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10 TeV
Multi-Wavelength
observations of the
Galactic Center
300 meV
Observational signatures
and characterisation
of the central black hole
D. Rouan
D. Rouan - LESIA - Obs. de Paris
16/03/05
The Galactic Centre
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At 8kpc, the GC region is totally hidden in the visible
by galactic dust (extinction by a factor 1 billion !)
• Fortunately it is seen in radio, infrared, X and g
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Star density : 10 million times the solar neighbourhood !
A complex area : ionized and
molecular gas, fast streams,
very hot gas, bubbles, relativistic
electrons, ...
Very young stars (106 years) and
evolved stars coexist in a
small volume
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L-M map (NACO)
D. Rouan - LESIA - Obs. de Paris
16/03/05
A supermassive Black Hole ?
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The GC area exhibits what is probably the most evident
concentration of dark mass
Coincident with the radio source Sgr A*
Given the small distance : the best candidate to test the
supermassive black hole paradigm
One might expect that Sgr A* should be a bright source,
yet it is underluminous at all wavelengths
by a factor of 10-9 with respect to Eddington luminosity
LEdd = 4 1037 W (= 1.3 1031 M/M for M = 3 106 M)
Lobs ≈ 1028 W
Any clue that indeed a BH is there or is unlikely is
welcome : this has been, and still is, the object of an
active multi-wavelengths quest
Recent review : Melia & Falcke (2001, ARAA)
D. Rouan - LESIA - Obs. de Paris
16/03/05
The radio view : Sgr A*
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D. Rouan - LESIA - Obs. de Paris
16/03/05
The radio view : Sgr A*
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• Extended emission (Yusef-Zadeh et al. 92):
• Mini spiral structure with 3 arms extending
on ≈ 3pc : Sgr A West
rotating at 150 km/s around Sgr A*
• A more diffuse ≈ spherical component
extending to the East : likely a young
(104 yr) SN remnant (Melia 02)
0.9pc
2cm
• A strong point source (Balik & Brown 74) : Sgr A*
• no infrared nor X counterpart until 2000-2
• Non-thermal radiation (synchrotron)
• variability :  2 typically (Brown & Lo, 82)
3pc
7pc
VLA 6cm
D. Rouan - LESIA - Obs. de Paris
16/03/05
Radio
• The minicavity : a spherical void of 0.08 pc
diameter, very close to Sgr A* : may be due
to a focused flow from it
• The mini-spiral is inside a cavity
delineated by a ring or shell of
molecular gas : hot gas and dust
inside are probably heated by
UV from the OB central cluster
• The overall dynamics in radio =>
suggests a point mass of 3 106 M
at the center (Genzel & Townes, 87)
• Once corrected from galactic
rotation, the proper motion of Sgr A*
is only 15 km/s (Reid et al. 99) : thus
at the very center of the Galaxy
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D. Rouan - LESIA - Obs. de Paris
16/03/05
Radio size and spectrum of Sgr A*
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Radio size : observations at 3 and 1.4 mm demonstrate that Sgr A*
size is below 0.1 mas = 0.8 AU = 11 RSchw (for M = 3 106 M)
Minimum size = 0.1 AU (1.2 RSchw) : set by maximum brightness
temperature at Compton limit (1012 K)
Spectrum :
• Power-law with a significant millimeter excess
• agrees well with synchrotron from plasma at 1011 K (Radiatively
Inefficient Accretion Flow)
Polarization :
• Linear and circular
• Variable (Bower et al 05)
D. Rouan - LESIA - Obs. de Paris
16/03/05
The X view
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Expected X luminosity if at 10% of the Eddington
luminosity = 4 1043 erg s-1
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Actually : Lx(2-10keV) < 1035 erg s-1
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The 109 discrepancy is one of the most challenging issue
in high energy astrophysics :
• Low accretion rate ?
• Extremely low radiative efficiency ?
• Anisotropy or strong absorption of the emission ?
