Distinguish whether 137.05 MHz detections are a distinct downlink or spillover from 137.50 ± 0.46 MHz

Determine whether the narrowband signals detected at 137.05 MHz (FWHM ≈ 13 kHz) from Starlink version 2 mini Ku-band and version 1.5 satellites constitute a separate downlink frequency from the previously reported 137.50 ± 0.46 MHz transmissions, or whether the apparent 137.05 MHz detections arise from spectral spillover caused by the extreme signal brightness into adjacent frequency channels.

Background

In this survey, the Engineering Development Array 2 (EDA2) detected strong narrowband signals pulsing every 100 seconds at 137.05 MHz with FWHM ≈ 13 kHz, reaching up to 7.7 MJy/beam. Prior work (Grigg et al. 2023) reported similar pulsing behavior around 137.50 ± 0.46 MHz but lacked the fine spectral resolution to classify the signals definitively as narrowband.

The proximity of 137.05 MHz to the previously reported 137.50 ± 0.46 MHz raises ambiguity: either there are two distinct downlink frequencies used by Starlink satellites, or the very bright transmissions at 137.50 ± 0.46 MHz spill into adjacent channels and manifest at 137.05 MHz in this dataset. Resolving this ambiguity is necessary to accurately attribute and mitigate Starlink downlinks at SKA-Low frequencies.