X-ray and optical light curves of Aql X-1. The start and end times of the four multi-wavelength observations listed in Table 1 are indicated with dotted lines. The start of each multi-wavelength observation is defined as the start time of the first observation within a set; for example, the start of the ATCA/VLT observations mark the start of obs 1/2a, respectively. Similarly, the end of each multi-wavelength observation is defined as the end time of the last observation within a set; for example, the end of the Swift/ALMA observations mark the end of obs 1/2a, respectively. Panel a: Swift/BAT 15–50 keV count rate. Panel b: Swift/XRT 0.3–10 keV count rate (count rates in PC mode could be affected by pile-up at count rates above ~1 cts s⁻¹). Panel c: Swift/XRT hardness ratio (1.5–10 keV/0.3–1.5 keV count rate). Panel d: Swift/(BAT/XRT) hardness ratio (15–50 keV/0.3–10 keV count rate). The hardness ratio is calculated for days on which there is a Swift/XRT point and a Swift/BAT point within one day. Panel e: Faulkes/LCO V and i^′ magnitudes.