Millimeter light curves of Sagittarius A* observed during the 2017 Event Horizon Telescope campaign

TL;DR

This study presents high-cadence millimeter wavelength light curves of Sagittarius A* during the 2017 Event Horizon Telescope campaign, revealing variability patterns, spectral properties, and the impact of an X-ray flare on source behavior.

Contribution

It provides unprecedented millimeter wavelength variability data of Sgr A*, analyzes its power spectral density, and assesses calibration uncertainties, advancing understanding of the black hole's emission processes.

Findings

Light curves show predominantly low variability, with increased activity after the X-ray flare.

PSD slope between -2 and -3 indicates red noise behavior on minute to hour timescales.

No time-lags between 213 and 229 GHz suggest low optical depth at event horizon scales.

Abstract

The Event Horizon Telescope (EHT) observed the compact radio source, Sagittarius A* (Sgr A*), in the Galactic Center on 2017 April 5-11 in the 1.3 millimeter wavelength band. At the same time, interferometric array data from the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array were collected, providing Sgr A* light curves simultaneous with the EHT observations. These data sets, complementing the EHT very-long-baseline interferometry, are characterized by a cadence and signal-to-noise ratio previously unattainable for Sgr A* at millimeter wavelengths, and they allow for the investigation of source variability on timescales as short as a minute. While most of the light curves correspond to a low variability state of Sgr A*, the April 11 observations follow an X-ray flare, and exhibit strongly enhanced variability. All of the light curves are consistent with a red noise process, with a power spectral density (PSD) slope measured to be between -2 and -3 on timescales between 1 min and several hours. Our results indicate a steepening of the PSD slope for timescales shorter than 0.3 h. The spectral energy distribution is flat at 220 GHz and there are no time-lags between the 213 and 229 GHz frequency bands, suggesting low optical depth for the event horizon scale source. We characterize Sgr A*'s variability, highlighting the different behavior observed just after the X-ray flare, and use Gaussian process modeling to extract a decorrelation timescale and a PSD slope. We also investigate the systematic calibration uncertainties by analyzing data from independent data reduction pipelines.