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

Abstract

We compile and analyse long term (~10 year) submillimetre (1.3, 0.87, 0.43\nmm, submm) wavelength light curves of the Galactic centre black hole,\nSagittarius A*. The 0.87 and 0.43 mm data are taken from the literature, while\nthe majority of the 1.3 mm light curve is from previously unpublished SMA and\nCARMA data. We use Monte Carlo simulations to show that on minute to few hour\ntime-scales the variability is consistent with a red noise process with a 230\nGHz power spectrum slope of 2.3+0.8-0.6 at 95% confidence. The light curve is\nde-correlated (white noise) on very long (month to year) times. In order to\nidentify the transition time between red and white noise, we model the light\ncurves as a stochastic damped random walk process. The models allow a\nquantitative estimate of this physical characteristic time-scale of 8-4+3 hours\nat 230 GHz at 95% confidence, with consistent results at 345 and 690 GHz. This\ncorresponds to ~10 orbital times or ~1 inflow (viscous) time at R = 3 Rs, a\ntypical radius producing the 230 GHz emission as measured by very long baseline\ninterferometry and found in theoretical accretion flow and jet models. This\ntime-scale is significantly shorter (longer) than those measured at radio\n(near-infrared, NIR) wavelengths, and is marginally inconsistent with the same\nvariability mechanism operating in the submm and NIR for the expected t ~ R^3/2\nscaling. It is crudely consistent with the analogous time-scale inferred in\nstudies of quasar optical light curves after accounting for the difference in\nemission radius. We find evidence that the submm variability persists at least\ndown to the ISCO, if not the event horizon. These results can be compared\nquantitatively with similar analyses at different wavebands to test for\nconnections between the variability mechanisms, and with light curves from\ntheoretical models of accreting black holes.\n