Probing into the shadow of the galactic center black hole with sub-millimeter VLBI Zhi-Qiang Shen (Shanghai Astronomical Observatory) In collaboration with: L. Huang, M. Cai, F. Yuan, S.-M. Liu K. Y. Lo, M.-C. Liang, P. T. P. Ho, J.-H. Zhao CHINA-US BILATERAL WORKSHOP ON ASTRONOMY 21-25 April 2008, Conference Hall at Sleeping Buddha Temple Hotel, Beijing, CHINA
outline Introduction to SgrA* s size measurements Discovery/Scattering/Elongation Detection of intrinsic size of SgrA* closure quantities/mm-vlbi/wavelength dependence Simulation of sub-mm VLBI image of SgrA* Summary 2008-4-29 2
(14-m RT in Goss, Brown & Lo 2003) Discovery paper in 1974 3 baselines between 3x26-m of GBI and the 14-m at 35 km angular resolution: 0.7 & 0.3 arcsec at 11 & 3.7 cm observed (apparent) source size: 0.1 arcsec total flux density: 0.6 Jy@11 cm; 0.8 Jy@3.7cm ; -> non-thermal
Discovery paper 3 baselines between 3x26-m of GBI and the 14-m at 35 km angular resolution of 0.7 & 0.3 arcsec at 11 & 3.7 cm observed (apparent) source size: 0.1 arcsec total flux density: 0.6 Jy@11 cm; 0.8 Jy@3.7cm low elevation (<10 deg) data should be ignored!
Θ = 1.4 ν = 1. 63 λ 2 2 obs arc sec GHz mas cm 0. 18±0. 03 0. 015±0. 004 First suggestion for lambda^2 scattering 3 baselines of early MERLIN MkIA-MkIII: 23.70 km @ 1.66 GHz (18 cm) -> 2.5 arc MkII-MkIII: 23.80 km @ 0.96 GHz (31 cm) -> 4.3 arc MkIA-Defford: 127 km @ 0.408 GHz (74 cm) -> 10 arc X Circular Gaussian diameter: 1. 5±0. 3 @ 31 cm; 0. 5 ±0. 1 @ 18 cm flux density: 0.56 ±0.06 Jy@18 cm; 0.26 ± 0.03 Jy@31 cm
VLBI observations of Sgr A* Interstellar scattering effect dominates the cm-vlbi images of SgrA* by lambda^2 with an apparent elongated shape in E-W Θ 2 2 2 2 2 obs = Θscat + Θint = Aλ ) ( + Θ 2 int wavelengths: 6.0, 3.6, 2.0, 1.35 and 0.7 cm (Lo, Shen, Zhao, Ho 1998)
Mm-VLBI observations of Sgr A* Most VLBA data of SgrA* are taken at low elevations (10-20 deg) where atmospheric effects are substantial large absorption due to spectral line transitions of water vapor and oxygen at mm poor spatial resolution in N-S (happen to be along the minor axis) short and variable coherence time limited sensitivity of RT (high Tsys and low efficiency) large uncertainty in the amplitude self-cal To improve, we need to use the closure amplitude! SC MK GBT FD KP PT LA OV HN NL BR
3mm CMVA observations six of the CMVA in April 1999 best-fit circular: 0.18±0.02 mas the measured closure phases consistent with zero The importance of the closure amplitude (!)
Science 304 (2004) 704 Intrinsic size detected at 7mm! More accurate measurements (using the closure amplitude analysis) Major axis (E-W) : 1.41 lambda^2 Intrinsic size: 0.70 mas @ 1.35cm; 0.24 mas @ 0.69 cm the intrinsic size as a function of wavelength
Major axis (E-W): 0.268 mas = 2.14 au =26.8 R sc 0.126 mas = 1.01 au =12.6 R sc Θ A = 1.39 ± 0.02 2 2 2 2 2 obs = Θscat + Θint = Aλ ) ( + Θ 2 int Intrinsic size of 1 AU at 3mm! Dynamic observations in November 2002 with the VLBA closure amplitude constraint refine 2-D scattering law: (1.39±0.02) lambda^2 (E-W); (0.69±0.06) lambda^2 (N-S) Size of 0.126 and 0.268 mas (or, 1 and 2 AU) at 3 and 7 mm, respectively
dark mass concentration of ~4.0x10 6 M within 90 AU stellar motion (Schödel et al. 2002; Ghez et al. 2005) 30 AU 40 AU proper motion of Sgr A* (Reid & Brunthaler 2004) Sgr A* mass >0.4x10 6 M within 1 AU! 2008-4-29 3mm VLBA (Shen et al. 2005) 11
3mm VLBA Observations (Shen et al. 2005) The best ever 3mm measurement in Nov 2002 indicates an intrinsic size of 0.126 mas, or, ~1 AU @ 8 kpc, or, ~13 R sc (4 x 10 6 M sun ). Mass density > 8.0 10 21 M pc -3 (10 12 times higher than any others) M SgrA* > 4 x 10 5 M sun
λ-dependence of the intrinsic size Θ int λ 1.09(+0.34/-0.32) ~26.7Rsc at 7mm ~12.6Rsc at 3.5mm emission is stratified shadow diameter: ~5Rsc Shadow size Event horizon Shen et al. 2005
Intrinsic size vs. wavelength VLA+PT at 17.5-25.2 cm in October 2004 best-fit (VLA+PT) major axis scat. 1.31 lambda^2 intrinsic size detected at various wavelengths derived an intrinsic size: lambda^(1.6±0.1)
