere between $6$ and $20$,GHz, complementary to those of current and proposed experiments targeting the measurement of cosmological gravitational waves. In this work, we study the benefits of combining these low-frequency observations with a representative future CMB experiment operating from the Southern Hemisphere. We find that the extra information can improve the knowledge of the underlying synchrotron spectral energy distribution (SED), with positive impacts on the robustness of measurement of the tensor-to-scalar ratio, $r$, against the complexity of low-frequency foregrounds.

Figures (15)

• Figure 1: Synchrotron SED used in the intensity component separation analysis, computed from GALPROP synch_galprop_model (blue), and the best-fit SED obtained after Planck's Commander component separation allowing for a frequency shift (orange) planck_int_diff_fg.
• Figure 2: Schematic diagram of the ELFS/SA Xband receiver.
• Figure 3: The ELFS/SA Xband feedhorn. Top-left: the feedhorn design. Top-right: simulated return loss. Bottom panels: the simulated co-polar and cross-polar beams.
• Figure 4: The ELFS/SA Xband optical system performance. Left: ray-tracing of the feedhorn at the focus of the SA telescope. Right: simulated beam pattern of the entire optical system.
• Figure 5: The ELFS/SA OMT. Left: the fabricated OMT. Right: measured OMT performance. Top-left and bottom-right: return loss at the output ports. Top-right and bottom-left: cross-coupling between the output ports. The white area highlights the working band.