A 21-cm power spectrum at 48 MHz, using the Owens Valley Long Wavelength Array

The Large-aperture Experiment to Detect the Dark Age (LEDA) was designed to measure brightness temperature fluctuations in the Cosmic Microwave Background due to 21-cm absorption/emission by neutral hydrogen in the early Universe. Measurements of 21-cm fluctuations will provide constraints on astrophysical processes during Cosmic Dawn ($z \approx 15-30$). LEDA uses observations made with the Owens Valley Long Wavelength Array (OVRO-LWA) in California, a compact imaging interferometer. We present a power spectrum of observations at 48 MHz ($z \approx 28$), and an analysis of the sensitivity of the OVRO-LWA telescope. Recently, Eastwood et al. published a power spectrum of OVRO-LWA observations generated from m-mode analysis, using statistical techniques to remove foregrounds. We use a method that isolates foreground power in a "wedge" in the power spectrum; outside of the wedge, in the "Cosmic Dawn" window, 21-cm fluctuations may be detected. Using 4 hrs of incoherently integrated OVRO-LWA observations, we measure a power of $\Delta^2(k) \approx 2 \times 10^{12}$ mK$^2$ in the Cosmic Dawn window. Given that a value of $\Delta^2(k) \approx 100$ mK$^2$ is predicted from theory, we conclude that our power spectrum is dominated by telescope thermal noise and systematic effects. By modelling the OVRO-LWA thermal noise, we show that OVRO-LWA has sufficient sensitivity for a 21-cm detection if 3000 hrs of observations are integrated using a mix of incoherent and coherent averaging. We show that OVRO-LWA will then be able to detect theoretically predicted Ly-$\alpha$ and X-ray peaks in 21-cm power spectra at Cosmic Dawn, according to theoretical models.