AGN 2006 “THE CENTRAL ENGINE OF ACTIVE GALACTIC NUCLEI” High-resolution millimeter-VLBI study of Sgr A* - A SMBH at the Galactic center Zhi-Qiang Shen (Shanghai Astronomical Observatory) In collaboration with: K. Y. Lo (NRAO), M.-C. Liang (Caltech), P. T. P. Ho (ASIAA), J.-H. Zhao (CfA) AGN 2006 “THE CENTRAL ENGINE OF ACTIVE GALACTIC NUCLEI” Xi’an, China, 16-21 October 2006

The extremely elongated orbit of S2 takes about 15.2 years to complete S2: young massive star 15x Sun's mass and 7x its diameter Orbital parameters for S2 Period: 15.2 yr Inclination: 46 deg Eccentricity: 0.87 Semi-major: 0.119 arcsec (5.5 l-d) Pericenter: 124 au (17 l-h) Central mass: 3.7x106 M⊙ 2019/2/22 Schödel, R. et al. 2002, Nature

Enclosed mass of 4.0x106 M⊙ within 2 light days Enclosed mass of 4.0x106 M⊙ within a radius of 45 AU (Ghez et al. 2005) 2019/2/22

Intrinsic Proper Motion of Sgr A* itself position residuals of Sgr A* wrt J1745-283 — galactic plane --- best fit intrinsic proper motion ( galactic plane) 8 km s-1  MSgrA* > 4 x 105 M⊙ galactic plane (Reid & Brunthaler 2004) Best Fit 2019/2/22

dark mass concentration of 4.0x106 M⊙ within 90 AU Sgr A* mass > 0.4x106 M⊙ within ? 2019/2/22

dark mass concentration of 4.0x106 M⊙ within 40 AU Sgr A* mass > 0.4x106 M⊙ within 1 AU ! 2019/2/22

VLBI Observations of Sgr A* Interstellar scattering effect dominates the cm-VLBI images of SgrA* by – law, with an apparent E-W elongated shape Need mm-VLBI! Observer SgrA* ISM 2019/2/22

Mm-VLBI observations of Sgr A* The mm-VLBI plagued by 2 facts southerly Dec of SgrA* (~ - 30o) northern lat. for most mm-VLBI antennas GBT SC MK PT LA FD KP HN NL OV BR GBT-SC SC-HN GBT-HN 2019/2/22

Mm-VLBI observations of Sgr A* The mm-VLBI plagued by 2 facts southerly Dec of SgrA* (~ - 30o) northern lat. for most mm-VLBI antennas lack of spatial resolution in N-S ( = minor axis) severe atmospheric effects on data calibration (large and variable opacity, short and variable coherence time) + compromised sensitivity at mm-band (high Tsys: >100 K at zenith; low antenna efficiency: < 45%) 2019/2/22

How to improve ? During observations Data analysis dynamic scheduling frequent pointing Data analysis closure amplitudes to constrain the model-fitting 2019/2/22

Model fitting using the Closure Amplitude Constraints (Shen et al 2 – minimization algorithm here, the visibility amplitude Aij is used, “good observable” - the closure amplitude is conserved by assuming an antenna-dependent gain Gi only. This is equivalent to the use of closure quantities! 2019/2/22

From the existing 7mm data Epoch Ctr Freq(+BW) GHz (+ MHz) S (Jy) Major axis (mas) Minor axis P.A (degree) Reduced chi^2 SC- HN Notes 1994.32 43.151 (64) 1.4 0.72 +/- 0.01 0.39 +/- 0.07 78 +/- 2 1.11 yes 1994.75 1.3 0.42 +/- 0.03 79 +/- 1 1.17 Bower & Backer 1998 1997.12 43.213 (32) 1.0 0.71 +/- 0.01 0.42 +/- 0.05 74 +/- 2 2.89 no Lo et al 1998; dual pol 1999.31 43.135 (32) 0.69 +/- 0.01 0.33 +/- 0.04 83 +/- 1 0.97 1.26 x 0.44 @ 7o 1999.39 1.5 0.44 +/- 0.02 1.59 1.35 x 0.48 @ 11o 1999.41 0.75 +/- 0.01 0.49+/- 0.05 70 +/- 3 0.85 39.135 (32) 1.6 0.86 +/- 0.01 0.54+/- 0.03 78 +/- 1 1.54 39 GHz 45.135 (32) 0.66 +/- 0.01 0.42 +/- 0.04 75 +/- 3 1.31 45 GHz 2001.58 42.8-43.1 (32) 0.9 0.74 +/- 0.01 0.47 +/- 0.14 77 +/- 6 3.41 Average over 7 epochs: major 0.72 +/- 0.02 mas minor 0.42 +/- 0.04 mas P.A. 77 +/- 3 deg 2019/2/22

comparison of size measurement with scattering angle @ 7mm Major axis P.A. Minor axis 1.42λ2 0.70λ2 either the scattering angle should be scaled-up (?) or the source intrinsic size comes to play (?) 2019/2/22

Scattering law revisited Quasi-simultaneous, five-band (6, 3.6, 2, 1.35 & 0.7 cm) VLBA observations Model fit with closure amplitude constraints major Θ ＝ (1.39±0.02) λ2 minor Θ ＝ (0.69±0.06) λ2 Shen et al. 2005 an even smaller (1.39 < 1.42) scattering size along the major axis！ 2019/2/22