D. Rouan - LESIA - Obs. de Paris
16/03/05
The X view = pre-Chandra/XMM era
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Until the advent of Chandra and XMM, the only X flux
detected revealed to be a combination of diffuse
emission and stellar sources :
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ROSAT : one source within 10” of SgrA* : Lx = 7 1035
erg s-1
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ASCA : bright diffuse emission of hot gas (10 keV)
associated to SgrA East shell : Lx = 1036 erg s-1
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BeppoSAX : diffuse emission identified 
upper limit for Sgr A* : Lx 2-10 keV= 1035 erg s-1
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GRANAT : Lx 35-150 keV < 6 1035 erg s-1
D. Rouan - LESIA - Obs. de Paris
16/03/05
And Chandra came...
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Chandra (Baganoff et al. 2000, 2003) :
• Astrometry : 0".16 (Tycho sources)
• 0.5-7 keV : diffuse emission + 119 point sources
• One source coincident with SgrA* within 0".27
1'.3 x 1'.5
D. Rouan - LESIA - Obs. de Paris
16/03/05
Sgr A* in X
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2-10 keV luminosity : 2.4 (1.8-5.4) 1033 erg s-1
Spectrum :
• Well fitted by an
absorbed power-law
N(E) = E-2.7 and
NH = 1023 cm-2
• Or by a plasma w kT = 2 keV
• Possible presence of
a Fe Ka line at 6-7 keV
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Extension:
• the source appears extended
w respect to point sources
• qintrinsic = (q2 - qpsf2)1/2 = 0".6  .024 pc
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Variability : statistically proven on 1h scale
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D. Rouan - LESIA - Obs. de Paris
16/03/05
Non-BH possible X sources
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Confusion w HeI/HI emission line stars (≈ LBV or WR star) ?
• No such star closer than 1.2"
• Soft spectrum of W-R stars : cannot penetrate through the deep
obscuration
Colliding winds of binary system including a W-R star ?
• Harder spectrum
• Variability on days to years rather than hours
Low mass YSO ?
• X-ray increase by 10-104 during first 107 years
• If 100 such stars within 0".5 of SgrA * : X luminosity could be
explained, but mass segregation and IMF would not favor such a
number
A cluster of X-ray binaries in the cusp ?
• Velocity dispersion (100 km s-1) : very few at a given time
• Collisions : short lifetime of a binary system
D. Rouan - LESIA - Obs. de Paris
16/03/05
X Flares
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First flare :
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Chandra Oct 2000
Baganoff et al. 01
Duration : 104 s
N(E)  E-1.0
Fastest variation : 10min
2-8 keV
10 minutes
2 hours
May 2002 campaign: ~0.6-1.2 flares/day
Baganoff et al. 2000, 2001,2003, Porquet et
al. 2003
D. Rouan - LESIA - Obs. de Paris
16/03/05
Flares spectrum
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Typical duration : 2500s
• Short scale : 10 min
•  a few RSchw
Hardness : 2 behaviours :
• Goldwurm et al. (03) - XMM :
flare with photon index G = 0.9,
thus harder than the G = 2.7 of
quiescent state
• Porquet et al. (03) - XMM :
a very bright flare remaining soft
(G = 2.5)
XMM : Porquet et al. (04)
XMM : Goldwurm et al. (03)
D. Rouan - LESIA - Obs. de Paris
16/03/05
The gamma view
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Soft g rays detected by EGRET
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Strong source of >100 MeV ≈ in Sgr A* direction
BUT recent re-analyze : EGRET source is
offset (probability to be Sgr A* < 5%)
INTEGRAL : hard-X & soft g rays
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20-40 and 40-100 keV map at 12' resolution
A hard source coincident within 1' w Sgr A*
20-40 keV : 1.9 ± 0.4 erg cm-2 s-1 (3.2 mcrab)
40-100 keV : 1.9 ± 0.4 erg cm-2 s-1 (3.4 mcrab)
Possible variability or flare (12) of 40 min :
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D. Rouan - LESIA - Obs. de Paris
16/03/05
The gamma view
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TeV emission detected by Whipple
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unique Cherenkov telescope
First evidence for TeV emission (97)
TeV g rays emission detected by HESS
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2/4 Cherenkov telescopes
g rays excess at 14" ± 30" from Sgr A*
Spectrum : E2 dE/dN = 2.5 10-8 E-.5 TeV m-2s-1
Conflict w CANGAROO measurements of larger flux
and softer spectrum => variability ? not
really predicted by various models
Hess
(Aharonian 04)
Whipple
(Kosack & al 04)
D. Rouan - LESIA - Obs. de Paris
16/03/05
The Infrared View
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Search for :
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dynamical signature
IR emission from disk, jet, accreting matter + variability, flares
Interaction of jet with its environment
Confusion is the issue  adaptive optics the solution ! qdiffr < 0.15"
NAOS/CONICA on Yepun VLT-ESO
Onera + Obs. de Paris + Obs. De Grenoble
D. Rouan - LESIA - Obs. de Paris
16/03/05
IR : 1- dynamical signature
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Follow-up of several stars during 10 years
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Very good radio/IR astrometry thanks to SiO masers of giant stars
Orbit of several stars belonging to the very central cluster (<1")
ESO program : MPE-Garching (Genzel et al.) + Lesia since 4 years
Keck program : A. Ghez
NAOS/CONICA measurements :
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Infrared wavefront sensor : IRS7, 6” at Nord : very good correction in K
angular resolution = 0.055"
Orbit of star S2
 gravity probe with closest approach
at 17 light-hour = 3  Sun-Pluto
 However beyond distance of tidal
disruption
Best mass distribution :
a point mass M = 3.6 106 M
+ stellar cluster Rc= 0.34 pc,
r = 4 106 M pc-3
Hard to avoid identifying SgrA*
with a Black Hole !
