An 8 h characteristic time-scale in submillimetre light curves of Sagittarius A

We compile and analyse long-term (a parts per thousand 10 yr) submillimetre (submm - 1.3, 0.87, 0.43 mm) wavelength light curves of the Galactic Centre black hole, Sagittarius A*. The 0.87 and 0.43 mm data are taken from the literature, while the majority of the 1.3 mm light curve is from previously unpublished SMA and CARMA data. We show that on minute to a few hour time-scales, the variability is consistent with a red noise process with a 230 GHz power-spectrum slope of beta = 2.3(-0.6)(+0.8) at 95 per cent confidence. The light curve is decorrelated (white noise) on long (month to year) times. We measure a transition time between red and white noise of tau = 8(-4)(+3) h at 230 GHz at 95 per cent confidence, with consistent results at 345 and 690 GHz. This corresponds to a parts per thousand 10 orbital times or a parts per thousand 1 inflow (viscous) time at R = 3R(s), a typical radius producing the 230 GHz emission as measured by very long baseline interferometry and found in theoretical accretion flow and jet models. This time-scale is shorter (longer) than those measured by some analyses of radio (near-infrared) light curves. It is roughly consistent with the analogous time-scale inferred in studies of quasar optical light curves after accounting for the difference in emission radius. We find evidence that the submm variability persists at least down to the innermost stable circular orbit, if not the event horizon. These results can be compared quantitatively with similar analyses at different wavebands to test for connections between the variability mechanisms, and with light curves from theoretical models of accreting black holes.