A power-law break in the near-infrared power spectrum of the Galactic center black hole

TL;DR

This study discovers a characteristic near-infrared variability timescale in Sgr A* that supports a linear mass scaling relation, challenging recent models linking timescale to luminosity and suggesting state-dependent scaling in black hole accretion.

Contribution

The paper provides the first near-infrared measurement of the characteristic timescale in Sgr A*, demonstrating linear mass scaling and proposing state-dependent variability relations.

Findings

Detected a 154-minute timescale in near-infrared variability.

Found inconsistency with luminosity-based scaling relations.

Supported linear mass scaling in the hard state of black holes.

Abstract

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.