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 mm, 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0.87 and 0.43 mm data are taken from the literature, while the majority of the 1.3 mm light curve is from previously unpublished SMA and CARMA data. We use Monte Carlo simulations to show that on minute to few hour time-scales the variability is consistent with a red noise process with a 230 GHz power spectrum slope of β = 2.3 +0.8 −0.6 at 95% confidence. The light curve is de-correlated (white noise) on very long (month to year) times. In order to identify the transition time between red and white noise, we model the light curves as a stochastic damped random walk process. The models allow a quantitative estimate of this physical characteristic time-scale of τ = 8 +3 −4 hours at 230 GHz at 95% confidence, with consistent results at 345 and 690 GHz. This corresponds to ≈ 10 orbital times or ≈ 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 significantly shorter (longer) than those measured at radio (near-infrared) wavelengths, and is marginally inconsistent with the same variability mechanism operating in the submm and NIR for the expected t ∝ R 3/2 scaling. It is crudely 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 ISCO, 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.

A Black Hole Mass-Variability Timescale Correlation at Submillimeter Wavelengths

“…Dexter et al (2014) at 230 GHz at 95% confidence, with consistent results at higher frequencies. This timescale is an order of magnitude larger than the period of the last stable orbit of a non-rotating black hole, which is most easily understood as resulting from accretion processes on a scale of a few to R 10 .…”

“…Both structure functions show increases in activity on short timescales followed by a plateau, similar to those of Sgr A * (Dexter et al 2014), but markedly different than those of typical calibrators in the database. For instance, the structure functions for 3C 84 and 279 rise smoothly without any apparent saturation timescale for t 10 year.…”

“…This model has been shown to successfully describe the optical light curves of quasars (Kelly et al 2009;MacLeod et al 2010), as well as the submm light curve of Sgr A * (Dexter et al 2014).…”

“…Kelly et al (2009) and Dexter et al (2014) used a Metropolis-Hastings algorithm to sample the likelihood over the model parameter space. We use this approach to measure the model parameters in cases where the upper limit to τ is smaller than the light curve duration T. However, this is not the case for the vast majority of the SMA calibrator light curves.…”

Time Domain Filtering of Resolved Images of Sgr A∗

“…Do et al 2009;Meyer et al 2009;Dexter et al 2014) and flaring emission (e.g. Baganoff et al 2001;Genzel et al 2003;Hornstein et al 2007;Eckart et al 2008;Marrone et al 2008;Porquet et al 2008;Dodds-Eden et al 2009;Yusef-Zadeh et al 2009;Trap et al 2011;Haubois et al 2012;Neilsen et al 2013).…”