High-resolution mm-VLBI Imaging of Sgr A*
Download
Report
Transcript High-resolution mm-VLBI Imaging of Sgr A*
High-resolution millimeterVLBI 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⊙
2015/7/16
Schödel, R. et al. 2002, Nature
2
Enclosed mass of 4.0x106 M⊙ within
a radius of 45 AU (Ghez et al. 2005)
50mas
2 light days
2015/7/16
3
Intrinsic Proper Motion of Sgr A* itself
position residuals of Sgr A* wrt J1745-283
— galactic plane
--- best fit
intrinsic proper motion ( galactic plane)
galactic
plane
8 km s-1
MSgrA* > 4 x 105 M⊙
(Reid & Brunthaler 2004)
Best Fit
2015/7/16
4
dark mass concentration of 4.0x106 M⊙ within 90 AU
Sgr A* mass > 0.4x106 M⊙
within ?
2015/7/16
5
dark mass concentration of 4.0x106 M⊙ within 40 AU
Sgr A* mass > 0.4x106 M⊙
within 1 AU !
2015/7/16
6
VLBI Observations of Sgr A*
Interstellar scattering effect dominates
2
the cm-VLBI images of SgrA* by – law,
with an apparent E-W elongated shape
2
2
2
2
obs
scat
int
( A2 ) 2 int
SgrA*
2015/7/16
ISM
Observer
7
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
SC
GBT
MK
FD
KP
PT
LA
OV
HN NL
BR
S GBT-SC
CH
N
GBT-HN
2015/7/16
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
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%)
2015/7/16
9
How to improve ?
During observations
dynamic scheduling
frequent pointing
Data analysis
closure amplitudes to
constrain the model-fitting
2015/7/16
10
Model fitting using the Closure Amplitude
Constraints (Shen et al. 2003)
2 – minimization algorithm
2 wij | Aij (t ) Gi (t ) Gj (t ) Aij (t ) |2
obs
t
mod
ij
here, the visibility amplitude Aij is used,
“good observable” - the closure amplitude
C ijkl
AA
A A
ij
kl
ik
jl
is conserved by assuming an antenna-dependent
gain Gi only.
This is equivalent to the use of closure quantities!
2015/7/16
11
From the existing 7mm data
Epoch
Ctr Freq(+BW)
GHz (+ MHz)
S
(Jy)
Major axis
(mas)
Minor axis
(mas)
P.A
(degree)
Reduced
chi^2
SCHN
1994.32
43.151 (64)
1.4
0.72 +/- 0.01
0.39 +/- 0.07
78 +/- 2
1.11
yes
1994.75
43.151 (64)
1.3
0.72 +/- 0.01
0.42 +/- 0.03
79 +/- 1
1.17
yes
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)
1.0
0.69 +/- 0.01
0.33 +/- 0.04
83 +/- 1
0.97
yes
1.26 x 0.44 @ 7o
1999.39
43.135 (32)
1.5
0.71 +/- 0.01
0.44 +/- 0.02
79 +/- 1
1.59
yes
1.35 x 0.48 @ 11o
1999.41
43.135 (32)
1.5
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)
1.5
0.66 +/- 0.01
0.42 +/- 0.04
75 +/- 3
1.31
45 GHz
42.8-43.1 (32)
0.9
0.74 +/- 0.01
0.47 +/- 0.14
77 +/- 6
3.41
2001.58
Average over 7 epochs: major 0.72 +/- 0.02 mas
minor 0.42 +/- 0.04 mas
P.A.
77 +/- 3 deg
2015/7/16
Notes
yes
12
comparison of size measurement
with scattering angle @ 7mm
2
2
2
2
obs
scat
int
( A2 ) 2 int
P.A.
Major axis
1.42λ2
Minor axis
0.70λ2
either the scattering angle should be scaled-up (?)
or the source intrinsic size comes to play (?)
2015/7/16
13
Quasi-simultaneous,
five-band (6, 3.6, 2,
1.35 & 0.7 cm) VLBA
observations
Model fit with closure
amplitude constraints
Shen et al. 2005
Scattering law revisited
major Θ ＝ (1.39±0.02)
λ2
minor Θ ＝ (0.69±0.06)
2
an λ
even
smaller (1.39 < 1.42) scattering size along the major axis！
2015/7/16
14
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
o
UN beam 1.11 mas x 0.32 mas @ 9
Super-resolution 0.2 mas
unresolved (no extended structure) → single component
zero closure phases → symmetrical structure
(~E-W) elongated emission → consistent withλ≥ 7mm data
2015/7/16
15
Model fitting results
major axis: 0.21 (+0.02 / -0.01) mas
minor axis: 0.13 (+0.05 / -0.13) mas
and PA: 79o (+12o / -33o)
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).
2015/7/16
16
2nd epoch 3mm VLBA Observation
512 Mbps; pointing check every 15 min
gust @ OV, tape (recording, playback) @ KP, PT
Shen et al. 2006a, (in prep.)
Observations on Sept 28, 2003
2015/7/16
17
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)
2002 Nov, VLBA
(Shen et al. 2005)
2003 Sept, VLBA
(Shen et al. 2006)
2015/7/16
circular
0.21(+0.02/-0.01),0.13(+0.05/-0.13), 79(+13/-33) 0.20 - 0.21
0.21(+0.01/-0.01),
0.00-0.13,
87(+12/-9)
0.20
18
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)
2015/7/16
19
Intrinsic size revealed
7mm:
3.5mm:
2 epochs of
VLBA+GBT in March 2004
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)
2015/7/16
Shen et al. 2005
20
λ-dependence of the intrinsic size
Θint ∝ λ1.09(+0.34/-0.32)
Event horizon
2015/7/16
Shen et al. 2005
21
testing RIAF model for SgrA* using
VLBI size measurements
Take into account the interstellar scattering
Yuan, Shen & Huang 2006
RIAF (Radiatively Inefficient Accretion Flow) model
VLBI size measurements at 7 and 3.5 mm (~2 & 1 AU)
2015/7/16
22
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.
2015/7/16
～５Rs
23
What can we learn from the
shadow of black hole?
1.
2.
3.
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.
2015/7/16
24
structural variability
Larger (than usual) deviations
3 sigma along the minor axis
Intrinsic sizes derived
Shen et al. 2004
structural variation detected at 7mm on 31 May 1999
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)
2015/7/16
25
possible geometry of the flare
active
region
~ 40 RSch
black hole
26.8RSch
hot accretion flow
7mm radio
emission
Shen et al. in prep.
2015/7/16
26
Summary
Thanks!
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.
2015/7/16
27