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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. The Galactic Centre.

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multi wavelength observations of the galactic center

10 TeV

Multi-Wavelength observations of the Galactic Center

300 meV

Observational signatures and characterisation

of the central black hole

D. Rouan

the galactic centre
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

D. Rouan - LESIA - Obs. de Paris

  • 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)

a supermassive black hole
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)

D. Rouan - LESIA - Obs. de Paris

the radio view sgr a
The radio view : Sgr A*

D. Rouan - LESIA - Obs. de Paris

the radio view sgr a1

0.9pc

2cm

3pc

7pc

VLA 6cm

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 strongpoint source(Balik & Brown 74) : Sgr A*
    • no infrared nor X counterpart until 2000-2
    • Non-thermal radiation (synchrotron)
    • variability : 2 typically (Brown & Lo, 82)

D. Rouan - LESIA - Obs. de Paris

radio
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

D. Rouan - LESIA - Obs. de Paris

  • 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
radio size and spectrum of sgr a
Radio size and spectrum of Sgr A*
  • 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)
  • 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)

D. Rouan - LESIA - Obs. de Paris

the x view
The X view
  • Expected X luminosity if at 10% of the Eddington luminosity = 4 1043 erg s-1
  • Actually : Lx(2-10keV) < 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 ?

D. Rouan - LESIA - Obs. de Paris

the x view pre chandra xmm era
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* : Lx2-10 keV= 1035 erg s-1
    • GRANAT : Lx35-150 keV < 6 1035 erg s-1

D. Rouan - LESIA - Obs. de Paris

and chandra came

1'.3 x 1'.5

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

D. Rouan - LESIA - Obs. de Paris

sgr a in x
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

D. Rouan - LESIA - Obs. de Paris

  • Extension:
    • the source appears extendedw respect to point sources
    • qintrinsic = (q2 - qpsf2)1/2 = 0".6  .024 pc
  • Variability : statistically proven on 1h scale
non bh possible x sources
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

D. Rouan - LESIA - Obs. de Paris

x flares

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

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

D. Rouan - LESIA - Obs. de Paris

flares spectrum

XMM : Porquet et al. (04)

XMM : Goldwurm et al. (03)

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)

D. Rouan - LESIA - Obs. de Paris

the gamma view
The gamma view
  • Soft g rays detected by EGRET
    • 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
    • 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 :

D. Rouan - LESIA - Obs. de Paris

the gamma view1

Whipple(Kosack & al 04)

Hess(Aharonian 04)

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 => variability ? not really predicted by various models

D. Rouan - LESIA - Obs. de Paris

the infrared view

Onera + Obs. de Paris + Obs. De Grenoble

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 < 0.15"

D. Rouan - LESIA - Obs. de Paris

NAOS/CONICA on Yepun VLT-ESO

ir 1 dynamical signature
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 (<1")
    • 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

D. Rouan - LESIA - Obs. de Paris

  • 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

excluded models
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

D. Rouan - LESIA - Obs. de Paris

  • Excluded Models :
    • 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

slide20

Clénet & al 04

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*

D. Rouan - LESIA - Obs. de Paris

comparison to predicted spectra
Comparison to predicted spectra

Yuan et al., 2003

NACO

NACO

D. Rouan - LESIA - Obs. de Paris

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…

ir 3 variability flashes

Clénet et al. 04

Ghez et al. 04

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

D. Rouan - LESIA - Obs. de Paris

detection of infrared flares
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 > X frequency(Chandra : 1.2 / day)
    • sub-period : 17 min

D. Rouan - LESIA - Obs. de Paris

flare or flash
Flare or Flash ?
  • In 2004 : several events detected
    • April: flare,
    • June : flare + short flash (<10 min),
    • Sept : flare
  • All observed in L' band (3.8 µm)

D. Rouan - LESIA - Obs. de Paris

Flare Sept 04

Flash Juin 04

Flare Juin 04

separation of flares and quiet mission
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

D. Rouan - LESIA - Obs. de Paris

  • 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 ?
flares what constraint do they bring
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

D. Rouan - LESIA - Obs. de Paris

  • 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)
a simultaneous x ir flare
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 < 15 min

D. Rouan - LESIA - Obs. de Paris

ir 4 interaction with environnement

Deconvolved L image

K, L, M Images

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

D. Rouan - LESIA - Obs. de Paris

  • Another red elongated source
    • further away
    • with a bow shock appearance
    • ≈ in the same direction
    • no counterpart at Paschen a
the overall picture
The overall picture
  • Taken from Aharonian 04
  • Not so far from energy equipartition ...

D. Rouan - LESIA - Obs. de Paris

summary
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 < 10 RSchw
    • Radio/X connection : Synchrotron Self Compton
    • No plausible alternate explanation

D. Rouan - LESIA - Obs. de Paris

summary1
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

D. Rouan - LESIA - Obs. de Paris

conclusion
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

D. Rouan - LESIA - Obs. de Paris

paschen a vs l m
Paschen a vs L-M

D. Rouan - LESIA - Obs. de Paris