the power law index of the intrinsic size? Θ obs =Θ scat +Θ int = ( ) + ( ) 2 2 2 2 2 2 Aλ Bλ β 6 pt (VLBA) 6 pt (VLBA) + 6pt (VLA) β = 1.80 + 0.04 0.77 Note: the PA of VLA results are 66, 70, 75, 75, 77, and 98. while those from VLBA are ~80. β = 1.63 + 0.19 0.35 depend on the accurate calibration of the scattering law! A new multi-wavelength (6,3.6,2,1.3,0.7 and 0.35 cm) VLBA imaging observation in Nov 2007
0.6 mm 1.3 mm Rotating Non-rotating (Falcke, Melia, Agol 2000) Black hole is strongly curved space(time) outside horizon - not just the horizon! 2008-4-29 16
shadow of Sgr A* The diameter of the shadow is about ~5 R sc for any BH. This corresponds to an angular size of about 50 μas for 4 million solar masses SMBH Sgr A*. (R sc = 0.08 AU = 10 μas) At 1 mm, we will be able to see the intrinsic source structure! 1. The extrapolated intrinsic size is ~3.5 R sc, or, ~35 μas. 2. The scattering size (decreasing as λ 2 )and is 14 μas. => SgrA* is the most important target for the future sub-mm VLBI experiment to test the GR effect.
Simulation of VLBI obs Accretion Flow What kind of emission model is appropriate? Black Hole At what band can VLBI obs see the shadow? Photon Trajectory Interstellar Scattering How does the light bend due to GR effect? VLBI Observation What is the scattering effect?
the RIAF Model with the Size Measurements (Yuan, Shen & Huang 2006) Calculating the intrinsic intensity profile from RIAF: not Gaussian distribution Assumptions: Schwarzschild BH; face-on RIAF Ray-tracing calculation + the relativistic effects (gravitational redshift; light bending; Doppler boosting) again not Gaussian Simulate the observed (apparent) size by taking into account the scattering broadening and compare it with observations RIAF
Intrinsic intensity Scattered Gaussian fit (Yuan, Shen & Huang 2006, ApJ) Detailed structure near the black hole can survive the ISM smoothing at 1.3 mm or shorter. It would be a good test of the presence of black hole. 7 mm 3.5 mm 1.3 mm
Simulated image of Sgr A* (Huang, Cai, Shen, & Yuan, 2007) North East RIAF model (Yuan et al. 2003) Observer i=90 deg 45 deg 0 deg 1.3 mm (ray-tracing) 1.3 mm (scattered) 3.5 mm (ray-tracing) 3.5 mm (scattered)
RIAF: a large inclination angle (i >60 deg, edge-on case)
Jet+nozzle Observer Jet+nozzle: a large viewing angle (i >50 deg) (also i>75 deg by Markoff, Bower & Falcke 2007)
Linear and Circular Polarization (Huang, Liu, Shen, Cai, Li, & Fryer, 2008) Two-temperature MRI-driven Accretion flow cm mm (i =40 deg) Bower et al.(2000); Aitken et al(2000); Macquart et al.(2006); Marrone et al.(2007); Eckart et al.(2006); Meyer et al.(2006) High linear polarization at sub-mm 90 o flip between sub-mm and NIR 2008-4-29 24
~32m[6xOVRO (10m)+9xBIMA(6m)] 2200 m, CA 0.8 mm SMTO (10m, 3186 m,tucson, 0.8 mm) potential sub-mm antennas for VLBI JCMT (15m, 4092 m, HI, 0.8 mm) LMT (50m, 4600 m, Mexico, 0.8 mm) ALMA (64x12m+, 5000 m) SMA (8x6m, 4200 m, HI) 2008-4-29 25
Visibility analysis
Visibility Slices Σ appear at a few 10 3 km σ Huang et al. (2007)
North visibility distribution is sensitive to the geometry Observer East Projected baseline length position angle inclination angle
conclusions SgrA*: the closest SMBH candidate with the largest angular size of its event horizon (1 mas = 8AU at D=8 kpc; R sc = 2GM /c² = 0.08 AU = 10 μas ) A wavelength-dependent intrinsic size of SgrA* s emitting region has been consistently seen with a diameter of about 1 AU at 3.5 mm, supporting its SMBH nature when combined with the mass estimated from both orbital motion and its proper motion. High quality radio interferometric images of SgrA* at multi-wavelengths from long cm to short mm bands are needed To get an accurate calibration of the scattering law To clarify the lambda dependence of the intrinsic source size To obtain an 2-D intrinsic source structure 2008-4-29 29
conclusions Ray-tracing calculations, when compared with the size measurements at 7 and 3mm, favor an edge-on accretion flow structure (large viewing angle in case of jet-nozzle model). Sub-mm VLBI (polarimetric) imaging: promising for resolving the shadow of Sgr A* and a test of the GR in strong field regime Visibility analysis: constrain the geometry of radio emitting region surrounding Sgr A* 2008-4-29 30
Thank you!
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