+ radio => 1019 Mpc-3
D. Rouan - LESIA - Obs. de Paris
16/03/05
Excluded models
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Recent refinement of orbits determination
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Ghez et al. 05 : simultaneous constraint from
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7 stars orbits
M = 3.7 ± 0.2 M
position accuracy : 1.3 mas
Closest approach : 40 AU !
Even more constraint on a point mass
Excluded Models :
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Dark stellar cluster (BD, neutron star, stellar BH ) : would impose a
central density = 1017-19 M pc-3  lifetime < 105 years  rejected
Ball of fermions (neutrinos, gravitinos, axinos, …)  finite size of 7000
UA > S2 perimelanophreas*  rejected
* From ancient greek : melano = black, phreas = well
16/03/05
IR : 2 - the thermal IR emission
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D. Rouan - LESIA - Obs. de Paris
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Detection at L' (3.8 µm) of a possible IR counterpart (Ghez & al 04,
Clénet & al 04), when S2 was nearby
First detection at M (4.8 µm) (Clénet et al. 04)
• Very red color
• Spectroscopy of S2 (Ghez, 2003) : O or B star  no confusion
• Since then, S2 moved : no more ambiguity
Astrometry : source w IR excess within 30 mas of SgrA*
Clénet & al 04
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16/03/05
Comparison to predicted spectra
D. Rouan - LESIA - Obs. de Paris
Yuan et al., 2003
NACO
NACO
Accretion disk :
synchrotron by thermal e+ inverse self-compton (X)
+ 5% of electrons
accelerated
Relativist Jet : synchrotron
(radio/IR) + inverse
self-compton (X)
Good agreement !
But…
D. Rouan - LESIA - Obs. de Paris
16/03/05
IR : 3 - variability, flashes
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• Ghez et al. 04, Clénet et al. 04: between August 02 and
June 03 : variation by a factor 2 of the L flux
• Excludes in practice any confusion w a
background star or a member of the young cluster
Clénet et al. 04
Ghez et al. 04
16/03/05
• May 03 : detection of a flare in H band (1.65µm)
(Genzel et al.)
• Followed by several (2 in K, 1 in L)
D. Rouan - LESIA - Obs. de Paris
Detection of infrared flares
• Flares Parameters :
• typical duration : 90 min
• frequency : 3 - 5 / day > X frequency
(Chandra : 1.2 / day)
• sub-period : 17 min
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D. Rouan - LESIA - Obs. de Paris
16/03/05
Flare or Flash ?
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In 2004 : several events detected
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April: flare,
June : flare + short flash (<10 min),
Sept : flare
All observed in L' band (3.8 µm)
Flare Sept 04
Flare Juin 04
Flash Juin 04
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Recent images : the quiet emission is resolved at ≈ 600 AU
The photo-centre moves : during a flare/flash it is precisely on Sgr A* while
the quiet emission is offset by 40 mas to the SW
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D. Rouan - LESIA - Obs. de Paris
16/03/05
Separation of flares and quiet mission
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The quiet emission could correspond to synchrotron of a jet and flares to
accretion events on the horizon of the BH
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Question : can a low luminosity jet be extended on ≈ 300 AU ?
