Peng et al. present 12 observations of the accreting millisecond X-ray pulsar Aql X-1, conducted between August 2022 and October 2023 using the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) at 1250 MHz. These observations span both the quiescent and X-ray outburst states, as determined through X-ray data from the Neutron Star Interior Composition Explorer (NICER) and the Monitor of All-sky X-ray Image (MAXI). Periodicity and single-pulse searches were performed for each observation, yet no pulsed radio signals were detected. The obtained upper limit flux densities range from 2.86 to 5.73 μJy, setting the most stringent constraints to date. Several mechanisms that could explain the non-detection are explored, with one possibility being that Aql X-1 enters a radio-ejection state during quiescence, where surrounding material absorbs the potential radio pulses.

The image above displays the X-ray light curves of Aql X-1 for 2022 (left panel) and 2023 (right panel). Black dots represent NICER background-subtracted light curves in units of ct per second (0.5–10.0 keV, 16s resolution), while red dots correspond to MAXI light curves in units of photons per second per cm² (2–20 keV, 1-day resolution). The grey dashed vertical lines indicate the MJDs of the FAST observations conducted in both years.