Event Horizon Telescope Next Steps:
EHT 2020s, ngEHT, and Space Extension

Kazu Akiyama
MIT Haystack Observatory
NRAO Jansky Fellow —> MIT Research Scientist

First M87 Results: Where are we now?
- Einstein’s GR has passed a new test at an extremely strong gravitational field- The strongest evidence for the presence of a supermassive black hole
- The M87 central black hole is most likely spinning- An AGN and associated jet are powered by a supermassive black hole
- The stellar dynamical mass is correct (6.5 billion masses)- Day-to-day variations on horizon scaleDawn of a New Era of Black Hole Astrophysics
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)First M87 Results: Remained Issues
Testing General Relativity
M87 Mass: the mass uncertainty is large~5 Rs

EHTC Paper+19 IV & VI
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)Testing General Relativity
M87 Mass: the mass uncertainty is large
Separate the photon ring emission from the surrounding accretion flow and jetApril 11654321Brightness Temp. (109 K)

0First M87 Results: Remained Issues

0EHTC+19 Paper I
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)First M87 Results: Remained Issues
Testing General Relativity
M87 Mass: the mass uncertainty is large
Separate the photon ring emission from the surrounding accretion flow and jet
Short time window (< a week): may significantly be affected by transient feature
April 5

April 6

April 10

April 1150 µas

EHTC+19 Paper IV
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)First M87 Results: Remained Issues
BH Magnetosphere, Accretion and Jet
Tracking evolving features in M87

April 5

April 6

April 10

April 1150 µas

EHTC+19 Paper I
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)First M87 Results: Remained Issues
BH Magnetosphere, Accretion and Jet
Tracking evolving features in M87
Black Hole Magnetosphere: Magnetic Flux (SANE / MAD)

EHTC+19 Paper I
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)First M87 Results: Remained Issues
BH Magnetosphere, Accretion and Jet
Tracking evolving features in M87
Black Hole Magnetosphere: Magnetic Flux (SANE / MAD)
Jet launching: how is the energy extracted from the black hole?
DR~10

VLBA+GBT 86 GHz(Hada et al. 2016)
DR>100Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)M87 Polarimetry: Coming Soon
Model (Convolved)

EHT 2017

Fractional Polarization

Counter Jet Model

Forward Jet Model

Model (Original)

Akiyama et al. 2017, Chael et al. 2016
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)Sgr A*: Another Horizon-scale Target
Best target for the GR test-

Mass & Distance are accurately measured(GRAVITY Collaboration+2018)

Current challenges-Time variations: on minutes time scales-Interstellar Scattering: much less dominant

GRAVITY Collaboration+2018
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)Weather forecast: Sgr A*An illustrative full-closure imaging simulation with EHT 2017 array

Model: Broderick & Loeb 2006, Imaging: Kotaro Moriyama
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)ntinued observations of both Sgr A* and M87 [16, 20, 17, 1, 33, 37], have made more
tailed measurements of these sources through expansion to new VLBI sites and fulllarimetric observations. In April 2017, the EHT made first observations with a full array
pable of imaging (Figure 1), and data analysis is proceeding with the expectation thatsults will be released this year.

EHT2020s: EHT within 3 years from now
2018:-Bandwidth: 32 Gbps -> 64 Gbps
New Station:GLT (under commissioning)

2020:
-

New Stations:
GLT (official)
NOEMA & Kitt Peak (KP)

2023:
-

New Station: Owens Valley (OVRO)
345 GHz Capability

gure 1: A map of the EHT. Stations active in 2017 and 2018 are shown with connecting
es Doeleman
and labeledetinal.yellow,
sites
incommission
arelabeledinred,andlegacy
sites
arelabeled
2019, Astro2020 white paper
2017:230GHz,
32Gbps,8stations@6sitesgray. Nearly-redundant baselines are overlaying each other, i.e. to ALMA/APEX and2023:
230&345GHz,
64Gbps,12stations@
10sites
MA/JCMT.

r the purposes of this white Kazu
paper,
we will "Active
assumeGalactic
that suchhorizon-scale
isAkiyama,
NucleusJets in the imaging
Event Horizon
Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT 2020s: Deeper, Shaper & Multi-frequency Images

