1. 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, October 2006

2. 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: yr
Inclination: 46 deg
Eccentricity: 0.87
Semi-major: 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

3. 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

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

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

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

7. 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

8. 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

9. 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

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

11. 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

12. 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
(64)
1.4
0.72 +/- 0.01
0.39 +/- 0.07
78 +/- 2
1.11
yes
1.3
0.42 +/- 0.03
79 +/- 1
1.17
Bower & Backer 1998
(32)
1.0
0.71 +/- 0.01
0.42 +/- 0.05
74 +/- 2
2.89 no
Lo et al 1998; dual pol
(32)
0.69 +/- 0.01
0.33 +/- 0.04
83 +/- 1
0.97
1.26 x 7o
1.5
0.44 +/- 0.02
1.59
1.35 x 11o
0.75 +/- 0.01
0.49+/- 0.05
70 +/- 3
0.85
(32)
1.6
0.86 +/- 0.01
0.54+/- 0.03
78 +/- 1
1.54
39 GHz
(32)
0.66 +/- 0.01
0.42 +/- 0.04
75 +/- 3
1.31
45 GHz
(32)
0.9
0.74 +/- 0.01
0.47 +/- 0.14
77 +/- 6
3.41
Average over 7 epochs: major / mas
minor / mas
P.A /- 3 deg
2019/2/22

13. 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

14. 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

15. 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 o
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

16. 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: (+0.02 / -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

17. 2nd epoch 3mm VLBA Observation
Observations on Sept 28, 2003
512 Mbps; pointing check every 15 min
OV, tape (recording, KP, PT
Shen et al. 2006a, (in prep.)
2019/2/22

18. 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)
2003 Sept, VLBA
(Shen et al. 2006)
0.21(+0.01/-0.01), , (+12/-9)
0.20
2019/2/22

19. 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), (+0.019/-0.020), 80.4(+/-0.8)
2004 March 20
0.725(+/-0.002), (+0.020/-0.018), 80.8(+0.6/-0.9)
2019/2/22

20. 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

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

22. 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

23. ～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

24. 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, 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

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

26. 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

27. 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