Intro High-order image Testing BH? BH alternatives
Using light to probe the vicinity ofsupermassive black holes
Frédéric Vincent1
1CNRS/Observatoire de Paris/LESIA
1/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
1 The goal of this talk
2 Strongly-lensed image features
3 Observing the secondary ring
4 Imaging BHs and alternatives
2/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
M87: low-luminosity galactic nucleus with kpc jet
3/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
EHT imageModel
Obs
4/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
EHT imageModel
Obs
Primary image
Primaryphoton
4/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
EHT imageModel
Obs
Primary image
Primaryphoton
Highly-lensed feature ("photon ring")
Highly-lensedphoton
4/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
EHT imageModel
Obs
Primary image
Primaryphoton
Highly-lensed feature ("photon ring")
Highly-lensedphoton
"Shadow"
Testing BH paradigm from EHT-like image
Definitions of “photon ring” and “shadow”Can we use these notions to test BH paradigm?
4/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
1 The goal of this talk
2 Strongly-lensed image features
3 Observing the secondary ring
4 Imaging BHs and alternatives
5/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Shadow/photon ring: simple introduction
6/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Absorbing sphere
6/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Event horizonprojected in
flat spacetime
6/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Event horizonprojected in
flat spacetime
Black hole shadow: photonscaptured by the BH
6/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Flat spacetime shadow
Flat spacetimeshadow
Absorbing sphere
6/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Black hole shadow
The shadow size
Event horizon
Photon orbit
A photon entering the photon orbit with pr<0will fall into the event horizon.
So the boundary of the shadow coincides withthe image of the photon orbit, called the photon ring.
6/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Shadow/photon ring: theory definition
Black hole shadow: photonscaptured by the BHPhoton ring: boundary
of the shadow
Event horizonprojected in
flat spacetime
Shadow and photon ring
Pure-gravitation, no-dirty-astrophysics definitionsGreat probes of gravity!... Really? To what extent?
6/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Black hole shadow in real life
Riazuelo 2007Marck 1996
Luminet 1979Bardeen 1972
7/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Black hole shadow in real life
Falcke+00
Observing the shadow: EHT
8/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Model
Gralla+19
lensing ring
photon ring
Observer
Equatorial plane
A more precise definitionWhat is the highly-lensed ring in the theory image?Gralla+19: lensing ring (ncross = 2), photon ring (ncross > 2)This is still pure-gravitation definition
9/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
6 4 2 0 2 4 6(M)
6
4
2
0
2
4
6
(M)
Sky projection (Johnson+20)
criticalcurve
project
on sky
Spherical orbit (Teo 2003)
Spherical photon orbitsWinding of photons → spherical photon orbitsCritical curve = image on sky of spherical photon orbitsi.e. of ncross = ∞ photonsRecap: ncross = 1 primary image; ncross = 2 lensing ring;ncross = 3+ photon ring; ncross = ∞ critical curve...what else?
10/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Sky plane
spherical orbit
criticalcurve
ncross=2
ncross=3
n=2n=3
n=∞
lensingring
...
Image spectroscopy
Highly-lensed feature of BH image =infinite set of pure-gravity-dictated subrings on skyTheoretical locus on sky, not directly observableThe flux distribution within this locus is not pure gravity
11/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Observer sky plane
criticalcurve n=2
n=3
n=∞
lensingring
...
0 2 4 6 8 10X (M)
4
2
0
2
4Z
(M)
To observer
Emittingmatter
Image spectroscopy
Observable = subsetwhere there is emissionSecondary ring: the partof these subrings wherethere is detectable flux(model-dependent)Well-posed question: whatis the secondary ring ofthat BH surrounded by thatparticular accretion model?
11/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Model-dependent definitionShadow = geodesics asymptotically approaching horizonand not visiting the flow
12/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
1 The goal of this talk
2 Strongly-lensed image features
3 Observing the secondary ring
4 Imaging BHs and alternatives
13/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Predicted size of secondary ringfor a spin=0.8M Kerr BH
Today’s observationsEHT image consistent with Kerr secondary ringBut also consistent with a lot of other things...
14/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Johnson+20, Sci. Adv., 6, 12
15/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Imageplane
Fourierplane
Detecting sharp features
Image = Gaussian primari + sharp featureFT = Gaussian + flatSo sharp feature should dominate at high freq
16/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Johnson+20, Sci. Adv., 6, 12
Tomorrow’s observationsDifferent rings dominate at different baselinesObservable question: Are there thin rings?This is still not a test of GR
17/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Image plane
Fourier plane
Gralla+20, arXiV:2008.03879
18/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Observable:first sub-ring
diameter
parameterslinked to intensity
profile
ring diameter
Gralla+20, arXiV:2008.03879
Fitting sub-ring diameters
Still a consistency checkIs it a test of GR?Very likely that other theories would lead to similar profiles
19/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
1 The goal of this talk
2 Strongly-lensed image features
3 Observing the secondary ring
4 Imaging BHs and alternatives
20/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
rin
θop
Sobj Sdisk
z
ne,in
Te,in
i M
line
of s
ight
Accretion flow modelGeometry: rin, θop
Physics: ne,in, Te,in, σ ∝ B2/ne
Emission: synchrotron radiationVelocity: Keplerian above ISCOBelow: radial or azimuthal flow
21/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Does M87* have an event horizon?Rotating boson starAssembly of spin-0 boson (e.g. Higgs)Behaves as a single quantum bodyDoes not collapse because of Heisenberg principleNo hard surface, no event horizon, no singularityNo photon spherical orbits to avoid stability issues
22/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
20
10
0
10
20 (M
)Kerr a = 0.8M, rin = 2.91M Boson star a = 0.8M, rin = 3.5M Boson star a = 0.8M
rin = rICO = 2.09M
20 10 0 10 20 (M)
20
10
0
10
20
(M)
20 10 0 10 20 (M)
20 10 0 10 20 (M)
0
21
42
0
16
32
0
19
38
Brig
htne
ss te
mpe
ratu
re (1
09 K)
0
6
12
0
4
8
0
5
10
Brig
htne
ss te
mpe
ratu
re (1
09 K)
Does M87* have an event horizon?Difference only due to MHD on current imagesFuture: tell the secondary ring?
23/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
BH BS
Detecting sharp features (inspired by Johnson+20)
Investigate the high-freq difference... in progress
24/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Model
Gralla+19
lensing ring
photon ring
Observer
Equatorial plane
Conclusion: highly-lensed features
Published terminology for highly-lensed features:lensing ring (ncross = 2), photon ring ncross > 2, photonsubrings, critical curve ncross = ∞. Pure gravitation.We introduce:secondary ring = all subringsAND depends on emission model (not pure gravitation)Theoretical locus on sky 6= observable
25/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Black hole Ad-hoc boson star
Conclusion: compact object natureThere is no “clean”, “pure-gravitation” probeYou must trust plasma physics to test the nature of acompact objectFascinating (but not fully clear yet) perspective: distinguishsharp features (space VLBI)
25/25 Frédéric Vincent M87*: Kerr or alternative?
Intro High-order image Testing BH? BH alternatives
Sgr
GRAVITY Collaboration, A&A 618, L10 (2018)
Conclusion: GRAVITYOrbital motion near horizon of Sgr A*Another fascinating probe of near-horizon physics
25/25 Frédéric Vincent M87*: Kerr or alternative?