230 GHz
EHT2017+GLT

230+345 GHz
EHT2017+GLT+KP+NOEMA+OVRO

Further new capabilities: Faraday Rotation Imaging
Active developments of multi-frequency (Chael+) and multi-scale (Akiyama+) Imaging
Simulations: Andrew Chael, Imaging: Kazu Akiyama
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT 2020s: Tracking Evolving Features

Regular monitoring observation capabilities on weekly scales
Simulations: Charles Gammie, George Wong et al., Imaging: Michael Johnson
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT 2020s: Precision Black Hole Astrophysics
M87

Sgr A*

Mass ? Spin ? Accretion flow types?
Viewing Geometry?
https://www.youtube.com/watch?v=0ymmnHlnDVY

Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT 2020s: Precision Black Hole Astrophysics
Are black hole images confusing for scientists and/or AI?
Doodle or Fried Chicken

https://barkpost.com/humor/doodle-or-fried-chicken-twitter/

Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT 2020s: Precision Black Hole Astrophysics
Current forecast: Horizon-scale images are much less confusing!

Van der Gucht et al. 2019
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)next generation Event Horizon Telescope (2023 -)Phase I: 2019-2023 (Array Design Phase + MIT Haystack as a new site)
Phase II: 2023- (Constructions of several new telescopes)
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)M87 ngEHT images: ~10 years from nowBlackburn et al. 2019; Doeleman et al. 2019 (Astro2020 Decadal Survey White Papers)
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)ngEHT: far more details

Astro2020 APC WP
Blackburn+Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT beyond the Earth: Why do we need?
Ground limitations:
Telescope Coverages-Not so many sites capable of sub-mm VLBI observations
Snapshot coverages will be highly limited by Earth rotation

Limiting snapshot imaging of Sgr A* and multi-epoch observations
Angular resolution-The maximum diameter = the Earth diameter
Frequency / wavelength: limited by atmosphere (up to ~350 GHz)
Sub-Rs scale imaging of M87/Sgr A*, more horizon-scale targets
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT beyond the Earth: Designing towards late 2020

1st KISS Workshop, Sep 2019
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT beyond the Earth: Designing towards late 2020
- Astro 2020 Science and APC white papers(Doeleman+, Haworth+, Pesce+)- Active International Discussions
Sep 2018: 1st International Space VLBI conference @ Netherland
Sep 2019: 1st KISS Workshop @ USA (invitation only)
Jan 2020: 2nd International Space VLBI conference @ USA (Next week!)
June 2020: 2nd KISS Workshop @ USA (invitation only)

1st KISS Workshop, Sep 2019
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT+LEO: Better Snapshot Imaging

Palumbo et al. 2019, ApJ
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)EHT+MEO/GEO: (sub-)Rs-scale Imaging of M87 / Sgr A*
Kawashima, Kino, & Akiyama
The Astrophysical Journal, 00:000000 (9pp), 2019 Month Day

Kawashima, Kino, & Akiyam

GRMHD simulations at a flaring state

1 Rs

for details about involved stations). (Left) uv-coverages of ground EHT stations.
T 2017 observations, while blue lines are additional baselines involving three sites
spacing for 10rg. (Right) uv-coverages of the EHT full array with additional spacee as in the left panel. The dashed line shows the fringe spacing for 2rg.

(Kawashima, Kino & Akiyama 2019, ApJ)

Figure 7 demonstrates the expected images in current andfuture EHT observations. In panel Kazu(a), theAkiyama,reconstructed
imagesGalactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)"Activefor the ﬁducial-mass model (M=6.2×109 Me) at 230 GHz

230 GHz flux density (Jy)Cen AOJ287

1NGC 1052

230 GHz nominal resolution (ground)10345 GHz nominal resolution (ground)

3C273

230 GHz nominal resolution (ground + GEO)

345 GHz nominal resolution (ground + GEO)

EHT+MEO/GEO: Horizon-scale Imaging of other SMBHs
M104 (Sombrero Galaxy)
Sgr A*

Ground Truth

with MEO/GEO Satellites

M87

M104
IC 1459

M81
IC 42960.11010 M
108 M

M84

NGC 4261
NGC 1399

106 M

5 µas

0.1

1
10Shadow diameter (µas)

