Event Horizon Telescope observations of Sgr A*

and the future of black hole imaging

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Freek Roelofs

Center for Astrophysics | Harvard & Smithsonian

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The EHT Collaboration

300+ members, 60 institutes, 20 countries in Europe, Asia, Africa, North and South America.

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Slide credit: E. Traianou, MPIfR

EHT Collaboration Meeting

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Granada, Spain

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June 2022

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The Event Horizon Telescope in 2017

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Image Credits: ALMA/ESO, Sven Dornbusch, Junhan Kim, Helge Rottmann, David Sanchez, Daniel Michalik, Jonathan Weintroub, William Montgomerie, Tom Lowe, Serge Brunier

SMT, Arizona

JCMT, Hawaii

APEX, Chile

IRAM 30m Spain

LMT, Mexico

Photos: ALMA, Sven Dornbusch, Junhan Kim, Helge Rottmann, David Sanchez, Daniel Michalik, Jonathan Weintroub, William Montgomerie, Tom Lowe, Serge Brunier

Slide: Sara Issaoun

ALMA, Chile

SMA, Hawaii

SPT, South Pole

6 different locations

6 single-dish telescopes

2 phased arrays

Our Multi-Wavelength Partners

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VERITAS

MAGIC

Fermi

Image credits: NSF/VERITAS, Juan Cortina, Vikas Chander, NASA, NASA/JPL-Caltech, NASA/CXC/SAO, NASA, ESO, P. Kranzler & A. Phelps, NRAO/AUI/NSF, HyeRyung, NAOJ, MPIfR/N. Tacken

Slide: Sara Issaoun

NuSTAR

Chandra

Swift

HESS

GBT

VLA

VERA

KVN

VLBA

VLT

GMVA

ALMA

The 2017 observations

Horizon-scale targets: M87* and Sagittarius A*

M87*

• Mass
• Distance
• Inclination
• Spin
• Astrophysical model

Sagittarius A*

• Mass
• Distance
• Inclination
• Spin
• Astrophysical model
• Variability
• Scattering

43 GHz with the Very Long Baseline Array/ Walker et al. 2018

22 GHz with the Very Large Array/ NRAO

Slide credit: S. Issaoun

First results: M87

Multi-wavelength view of M87

EHT 2021 Summer Collaboration Meeting

The supermassive black hole Sagittarius A*

20 as

Gravity Collaboration+ 2018

Closest supermassive black hole

• Mass: 4.1x10⁶ solar masses
• Distance: 8.1 kpc

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(Gravity Collaboration+ 2019, Do+ 2019)

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Image Credits:

X-ray: NASA/CXC/UCLA/Z. Li et al

Radio 22 GHz: NRAO/VLA

S-stars: UCLA Galactic Center Group (Keck), Genzel et al. (2010), Yuan et al. (2003)

S2: Gravity Collaboration+ 2018, ESO/Gravity

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Slide Credit: S. Issaoun

JVLA, 1.3cm

The supermassive black hole Sagittarius A*

20 as

Gravity Collaboration+ 2018

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What does Sgr A* look like?

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Expected size of the shadow of Sgr A*:

~50 𝜇as ~ 5 Schwarzschild radii

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(Falcke+2000, Doeleman+2008, Fish+2011,

Johnson+2015, Fish+2016, Lu+ 2018)

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What is the orientation of the black hole?

Is it spinning?

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Long-standing debate: what emission process dominates in the radio (disk versus jet)?

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Image Credits:

X-ray: NASA/CXC/UCLA/Z. Li et al

Radio 22 GHz: NRAO/VLA

S-stars: UCLA Galactic Center Group (Keck), Genzel et al. (2010), Yuan et al. (2003)

S2: Gravity Collaboration+ 2018, ESO/Gravity

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Slide Credit: S. Issaoun

JVLA, 1.3cm

The SED of Sgr A* is highly variable, simultaneous coverage is crucial for modeling

Slide credit: D. Haggard, McGill / EHT MWL Science WG

The supermassive black hole Sagittarius A*

Our 2017 Multi-Wavelength Effort on Sagittarius A*

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Figure credit: M. Johnson, J. Farah, SAO/EHT MWL Science WG.

First results: Sgr A*

A Tale of Two Black Holes

55 million light-years away from us

6.5 billion solar masses

27 thousand light-years away from us

4 million solar masses

A Tale of Two Black Holes

Visualization by L. Medeiros, IAS/ xkcd

From recordings to image

What is imaging?

Aperture synthesis: Earth rotation helps us fill our virtual mirror and combine data on multiple temporal and spatial scales

M87 becomes visible to stations as Earth rotates

Our coverage, or “virtual mirror” fills up

Our data, with the help of scan-averaging, give high SNR information on multiple spatial scales that improves our image

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Animation credit: D. Palumbo, Harvard/ M. Wielgus, MPIfR

The imaging process

Data calibration

Blind imaging in independent teams

Pipeline building per independent software

Image validation

Synthetic Models

Consistency with calibrator analysis

Goodness of fit to data

Structure robustness

Step 1

Step 2

Step 3

Feedback on data quality

Slide credit: S. Issaoun

Event-horizon scale variability in Sgr A* data

Visibility amplitudes

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Closure phases

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Q-metric: Roelofs et al. (2017)

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Broderick et al. (2022)

Georgiev et al. (2022)

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The detected variability is most statistically significant on baselines between ~2.5 and 6 Gλ.

Imaging a variable source

Visualization by C. Fromm, U. Wurzburg/ L. Rezzola U. Frankfurt

Variability mitigation and geometric modeling

1. “Snapshot” approach fits to short segments of data over which the source is static, and then averages these independent fits�

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• “Full-track” approach fits the average source structure alongside a parameterized model for the variability about that average source structure�

The detected variability is most statistically significant on baselines between ~2.5 and 6 Gλ.

