Simulation and Data Processing of Beamforming Experiments with Four 21CMA Stations

Abstract

We present an end-to-end simulation and data-processing framework for digital beamforming experiments conducted with four stations of the 21 Centimeter Array (21CMA). Motivated by the need to characterize instrumental systematics, such as those arising from station-level digital beam synthesis and two-stage channelization, and to validate the data-processing pipeline framework for a future upgraded 21CMA with beamforming capability across all stations, we simulate interferometric visibilities using realistic four-station layouts with radio interferometer simulation software OSKAR. Two representative pointings are considered: a bright, complex Cassiopeia A field and a near-north celestial pole (NCP) calibration field. The sky model combines cataloged point sources with a diffuse Galactic component from the Global Sky Model (GSM), and frequency-dependent thermal noise is injected. We further quantify the imprint of two-stage channelization by comparing an ideal beamformer with a coarse-channel phase approximation, demonstrating that off-axis sources exhibit a characteristic piecewise-linear spectral modulation across coarse-channel boundaries. A data-processing pipeline, including Radio Frequency Interference (RFI) mitigation, calibration, imaging, and mosaicking steps consistent with current low-frequency radio astronomy practice, is constructed. The resulting synthetic images and background root-mean-square (RMS) noise measurements demonstrate the feasibility of adapting established 21CMA calibration and imaging strategies to digital beamforming modes, and provide a framework that can be further developed for beam-aware processing in future full-scale 21CMA beamforming observations.

Figures (8)

• Figure 1: Simulated 21CMA station beam patterns at 100 MHz for the two target fields used in this work. The beams are computed for the four–station sub–array (E13, E03, W02 and W09) and are normalized to unity at the respective phase centers.
• Figure 2: Simulated 24-hour $uv$ coverages at 100 MHz for the NCP and Cassiopeia A fields, using the four 21CMA stations E13, E03, W02 and W09. Baselines are plotted in units of observing wavelength.
• Figure 3: Input point-source sky models used for the simulated 21CMA observations. The panels show the distribution of cataloged radio sources in equatorial coordinates (J2000) for the Cassiopeia A field (left; 5$^\circ$ field of view) and the near--north celestial pole calibration field (right; 8$^\circ$ field of view). The source flux is encoded by color. A diffuse Galactic component from a GSM realization and thermal noise are also included in the simulations but are not shown here.
• Figure 4: Two-stage channelization DBF imprint on the station power response for the NCP and Cas A pointings using the measured 127-antenna station layout. Curves show $10\log_{10}g(\nu)$ (dB) over a 5 MHz band: dashed lines correspond to the ideal case ($\nu'=\nu$), and solid lines to the two-stage approximation where the beamforming phase is evaluated at a coarse-channel center frequency $\nu'$ that is held fixed within each coarse channel ($B=781.25$ kHz; faint vertical lines mark coarse-channel boundaries). Top: on-axis source. Bottom: source offset by $2^{\circ}$ towards zenith.
• Figure 5: Per-channel RMS noise as a function of frequency for the 21CMA observations.