Ground EHT resolution

100

Dom Pesce

Fish, Shea & Akiyama 2019

Figure5:(Leftpanel)
Typical230
GHz
fluxdensity
versuspredictedblackholeshadow
Doelemanetal.
2019,Astro2020
whitepaper
Figure 5: (Left panel) Typical 230 GHz flux density versus predicted black hole shdiameter for a number of current and potential future EHT targets; the size of each point
diameterforanumberofcurrentandpotential
futureEHT
targets;
thesizeofeachKazu
Akiyama,"Active Galactic
Nucleus
JetsThein the Event
Horizonscience
Telescopetargets,Era”, Tohoku
University,
2020/01/20
scales withtheestimated
SMBHmass.
primarySgrA* andM87,(Mon)areFar more accurate shadow measurements?
2Michael D. Johnson et al.

Hung-Yi Pu

Johnson et al. 2019
Figure 1. Time-averaged image of a GRMHD simulation of M87
with parameters chosen to be consistent with the 2017 EHT data.
Photonshell Telescopeand photon ringa Kerr blackhole withKazu
Akiyama,"Activeflux
Galactic
Nucleus JetsFigurein the 2.
Event
HorizonEra”,ofTohoku
University,
2020/01/20 (Mon)This model corresponds
to thehigh magnetic“Magneticallyspin a/M = 0.94. Top: cross section of the photon shell in the (r, ✓)Arrested Disk” accretion state with parameters M = 6.2 ⇥ 109 M ,Far more accurate shadowUniversal
measurements?
Interferometric Signatur
2Michael D. Johnson et al.

el D. Johnson et al.

r the, and051020I (x)I (x )

100-40-30-20-10010

Relative Position (as)3050n=340

20n=230

n=120

1010

Figure 1. Time-averaged image of a GRMHD simulation of M87
with parameterschosento be40
consistent with the 2017 EHT data.
2030
This model corresponds to the high magnetic flux “MagneticallyArrested Disk” accretion state with parameters M = 6.2 ⇥ 109 M ,a/M = 0.94, ✓obs = 163 , rhigh = 10, and mass accretion rate
matching the 1.3 mm flux density (see Paper V for details). The
spin axis points left when projected onto the image. The time average was performed over 100 snapshots produced from uniformlyspaced GRMHD fluid samples over a time range of 1000M (approximately 1 year). Though visually prominent, the thin, bright ring
contains only ⇠20% of the total image flux density.

n=3
its relation to the photon ring. We also present a decomposition of the photon ring into subrings indexed byhalf-orbit number and derive their self-similar structure,
which is universally governed by Lyapunov exponents
n=2that characterize orbital instability. Next, in §3, we derive generically expected interferometric signatures of thephoton ring. We show that its subrings produce a cascade of damped oscillations on progressively longer baselines, with the visibility of each subring conveying precise
n =its
1 diameter, width, and angular proinformation aboutfile. Finally, in §4, we discuss observational prospects fordetecting these signatures with extensions of the EHT.
In particular, we highlight the possibility of detecting theleading n = 1 subring using a station in low Earth orbit,
the n = 2 subring using a station on the Moon, and the0n = 3 subring using an =stationin the Sun-Earth L2 orbit.

Visibility Amplitude (Jy)

30

101

105

10-1

100

104

n=1103

u-3/2

102

n=2
n=3

101
100

Figure 2. Photon shell and photon ring of a Kerr black hole withspin a/M = 0.94. Top: cross section of the photon shell in the (r, ✓)plane in Boyer-Lindquist coordinates. The spin axis is vertical andthe color varies with r. The intersection of an observer’s line of3 the photon shell boundaries at r = r± determines thesight withvisible subregion of the photon shell. Bottom: photon ring on thescreen of an observer at varying inclinations ✓obs relative to the(left-pointing) spin axis. The color coding on the ring denotes thematching radius on the shell from which the photon emanated.
The photon shell r  r  r+ is only visible in its entirety to theedge-on (✓obs = 90 ) observer. The face-on (✓obs = 0 ) observer
only receives photons from the white r = r0 orbit. The ✓obs = 17observer sees the portion of the shell delineated by the dashed lines.