D. Rouan - LESIA - Obs. de Paris
16/03/05
Flares : what constraint do they bring?
• Spectrum looks « blue »
• Energy in IR flares ≈ X
• tvar= few min  r < 10 Rschw
• If synchrotron : accelerating event
(g= 103), but issue of blue spectrum
• If free-free (or BB) :accretion event of
m = few 1019 g (≈ comet)
• Polarization should bring an answer
• Matter of the disk should accumulate on
the LSO (Last Stable Orbit) :
• in Schwarzschild metric : T = 27 min
• In Kerr metric (rotating BH) : T= 17 min,
if J/(GM/c) = 0.52  maximum spin
• Proposal (Genzel et al. 03) : the 17 min
pseudo-period could be the LSO  the BH
one = 13 min
• Could be the 1st measure of a BH spin,
one of the 3 parameters caracterizing a
BH (masse M, spin J, charge Q)
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D. Rouan - LESIA - Obs. de Paris
16/03/05
A simultaneous X / IR flare
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Simultaneous observation of a flare in X (Chandra) and IR (NACO)
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Eckart et al. (04) :
Well explained by SSC (Synchrotron Self Compton) from a component
at a few RSchw
Sn  n-1.3
Time Lag < 15 min
D. Rouan - LESIA - Obs. de Paris
16/03/05
IR : 4 - Interaction with environnement ?
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Deconvolved L image
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A jet colliding the ISM should leave
traces : host dust, shock signature
A very red source close to SgrA* (.025 pc)
• elongated to SgrA*
• Tcol = 650-800 K : hot dust
Another red elongated source
• further away
• with a bow shock appearance
• ≈ in the same direction
• no counterpart at Paschen a
K, L, M Images
16/03/05
The overall picture
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D. Rouan - LESIA - Obs. de Paris
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Taken from Aharonian 04
Not so far from energy equipartition ...
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D. Rouan - LESIA - Obs. de Paris
16/03/05
Summary
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At all wavelengths from gamma to radio, there are now
compelling evidences that a massive black hole is sitting
at the very center of the Galaxy.
Radio :
• unresolved source at scale of 1 UA (=11 Rschw),
• Tbrightness  size  .1 AU (1. Rschw)
• Spectrum ≈ synchrotron from plasma at 1011K
• Dynamics of the gas  compact mass of 3 106 M
• Very small proper motion
X rays :
• A counterpart to Sgr A* within 0.2"
• Very intense flares and variability : d < 10 RSchw
• Radio/X connection : Synchrotron Self Compton
• No plausible alternate explanation
D. Rouan - LESIA - Obs. de Paris
16/03/05
Summary
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Gamma rays :
• INTEGRAL : 20-110 keV source coincident w Sgr A*
• HESS : TeV emission coincident w Sgr A*
Infrared :
• Stellar orbits determination within 1 arcsec:
 Center of mass position accuracy : 1.3 mas
 Mass distribution implies a point mass of 3.7 M
 40 AU closest encounter excludes a dark cluster
• IR emission :
 3.8 and 4.8 µm IR source : on Sgr A* within 0.01"
 Flux level fits very well expected spectrum
 Flares and flashes from 1.6 to 3.8 µm : on Sgr A*
 Simultaneous X and IR flare
 Quiet emission : slightly extended and offset
• Possible traces of a jet interaction with MIS
D. Rouan - LESIA - Obs. de Paris
16/03/05
Conclusion
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The last 4 years brought an harvest exciting key
observational results (X, Gamma, IR)
The supermassive BLACK HOLE PARADIGM at center
of galaxies is now HARDLY ESCAPABLE
All results point to an EXTRAORDINARY LOW
LUMINOSITY of the GC BH environment. WHY ?
The FLARE phenomenon is likely THE KEY TO REACH
THE HORIZON of the BH
Need for :
• Simultaneous observations in g, X, IR, radio
 Should constrain models on flare mechanism
• Even higher resolution :
 interferometry in the IR
 XEUS, ...
• More predictions from models to test observationally
D. Rouan - LESIA - Obs. de Paris
16/03/05
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D. Rouan - LESIA - Obs. de Paris
16/03/05
Paschen a vs L-M
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