The Cycles of Alpha Centauri: Double Dipping of AB

In mid-2023, the Sunlike dwarfs of nearby alpha Centauri (HD 128620 ("A"): G2 V; HD 128621 ("B"): K1 V; hereafter "AB") coincidentally both entered coronal (T approximate to 1-3 MK) low states in their long-term X-ray cycles, as captured by the Chandra Observatory and its High Resolution Camera (HRC-I). The assessment benefited from a sensitivity degradation model for HRC-I based on a "check star," the F subgiant Procyon (alpha Canis Minoris A; HD 61421: F5 IV-V), further validated by Hubble Space Telescope time series of the Fe xii 1241.985 angstrom and 1349.396 angstrom coronal forbidden lines (T approximate to 1.6 MK) of all three stars. The AB starspot-cycle periods noted previously, 19 and 8 yr, appear to be holding. These deviate substantially from the 11 yr solar example, despite only modest differences in the stellar properties. The new cycle-minimum L X/L BOL of alpha Cen A is similar to that of the previous minimum observed by Chandra in 2005-2010, and close to solar-MIN X-ray levels in 2009 and 2019, with implications for the "basal" coronal flux of low-activity early G stars. The recent alpha Cen B X-ray MIN is similar to that in 2016 but higher than an earlier one in 2007-2009. Significantly, the B X-ray minima (in L X/L BOL) are comparable to the solar Cycle 24 maximum, suggesting plentiful starspots at the B-cycle MIN (contrary to the solar case). Tangentially, the current AB low states favor Doppler-reflex detections of planets, as opposed to the coronal high states, when elevated chromospheric activity can add nuisance signals to radial velocity time series.