First 3mm VLBI image of SgrA* Nov 3, 2002 (dynamic scheduling since Feb 2001) 512 Mbps (highest recording rate) + Frequent pointing check (every 15 min) Very good detections among 5 antennas (FD/KP/LA/OV/PT), plus some to NL UN beam 1.11 mas x 0.32 mas @ 9o Super-resolution 0.2 mas unresolved (no extended structure) → single component zero closure phases → symmetrical structure (~E-W) elongated emission → consistent withλ≥ 7mm data 2019/2/22

Model fitting results Contour plot showing the confidence intervals of minor axis: 0.13 (+0.05 / -0.13) mas and PA: 79o (+12o / -33o) major axis: 0.21 (+0.02 / -0.01) mas Contour plot showing the confidence intervals of 68.3% and 90.0%. Surface plot of Chi^2 as a function of both minor axis and PA (with a fixed major axis size of 0.21 mas). 2019/2/22

2nd epoch 3mm VLBA Observation Observations on Sept 28, 2003 512 Mbps; pointing check every 15 min gust @ OV, tape (recording, playback) @ KP, PT Shen et al. 2006a, (in prep.) 2019/2/22

Elliptical Gaussian Model (major, minor, pa) Apparent SgrA* structure at 3mm: elongated roughly along E-W with a major axis size of 0.21 mas Elliptical Gaussian Model (major, minor, pa) circular 2002 Nov, VLBA (Shen et al. 2005) 0.21(+0.02/-0.01),0.13(+0.05/-0.13), 79(+13/-33) 0.20 - 0.21 2003 Sept, VLBA (Shen et al. 2006) 0.21(+0.01/-0.01), 0.00-0.13, 87(+12/-9) 0.20 2019/2/22

Two epochs of 7mm VLBA + GBT (512 Mbps) observations in March 2004 Fitted Apparent SgrA* structure at 7mm apparent major (mas), minor (mas), pa (deg) 2004 March 08 0.722(+/-0.002), 0.395(+0.019/-0.020), 80.4(+/-0.8) 2004 March 20 0.725(+/-0.002), 0.372(+0.020/-0.018), 80.8(+0.6/-0.9) 2019/2/22

Intrinsic size revealed 7mm: 2 epochs of VLBA+GBT in March 2004 3.5mm: 2 epochs of VLBA in Nov 2002 and Sept 2003 Major axis (E-W): 0.268 mas = 2.14 au =26.8 Rsc 0.126 mas = 1.01 au =12.6 Rsc brightness temperature Tb > 1.2 x 1010 K dark mass density (>4 × 105 M◉ within 1 AU) > 6.5 × 1021 M◉ pc-3 (SMBH) Shen et al. 2005 2019/2/22

λ-dependence of the intrinsic size Shadow size Event horizon 2019/2/22 Shen et al. 2005

testing RIAF model for SgrA* using VLBI size measurements RIAF (Radiatively Inefficient Accretion Flow) model VLBI size measurements at 7 and 3.5 mm (~2 & 1 AU) Take into account the interstellar scattering Yuan, Shen & Huang 2006 2019/2/22

～５Rs Shadow Images 　　 ～５Rs Whatever the model looks like the shadow is always visible! If there is a black hole, we are going to see it. 2019/2/22

What can we learn from the shadow of black hole? The diameter of the shadow is about 10 gravitational radii or 5 Schwarzschild radii for any BH. This corresponds to an angular size of about 50 micro arcsec for 4 million solar masses SMBH Sgr A*. The extrapolated intrinsic size at 1.0 mm is about 3.5 Rsc, or, 35 micro arcsec. The scattering size will decrease as the 2 and is as small as <15 micro arcsec at 1.0 mm. SgrA* will be the most important target for the future sub-mm VLBI experiment to test the GR effect. 2019/2/22

structural variability structural variation detected at 7mm on 31 May 1999 Larger (than usual) deviations 3 sigma along the minor axis Intrinsic sizes derived 0.334 +/- 0.042 mas (EW) 0.359 +/- 0.095 mas (NS) First detection of intrinsic size along minor axis of 2.87 AU Symmetric Increased (cf. 0.268+/- 0.025 mas) by ~25% to ~34% (Shen et al. 2006b in prep) Shen et al. 2004 2019/2/22

possible geometry of the flare hot accretion flow active region 7mm radio emission black hole 26.8RSch ~ 40 RSch Shen et al. in prep. 2019/2/22

Thanks! Summary We’ve refined the scattering size vs. the observing wavelength relation as 1.39 mas at 1cm compared to 1.42 mas from the old one along the major axis. We obtained the 1st 3mm VLBI image of SgrA* with the dynamically scheduled VLBA observation. The consistent E-W elongated apparent structure is seen. This is confirmed by the 2nd 3mm VLBA observation. Both 3 and 7mm data show that the intrinsic size has come to play with the scattering size. The inferred intrinsic size is about 1 and 2.1 AU at 3 and 7mm, respectively. The inferred lower limit to the mass density of Sgr A* is 6.5×1021 M◉ pc-3, supporting its SMBH hypothesis. A structural variation was detected, showing an intrinsically symmetric structure that increases more than 25-34% in its size. 2019/2/22