Using light to probe the vicinity of supermassive black holes · 2020. 10. 7. · Black hole shadow...

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?


1 The goal of this talk

2 Strongly-lensed image features

3 Observing the secondary ring

4 Imaging BHs and alternatives



M87: low-luminosity galactic nucleus with kpc jet



EHT imageModel

Obs



EHT imageModel

Obs

Primary image

Primaryphoton



EHT imageModel

Obs

Primary image

Primaryphoton

Highly-lensed feature ("photon ring")

Highly-lensedphoton



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?



Shadow/photon ring: simple introduction



Absorbing sphere



Event horizonprojected in

flat spacetime




flat spacetime

Black hole shadow: photonscaptured by the BH



Flat spacetime shadow

Flat spacetimeshadow

Absorbing sphere



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.



Shadow/photon ring: theory definition

Black hole shadow: photonscaptured by the BHPhoton ring: boundary

of the shadow


flat spacetime

Shadow and photon ring

Pure-gravitation, no-dirty-astrophysics definitionsGreat probes of gravity!... Really? To what extent?



Black hole shadow in real life

Riazuelo 2007Marck 1996

Luminet 1979Bardeen 1972



Black hole shadow in real life

Falcke+00

Observing the shadow: EHT



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



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?



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



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?



Model-dependent definitionShadow = geodesics asymptotically approaching horizonand not visiting the flow



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...



Johnson+20, Sci. Adv., 6, 12



Imageplane

Fourierplane

Detecting sharp features

Image = Gaussian primari + sharp featureFT = Gaussian + flatSo sharp feature should dominate at high freq



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



Image plane

Fourier plane

Gralla+20, arXiV:2008.03879



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



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



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



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?



BH BS

Detecting sharp features (inspired by Johnson+20)

Investigate the high-freq difference... in progress



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



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)



Sgr

GRAVITY Collaboration, A&A 618, L10 (2018)

Conclusion: GRAVITYOrbital motion near horizon of Sgr A*Another fascinating probe of near-horizon physics


Using light to probe the vicinity of supermassive black holes · 2020. 10. 7. · Black hole shadow...

Documents

Transcript of Using light to probe the vicinity of supermassive black holes · 2020. 10. 7. · Black hole shadow...