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We compile and analyse long term (~10 year) submillimetre (1.3, 0.87, 0.43mm, submm) wavelength light curves of the Galactic centre black hole,Sagittarius A*. The 0.87 and 0.43 mm data are taken from the literature, whilethe majority of the 1.3 mm light curve is from previously unpublished SMA andCARMA data. We use Monte Carlo simulations to show that on minute to few hourtime-scales the variability is consistent with a red noise process with a 230GHz power spectrum slope of 2.3+0.8-0.6 at 95% confidence. The light curve isde-correlated (white noise) on very long (month to year) times. In order toidentify the transition time between red and white noise, we model the lightcurves as a stochastic damped random walk process. The models allow aquantitative estimate of this physical characteristic time-scale of 8-4+3 hoursat 230 GHz at 95% confidence, with consistent results at 345 and 690 GHz. Thiscorresponds to ~10 orbital times or ~1 inflow (viscous) time at R = 3 Rs, atypical radius producing the 230 GHz emission as measured by very long baselineinterferometry and found in theoretical accretion flow and jet models. Thistime-scale is significantly shorter (longer) than those measured at radio(near-infrared, NIR) wavelengths, and is marginally inconsistent with the samevariability mechanism operating in the submm and NIR for the expected t ~ R^3/2scaling. It is crudely consistent with the analogous time-scale inferred instudies of quasar optical light curves after accounting for the difference inemission radius. We find evidence that the submm variability persists at leastdown to the ISCO, if not the event horizon. These results can be comparedquantitatively with similar analyses at different wavebands to test forconnections between the variability mechanisms, and with light curves fromtheoretical models of accreting black holes.

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