Frequency (GHz)Tb (109 K)

40

Tb (109 K)

ds thephoin an
conof theuring
he seturing
blackdomhole.
ution
y eachapart,ed onemisisibly
peaklayers
ading

-5

50

Tb (109 K)

ccupy
nteredroundof the-10

10se
line

rthsa
B

EO

OEL

M

0.2OEGEaM

noo

L2

0.5
1.0

EHT

2.0

2.1. Photon Shell

The2photon shell, illustrated in Figure 2, is the region
10of a black hole spacetime
10 containing bound null geodesics 1011
10or “bound orbits” that neither escape to infinity nor fallacross the event horizon. For Schwarzschild, the photon
shell is the two-dimensional sphere at r = 3M and any✓, , and t. For Kerr, this two-dimensional sphere fattens
to a three-dimensional spherical shell. It is best describedusing Boyer-Lindquist coordinates, in which the metricof a Kerr black hole of mass M and angular momentum
J = aM (with 0  a  M ) is

Wavelength (mm)s arists bebit atmustow(12)Angular Resolution 1/u (as)Relative Position (M/D)

1012Baseline Length u ()Figure 5. Top: Schematic showing visibility amplitude as a funcSpace
VLBIwith
Heterodyne
receiversof theOrigin
Space
Telescope
atflux
L2
00tionofbaseline
lengthforaphotonring
with
d=
40
µas
and-17-16-15
17
18
1920
This section describes the shell of unstable bound pho⌃
Relative Position (as)RelativePosition(as)ds comparable=dt a sin ✓ d+ todr(1a)ton orbits
surroundinga black hole, its lensed photondensity
M87.
Theblack,
red,andblue
visibilities⌃Johnsonet al. 2019, ApJ
ring image, the photon subrings labeled by half-orbit⇥⇤Figure 3. Top: Brightness cross sections forthetime-averagedsin✓number, and the angle-dependent Lyapunov exponents
correspondphotons
withhalf-orbit numbers n = 1, 2, and 3.
+⌃ d✓to+r +a da dt,
GRMHD image shown in Figure 1. The blue/redcurves⌃
that govern
theshow
subringcrossbrightness ratio asymmetry. Previous
treatmentsof these structures include Bardeen
sections perpendicular/parallel to the projectedspinaxis. Bottom:=r2M
r + a , ⌃ = r + a cos ✓.
(1b)
Bottom:Frequency-dependent
rangeofEarthbaselines
andrepreetal.
(1972);
Luminet(1979);
Teo(2003);
Grallaetal.
Kazu
Akiyama,"ActiveGalactic
NucleusJets
intheEventHorizon
Telescope
Era”,
TohokuUniversity,
2020/01/20(Mon)
Decomposition of the left perpendicular peak and the right parallel(2019). Observational aspects of these features follow in
These coordinates have the special property that all
peak into subrings indexed by the number n §3
ofandphoton
half-orbits
sentativebaselines.
Earth-space baselines shown are§4.
bound orbits lie Earth-spaceat some fixed value of r in therange
n=02. PHOTON SHELL AND PHOTON RING22222222222222

Weather Forecast of the EHT era: Awesome!
EHT in the next three years:- Sgr A* & M87 Polarimetry- Doubled bandwidth (2018-): Higher sensitivity & Rotation Measure Imaging- Four new stations until 2023: Higher dynamic range (few/several 100s)- Higher angular resolution: 345 GHz (0.87 mm)- Time domain: Monitoring Capability
mid-late 2020s: Expanding the Array to the ngEHT
Higher Dynamic Range (~1000s), Better sensitivity
late 2020s-2030s: Expanding the ngEHT to space:
Better snapshot, sub-Rs-scale imaging, more horizon-scale targets
Kazu Akiyama, "Active Galactic Nucleus Jets in the Event Horizon Telescope Era”, Tohoku University, 2020/01/20 (Mon)