Preprint · arXiv · November 2, 2026
Estimating LuSEE-Night's Sensitivity to Cα Radio Recombination Lines
Fatima Yousuf, Aurelia Gerber, Zack Li, Akshatha Vydula, Stuart D. Bale, David W. Barker
Abstract
Carbon radio recombination lines (CRRLs) arising from the cold neutral medium (CNM) of the interstellar medium are potential foregrounds in the 1-50 MHz band that LuSEE-Night will observe from the lunar far side. CRRLs are sensitive probes of the temperature, density, and radiation field of the CNM, and LuSEE-Night will provide sky-averaged measurements in a poorly explored frequency range. We model CRRLs across this band, accounting for doppler, collisional, and radiation broadening, and we estimate line strengths using two independent methods - fixing either the peak or the frequency-integrated optical depth - applied both uniformly across the sky and with region-dependent values informed by prior low-frequency observations. We simulate LuSEE-Night's expected response to these CRRLs and further convolve the beam-diluted spectra with LuSEE-Night's polyphase-filter-bank spectrometer response, including its zoomed-mode frequency resolution of $\sim$390 Hz, to obtain the expected observed brightness temperature. Using these simulated spectra, we estimate the integration time required to detect individual CRRLs at SNR=5 across the six sky models considered. Required integration times range from as short as 30 hours (in the most optimistic model where the optical depth across the sky is $\sim$10$^{-3}$) to 27.6 years (in the most conservative models where the optical depth across the sky is $\sim$10$^{-4}$). Stacking the 361 simulated C$α$ lines between 10 and 50 MHz reduces the integration time needed for an SNR=5 detection to $\sim$11 minutes and $\sim$151 hours respectively, indicating that LuSEE-Night has realistic chances of making a stacked detection of C$α$ RRLs within a realistic mission lifetime, even under the most conservative optical-depth assumptions.