Millimeter Light Curves of Sagittarius A* Observed during the 2017 Event Horizon Telescope Campaign

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 mm 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 minute and several hours. Our results indicate a steepening of the PSD slope for timescales shorter than 0.3 hr. 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.

Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI

“…The light curve of Sgr A * during the observing nights throughout the 2017 EHT campaign is dominated by fluctuations on the longest timescales (Paper V; Wielgus et al 2022). This is consistent with other studies that cover much longer time spans, which suggest a potential break at a timescale between 2 and 8 hr (Dexter et al 2014;Witzel et al 2018Witzel et al , 2021.…”

“…Typically, there is not a single variability timescale, but rather a variability spectrum, with some components of the variability appearing well within the observing window and others extending beyond it. This multitude of timescales is evident, e.g., in the variability properties of the light curve of Sgr A * during the 2017 EHT observations (Wielgus et al 2022) and suggested by the GRMHD simulations described in the previous section (Georgiev et al 2022). More important is the presence of correlations between measured visibilities, which will be present even in the absence of spatial correlations in the image.…”

“…To date, all sources observed with the EHT during its 2017 campaign have been spatially resolved (Kim et al 2020;Janssen et al 2021), and many have exhibited structural variability across days. The most extreme of these variable sources is Sgr A * , which exhibits substantial variability across the electromagnetic spectrum (see, e.g., Boyce et al 2019;Witzel et al 2021;Wielgus et al 2022, and references therein) and on horizon scales (see, e.g., Fish et al 2011;Gravity Collaboration et al 2018). The mass of Sgr A * is M ≈ 4 × 10 6 M e (Do et al 2019;Gravity Collaboration et al 2019, corresponding to a gravitational timescale of t g = GM/c 3 ≈ 20 s, which is much shorter than the ∼10 hr duration of a typical EHT observing track.…”

A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows

“…Below a break that lies between 2 and 8 hr, the light-curve variability is consistent with a red-noise process, having a power-law spectrum that is dominated by the longest timescales. At 1.3 mm wavelengths, the typical degree of variability is observed to be less than 10% during the 2017 EHT observing campaign (Wielgus et al 2022). The EHT observations of Sgr A * imply a similar degree of variability in the source structure (Paper IV).…”

“…Additionally, two of the EHT sites, the Atacama Large Millimeter/submillimeter Array (ALMA) and the Submillimeter Array (SMA), are themselves connected element interferometric arrays, probing linear scales of ∼10 6 Schwarzschild radii. While the phased-up signal is used for the VLBI EHT observations (Goddi et al 2019;Paper II), the internal correlations between the individual telescopes can be used to generate the total flux light curves, representing the time-dependent total flux of the compact Sgr A * source (Wielgus et al 2022), unresolved at spatial scales of ∼100 kλ. These observations effectively correspond to ( ) t 0, 0, for a compact Sgr A * source.…”