Detection of Intrinsic Source Structure at ∼3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*

Abstract

We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A*, at 1.3 mm (230 GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional uv coverage in the N–S direction, and leads to a spatial resolution of ∼30 μas (∼3 Schwarzschild radii) for Sgr A*. The source is detected even at the longest baselines with visibility amplitudes of ∼4%–13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of ∼3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3 mm VLBI observations with an expanded array and better sensitivity will allow more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.

Metrics and Motivations for Earth–Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope

“…Further, the current EHT array is missing short baseline coverage, and an orbit that never crosses the face of the Earth as seen from Sgr A* would form short baselines primarily with sites that had just come into view; these sites would be looking through the largest possible amount of atmosphere, and thus short-baseline coverage would be less sensitive than for other orbital orientations. Baselines from the ground to a single "face-on" orbiter still have thermal noise approximately equal to those between ground sites as shown in Figure 3, and significantly lower than the long-baseline flux densities of Sgr A* at 230 GHz (Lu et al 2018), suggesting that even long-baseline observations with orbiters will produce detections.…”

“…We choose a desired thermal noise of 10 mJy based on longbaseline (∼ 7 Gλ) correlated flux densities of tenths of Janskys observed for Sgr A* (Lu et al 2018). This approximate mean sensitivity over a full observing track yields a required τ min ≈ 1 second for space-ALMA baselines.…”

“…We compare these values with the relevant temporal and angular scales for Sgr A* and M87. For each source, we assume a conservative FOV of 180 µas, which is a small factor larger than the Gaussian image fullwidth at half of maximum inferred from previous EHT observations of each source (Doeleman et al 2008;Fish et al 2011;Doeleman et al 2012;Johnson et al 2015;Akiyama et al 2015;Lu et al 2018). To bracket the representative timescales of each source, we use the ISCO period at zero spin (P a=0 = 12 √ 6πt G ≈ 92.3t G ) and at maximal spin (P a=1 = 4πt G ≈ 12.6t G ) for the gravitational time t G = GM/c 3 and black hole mass M. For the maximal representative angular scale, we use the expected Schwarzschild shadow diameter (2 √ 27r G /D ≈ 10.4r G /D, where D is the distance from the observer to the black hole and r G = GM/c 2 ).…”

First M87 Event Horizon Telescope Results. II. Array and Instrumentation

“…For SgrA * , the black hole candidate at the Galactic center with a presumed mass of M 4 10 6  (Balick & Brown 1974;Ghez et al 2008;Genzel et al 2010;GRAVITY Collaboration et al 2018a), the 1.3 mm emission has been measured to have a size of 3.7 R s (Doeleman et al 2008;Fish et al 2011). More recently, full polarimetric VLBI observations at 1.3 mm wavelength have revealed ordered and time-variable magnetic fields within SgrA * on horizon scales (Johnson et al 2015), and extension to longer baselines has confirmed compact structure on ∼3 R s scales (Lu et al 2018). These results, obtained with three-and four-site VLBI arrays consisting of the former Combined Array for Research in Millimeter-wave Astronomy (CARMA) in California, the Submillimeter Telescope (SMT) in Arizona, the James Clerk Maxwell Telescope (JCMT) and Submillimeter Array (SMA) facilities on Maunakea in Hawaii, and the Atacama Pathfinder Experiment (APEX) telescope in Chile, demonstrated that direct imaging of emission structures near the event horizon of SMBH candidates is possible in principle.…”

A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

“…It has long been a target of radio very long baseline interferometry (VLBI, Lo et al 1975;Backer 1978;Krichbaum et al 1998;Bower et al 2006). With 1.3mm VLBI, the source size is as compact as 8 − 10r g (Doeleman et al 2008;Fish et al 2011;Lu et al 2018;Johnson et al 2018), where r g = GM/c 2 6 × 10 11 cm is the gravitational radius.…”