Imaging an Event Horizon: Mitigation of Scattering Toward Sagittarius A*

Fish, Vincent L.; Johnson, Michael D.; Lu, Ru-Sen; Doeleman, Sheperd S.; Bouman, Katherine L.; Zoran, Daniel; Freeman, William T.; Psaltis, Dimitrios; Narayan, Ramesh; Pankratius, Victor; Broderick, Avery E.; Gwinn, C. R.; Vertatschitsch, Laura

doi:10.1088/0004-637x/795/2/134

Cited by 79 publications

(87 citation statements)

References 55 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Unfortunately, the resolution of our observations is not sufficient to probe structures in accretion flows that are expected to occur on scales 10 R S (cf., e.g., Broderick et al 2011); those observations will probably have to await the Event Horizon Telescope (cf. Fish et al 2014). When referring to radio galaxies for comparison, especially to M 87, which has a central black hole with small angular diameter (≈10 μas) second only to Sgr A*, a distance of tens of Schwarzschild radii appears to be critical for the formation of AGN jets.…”

Section: Discussionmentioning

confidence: 99%

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

Park

Trippe

Krichbaum

et al. 2015

A&A

View full text Add to dashboard Cite

The gas cloud G2 that falls toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is assumed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for a possible transient asymmetric structure -such as jets or winds from hot accretion flows -around Sgr A* that might be caused by accretion of material from G2. The interferometric closure phases remained zero within errors during the observation time. We therefore conclude that Sgr A* did not show significant asymmetric (in the observer frame) outflows in late 2013. Using simulations, we constrain the size of the outflows that we could have missed to ≈2.5 mas along the major axis and ≈0.4 mas along the minor axis of the beam, corresponding to approximately 232 and 35 Schwarzschild radii, respectively; we thus probe spatial scales on which the jets of radio galaxies are thought to convert magnetic into kinetic energy. Because probably less than 0.2 Jy of the flux from Sgr A* can be attributed to accretion from G2, the effective accretion rate is ηṀ 1.5 × 10 9 kg s −1 ≈ 7.7 × 10 −9 M ⊕ yr −1 for material from G2. Exploiting the relation of kinetic jet power to accretion power of radio galaxies shows that the rate of accretion of matter that is finally deposited in jets is limited toṀ 10 17 kg s −1 ≈ 0.5 M ⊕ yr −1 . Accordingly, G2 appears to be mostly stable against loss of angular momentum and subsequent (partial) accretion at least on timescales 1 yr.

show abstract

Section: Discussionmentioning

confidence: 99%

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

Park

Trippe

Krichbaum

et al. 2015

A&A

View full text Add to dashboard Cite

show abstract

“…The scattering is especially strong along the line of sight to the Galactic Center, and although interstellar scattering is subdominant to the intrinsic structure at l = 1.3 mm, it remains an important consideration for the EHT (see Fish et al 2014). Nevertheless, interstellar scattering is not likely to be an important consideration for our proposed methods.…”

Section: Effects From Interstellar Scatteringmentioning

confidence: 99%

“…Chief among these is synthesis imaging (see, e.g., Fish et al 2014;Lu et al 2014). Yet, with a mass of  M 4 10 6 (Ghez et al 2008;Gillessen et al 2009), Sgr A * has a gravitational timescale of only » GM c 20 s 3 and an orbital period at the innermost stable circular orbit of only 4-30 minutes, depending on spin (Bardeen et al 1972).…”

Section: Introductionmentioning

confidence: 99%

Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance

Johnson

Loeb

Shiokawa

et al. 2015

ApJ

Self Cite

View full text Add to dashboard Cite

We show that interferometry can be applied to study irregular, rapidly rotating structures, as are expected in the turbulent accretion flow near a black hole. Specifically, we analyze the lagged covariance between interferometric baselines of similar lengths but slightly different orientations. For a flow viewed close to face-on, we demonstrate that the peak in the lagged covariance indicates the direction and angular velocity of the emission pattern from the flow. Even for moderately inclined flows, the covariance robustly estimates the flow direction, although the estimated angular velocity can be significantly biased. Importantly, measuring the direction of the flow as clockwise or counterclockwise on the sky breaks a degeneracy in accretion disk inclinations when analyzing timeaveraged images alone. We explore the potential efficacy of our technique using three-dimensional, general relativistic magnetohydrodynamic simulations, and we highlight several baseline pairs for the Event Horizon Telescope (EHT) that are well-suited to this application. These results indicate that the EHT may be capable of estimating the direction and angular velocity of the emitting material near Sgr A * , and they suggest that a rotating flow may even be utilized to improve imaging capabilities.

show abstract

“…With ultra-high angular resolution (up to ∼20 μas) and sufficient sensitivity, the forthcoming millimeter/sub-millimeter VLBI observations at  l 1.3 mm (  n 230 GHz), from the Event Horizon Telescope (EHT), should obtain the first image of such a shadow region (Fish et al 2014;Lu et al 2016). The emission from the accretion flow in the vicinity of the black hole, before reaching a distant observer, will experience considerable energy shifts due to both the motions of the emitting fluid medium and the strong gravity near the black hole (Broderick & Blandford 2004;Younsi et al 2012).…”

Section: Introductionmentioning

confidence: 99%

The Effects of Accretion Flow Dynamics on the Black Hole Shadow of Sagittarius A*

Akiyama

Asada

2016

ApJ

View full text Add to dashboard Cite

A radiatively inefficient accretion flow (RIAF), which is commonly characterized by its sub-Keplerian nature, is a favored accretion model for the supermassive black hole at the Galactic center, Sagittarius A * . To investigate the observable features of an RIAF, we compare the modeled shadow images, visibilities, and spectra of three flow models with dynamics characterized by (i) a Keplerian shell that is rigidly rotating outside the innermost stable circular orbit (ISCO) and infalling with a constant angular momentum inside ISCO, (ii) a sub-Keplerian motion, and (iii) a free-falling motion with zero angular momentum at infinity. At near-millimeter wavelengths, the emission is dominated by the flow within several Schwarzschild radii. The energy shift due to these flow dynamics becomes important and distinguishable, suggesting that the flow dynamics are an important model parameter for interpreting the millimeter/sub-millimeter very long baseline interferometric observations with the forthcoming, fully assembled Event Horizon Telescope (EHT). As an example, we demonstrate that structural variations of Sagittarius A * on event horizon-scales detected in previous EHT observations can be explained by the nonstationary dynamics of an RIAF.

show abstract

Imaging an Event Horizon: Mitigation of Scattering Toward Sagittarius A*

Cited by 79 publications

References 55 publications

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance

The Effects of Accretion Flow Dynamics on the Black Hole Shadow of Sagittarius A*

Contact Info

Product

Resources

About