The Intrinsic Structure of Sagittarius A* at 1.3 cm and 7 mm

Abstract

Sagittarius A* (Sgr A*), the Galactic Center supermassive black hole (SMBH), is one of the best targets in which to resolve the innermost region of an SMBH with very long baseline interferometry (VLBI). In this study, we have carried out observations toward Sgr A* at 1.349 cm (22.223 GHz) and 6.950 mm (43.135 GHz) with the East Asian VLBI Network, as a part of the multiwavelength campaign of the Event Horizon Telescope (EHT) in 2017 April. To mitigate scattering effects, the physically motivated scattering kernel model from Psaltis et al. (2018) and the scattering parameters from Johnson et al. (2018) have been applied. As a result, a single, symmetric Gaussian model well describes the intrinsic structure of Sgr A* at both wavelengths. From closure amplitudes, the major-axis sizes are ∼704 ± 102 μas (axial ratio ∼ 1.19 − 0.19 + 0.24 ) and ∼300 ± 25 μas (axial ratio ∼1.28 ± 0.2) at 1.349 cm and 6.95 mm, respectively. Together with a quasi-simultaneous observation at 3.5 mm (86 GHz) by Issaoun et al. (2019), we show that the intrinsic size scales with observing wavelength as a power law, with an index ∼1.2 ± 0.2. Our results also provide estimates of the size and compact flux density at 1.3 mm, which can be incorporated into the analysis of the EHT observations. In terms of the origin of radio emission, we have compared the intrinsic structures with the accretion flow scenario, especially the radiatively inefficient accretion flow based on the Keplerian shell model. With this, we show that a nonthermal electron population is necessary to reproduce the source sizes.

First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration

“…The substructure manifests in the visibility domain as "refractive noise," which is an additive complex noise component with broad correlation structure across baselines and time (Johnson & Narayan 2016). Using observations of Sgr A * from 1.3 mm to 30 cm, Johnson et al (2018) have shown that the combined image broadening and substructure strongly constrain the power spectrum of density fluctuations, which is consistent with later observations that occurred close to the 2017 EHT observations of Sgr A * described in this work (Sections 4.1.1 and 4.1.2) and in Issaoun et al (2019bIssaoun et al ( , 2021 and Cho et al (2022).…”

“…The measured flux densities from Sgr A * at these frequencies are 1.07 ± 0.11 Jy and 1.35 ± 0.14 Jy, respectively (Table 2). Two of the observations on 2017 April 3 and 4 are (quasi-)simultaneous with the Global 3 mm VLBI Array observations (Section 4.1.2), as well as the EHT sessions (Cho et al 2022). These measurements provide an estimated size and flux density of Sgr A * at 1.3 mm via extrapolation of power-law models (i.e., the intrinsic size scales with observing wavelength as a power law with an index of ∼1.2 ± 0.2).…”

“…These variations introduce substructure in the image that is not intrinsic to the source (Goodman & Narayan 1989;Johnson & Gwinn 2015;Johnson & Narayan 2016;Psaltis et al 2018). Scatteringinduced substructure was first discovered in images of Sgr A * at 1.3 cm by Gwinn et al (2014) and later observed at other wavelengths (Johnson et al 2018;Issaoun et al 2019bIssaoun et al , 2021Cho et al 2022), giving additional constraints on the scattering properties of Sgr A * . This substructure is caused by modes in the scattering material on scales comparable to the image extent, so scattering models with identical scatter broadening may still exhibit strong differences in their scattering substructure.…”

Applications of the Source-Frequency Phase-Referencing Technique for ngEHT Observations

“…According to their results, a clear difference in the core shift between the two scenarios is shown. Especially at a small inclination angle, as has been suggested in recent studies [20][21][22], the expected core shift at 22-43 GHz is 10 µas in the accretion disk model while it is 100 µas in the jet model. Our preliminary core-shift measurements with the Korean VLBI Network (KVN) and the Very Long Baseline Array (VLBA) at the same frequencies show ∼100 µas (I.…”

Magnetic Reconnection in Black Hole Magnetospheres: Lepton Loading into Jets, Superluminal Radio Blobs, and Multiwavelength Flares

“…We find that F radio for Sgr A * is ∼0.1 mJy, which may be challenging to detect. VLBI images on Sgr A * are likely affected by the refractive scattering effect in the foreground plasma, which effectively adds 1-10 mJy noises to the data (Cho et al 2022). Brinkerink et al (2019) reported a possible blob-like feature in their 86 GHz VLBI Sgr A * image with a flux density of ∼10 mJy.…”