MultiWavelength observations of the Galactic Center

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Multi-Wavelength observations of the Galactic
Center
10 TeV
300 meV
Observational signatures and characterisation
of the central black hole
D. Rouan
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The Galactic Centre

• 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

• 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

L-M map (NACO)
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Distance Sun Proxima Centauri
And one can see only the very luminous ones !
1,3 light-years
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A supermassive Black Hole ?

• 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
  wavelengthsby a factor of 10-9 with respect to
  Eddington luminosityLEdd 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)

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The radio view Sgr A
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The radio view Sgr A

• 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)

• 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)

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Radio

• The minicavity a spherical void of 0.08
  pcdiameter, 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
  dustinside are probably heated byUV from the OB
  central cluster

• The overall dynamics in radio gtsuggests 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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Radio size and spectrum of Sgr A

• 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)

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The X view

• Expected X luminosity if at 10 of the Eddington
  luminosity 4 1043 erg s-1
• Actually Lx(2-10keV) lt 1035 erg s-1
• 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 ?

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The X view pre-Chandra/XMM era

• Until the advent of Chandra and XMM, the only X
  flux detected revealed to be a combination of
  diffuse emission and stellar sources
• ROSAT one source within 10 of SgrA Lx 7
  1035 erg s-1
• ASCA bright diffuse emission of hot gas (10
  keV) associated to SgrA East shell Lx 1036
  erg s-1
• BeppoSAX diffuse emission identified ? upper
  limit for Sgr A Lx 2-10 keV 1035 erg s-1
• GRANAT Lx 35-150 keV lt 6 1035 erg s-1

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And Chandra came...

• 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

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Sgr A in X

• 2-10 keV luminosity 2.4 (1.8-5.4) 1033 erg s-1
• Spectrum
• Well fitted by an absorbed power-lawN(E)
  E-2.7 andNH 1023 cm-2
• Or by a plasma w kT 2 keV
• Possible presence ofa Fe Ka line at 6-7 keV

• Extension
• the source appears extended w respect to point
  sources
• qintrinsic (q2 - qpsf2)1/2 0".6 ? .024 pc
• Variability statistically proven on 1h scale

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Non-BH possible X sources

• 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

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X Flares

• First flare
• Chandra Oct 2000
• Baganoff et al. 01
• Duration 104 s
• N(E) ? E-1.0
• Fastest variation 10min

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Flares spectrum

• 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)

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The gamma view

• Soft g rays detected by EGRET
• Strong source of gt100 MeV in Sgr A direction
• BUT recent re-analyze EGRET source is offset
  (probability to be Sgr A lt 5)

• INTEGRAL hard-X soft g rays
• 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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The gamma view

• TeV emission detected by Whipple
• unique Cherenkov telescope
• First evidence for TeV emission (97)

• TeV g rays emission detected by HESS
• 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 gt variability ? not really
  predicted by various models

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The Infrared View

• Search for
• 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 lt 0.15"

NAOS/CONICA on Yepun VLT-ESO
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IR 1- dynamical signature

• Follow-up of several stars during 10 years
• Very good radio/IR astrometry thanks to SiO
  masers of giant stars
• Orbit of several stars belonging to the very
  central cluster (lt1")
• ESO program MPE-Garching (Genzel et al.)
  Lesia since 4 years
• Keck program A. Ghez
• NAOS/CONICA measurements
• 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 gt 1019 M?pc-3
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Orbit of S2 star in Aug 2003
2.31.2 -3.1 1.2
3.590.29(0.59) 15.56 0.35 2002.330.016
0.881 0.007 45.0 1.6 -48.1 2.3
245.4 1.7 4.63 0.10 0.551 0.010
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Excluded models

• Recent refinement of orbits determination
• Ghez et al. 05 simultaneous constraint from7
  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
• Dark stellar cluster  (BD, neutron star, stellar
  BH ) would impose a central density 1017-19
  M? pc-3 ? lifetime lt 105 years ? rejected
• Ball of fermions (neutrinos, gravitinos, axinos,
  ) ? finite size of 7000 UA gt S2
  perimelanophreas ? rejected

From ancient greek melano black, phreas
well
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IR 2 - the thermal IR emission

• 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

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Comparison to predicted spectra
Yuan et al., 2003
NACO
NACO
Relativist Jet synchrotron(radio/IR) inverse
self-compton (X)
Accretion disk synchrotron by thermal e-
inverse self-compton (X)
5 of electrons accelerated
Good agreement !
But
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IR 3 - variability, flashes

• 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

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Detection of infrared flares

• May 03 detection of a flare in H band
  (1.65µm) (Genzel et al.)
• Followed by several (2 in K, 1 in L)

• Flares Parameters
• typical duration 90 min
• frequency 3 - 5 / day gt X
  frequency(Chandra 1.2 / day)
• sub-period 17 min

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Flare or Flash ?

• In 2004 several events detected
• April flare,
• June flare short flash (lt10 min),
• Sept flare
• All observed in L' band (3.8 µm)

Flash Juin 04
Flare Sept 04
Flare Juin 04
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Separation of flares and quiet mission

• 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

• The quiet emission could correspond to
  synchrotron of a jet and flares to accretion
  events on the horizon of the BH
• Question can a low luminosity jet be extended
  on 300 AU ?

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Flares what constraint do they bring?

• Spectrum looks  blue 
• Energy in IR flares X
• tvar few min ? r lt 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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A simultaneous X / IR flare

• Simultaneous observation of a flare in X
  (Chandra) and IR (NACO)
• Eckart et al. (04)
• Well explained by SSC (Synchrotron Self Compton)
  from a component at a few RSchw
• Sn ? n-1.3
• Time Lag lt 15 min

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IR 4 - Interaction with environnement ?

• 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

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The overall picture

• Taken from Aharonian 04
• Not so far from energy equipartition ...

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Summary

• 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 lt 10
  RSchw
• Radio/X connection Synchrotron Self Compton
• No plausible alternate explanation

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Summary

• 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

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Conclusion

• 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

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Paschen a vs L-M