Ring

Angular Profile

(Fourier Modes)

Modeling and imaging results

The detected variability is most statistically significant on baselines between ~2.5 and 6 Gλ.

We quantify various morphological parameters using both geometric modeling and image-domain feature extraction techniques

• Ring diameter: 51.8 ± 2.3 μas
• Ring thickness: FWHM between ~30-50% of the ring diameter
• other morphological quantities – magnitude and orientation of the asymmetry, depth of the central brightness depression – are poorly constrained and depend on the measurement method

Slide credit: H. Shiokawa

Simulating a black hole and its environment: GRMHD

Ray tracing

GRMHD + ray tracing parameters

Mass

Distance

Astrophysics

• Electron temperature distribution
• Magnetic field strength (MAD/SANE)

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Inclination

Spin

Scattering

M. Moscibrodzka, Radboud U.

Moscibrodzka & Gammie 2018

Sgr A* GRMHD library

Animation Credit: B. Prather / UIUC

Gravitational radius and black hole mass

Calibrating size measurements using synthetic data generated from GRMHD simulations:

• θ_g = G M / (c² D) = 4.8 (+1.4,-0.7) μas
• Uncertainty arises from
• Model flexibility needed to capture structural variability in the source
• Broad morphological diversity of the GRMHD calibration suite (reflecting the unknown inclination of Sgr A*)

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Combining the gravitational radius constraint with an independent distance measurement from maser parallaxes (Reid et al. 2019), we determine the mass of Sgr A* to be 4.0 (+1.1,-0.6) x 10⁶ M_☉

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Both the gravitational radius and mass measurements are consistent with the more precise constraints obtained from stellar orbits (Do et al. 2019, Gravity Collaboration et al. 2019, 2020)

Testing General Relativity

Even though tests with gravitational waves and black hole images span black hole masses that are different by 8 orders of magnitude, they are all consistent with the GR predictions that all black holes are described by the same metric, independent of their mass.

Future Arrays: Synthetic Data

Plus:

→ Telescope locations

→ Observation schedule

→ Calibration pipeline

Aperture efficiency

Absorption, emission, turbulence, and delays in the atmosphere

→resulting in lower signal-to-noise ratios, phase corruptions, and amplitude systematics

Instrumental (thermal) noise, gains, and leakage

Antenna pointing offsets

→resulting in systematic amplitude gain offsets

Antenna diameter

Source model

Adapted from diagram by S. Issaoun

Data generation codes:

• eht-imaging (Chael+ 2016, 2018)
• SYMBA (Roelofs+ 2020, Janssen+ 2019, Natarajan+ 2022)

Array expansion

Davelaar et al. (2019)

Roelofs, Janssen et al. (2020)

Gamsberg, Namibia

GRMHD and hotspot reconstructions

Noemi La Bella et al., in prep.

Western

Eastern + Africa

The next-generation Event Horizon Telescope

230 GHz

Spans ~2 weeks

Observes ~7 nights/year

EHT Sites

The next-generation Event Horizon Telescope

86+230+345 GHz

Spans 3+ months

Observes 60+ nights/year

EHT Sites

ngEHT Sites (Phase 1)

The next-generation Event Horizon Telescope

86+230+345 GHz

Spans 3+ months

Observes 60+ nights/year

EHT Sites

ngEHT Sites (Phase 2)

ngEHT threshold science goals

M. Johnson

P. Tiede

ngEHT objective science goals

M. Johnson

ngEHT Analysis Challenges: process overview

• Start from a “ground truth” source model
• Can be tailored to a specific science case
• Can be multi-frequency
• Static or time-variable�
• Simulate ngEHT observations of the source model
• Using different arrays/design choices
• Adding in realistic observation and calibration effects�
• Participants analyze the synthetic datasets, trying to maximize the science output
• Image and movie reconstruction
• Model fitting and parameter estimation
• Any other analysis�
• Submissions are evaluated using quantitative metrics

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C. Fromm

Roelofs et al. 2022 (in prep.)

https://challenge.ngeht.org/

The next-generation EHT: Reference Array

EHT2022 + 10 additional sites (incl. HAY, OVRO)

ngEHT reference array 1 (A. Raymond)

Performs well on uv-coverage metrics

6 m dishes, 8 GHz bandwidth, median April weather

Roelofs et al. 2022 (in prep.)

ngEHT Analysis Challenges: M87

Roelofs et al. 2022 (in prep.)

86 GHz

230 GHz

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Model by K. Chatterjee and R. Emami

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Reconstructions by P. Arras and J. Knollmueller using Resolve Bayesian imaging algorithm

Weekly observations of M87 for 5 months

ngEHT Analysis Challenges: Sgr A*

Roelofs et al. 2022 (in prep.)

Model by P. Tiede

Reconstructions by A. Fuentes

Intra-hour variability of Sgr A*

10-minute scans with 10-minute gaps

Space VLBI: Event Horizon Imager

Two or three satellites in circular orbits with slightly different radius

Dense spiral-shaped uv-coverage

Frequency up to 690 GHz

4 μas nominal resolution

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Roelofs et al. (2019)

Image reconstruction and parameter estimation

Davelaar et al. (2019)

Roelofs et al. (2019)

Sub-percent precision mass measurements → GR tests

Fitting EHT 2017 M87 GRMHD library to simulated data

Roelofs et al. (2021)

Constraints on black hole spin and other GRMHD parameters

Summary

• We have imaged the black hole shadow of M87 and Sgr A*
• Expansions of the array are currently happening
• The ngEHT will a be transformative improvement, opening new windows in angular resolution, sensitivity, frequency coverage, and time coverage
• On the longer term, a Space VLBI mission will take high-precision black hole imaging even further