A power-law break in the near-infrared power spectrum of the Galactic
  center black hole
  Proposed scaling relations of a characteristic timescale in the X-ray power
spectral density of galactic and supermassive black holes have been used to
argue that the accretion process is the same for small and large black holes.
Here, we report on the discovery of this timescale in the near-infrared
radiation of Sgr A*, the 4x10^6 solar mass black hole at the center of our
Galaxy, which is the most extreme sub-Eddington source accessible to
observations. Previous simultaneous monitoring campaigns established a
correspondence between the X-ray and near-infrared regime and thus the
variability timescales are likely identical for the two wavelengths. We
combined Keck and VLT data sets to achieve the necessary dense temporal
coverage, and a time baseline of four years allows for a broad temporal
frequency range. Comparison with Monte Carlo simulations is used to account for
the irregular sampling. We find a timescale at 154 (+124 -87) min (errors mark
the 90% confidence limits) which is inconsistent with a recently proposed
scaling relation that uses bolometric luminosity and black hole mass as
parameters. However, our result fits the expected value if only linear scaling
with black hole mass is assumed. We suggest that the luminosity-mass-timescale
relation applies only to black hole systems in the soft state. In the hard
state, which is characterized by lower luminosities and accretion rates, there
is just linear mass scaling, linking Sgr A* to hard state stellar mass black
holes.
