ultra-high angular resolution vlbi - science website · 2014-07-30 · ultra-high angular...
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![Page 1: Ultra-High Angular Resolution VLBI - Science Website · 2014-07-30 · Ultra-High Angular Resolution VLBI enabled by mm-VLBI Tuesday, May 20, 14. The quest for high resolution VLBI](https://reader030.vdocument.in/reader030/viewer/2022041019/5ece2721cebd7c0f84041042/html5/thumbnails/1.jpg)
MIT Haystack Observatory
Rusen Lu (路如森)[email protected]
Ultra-High Angular Resolution VLBI
Tuesday, May 20, 14
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MIT Haystack Observatory
Rusen Lu (路如森)[email protected]
Ultra-High Angular Resolution VLBI
enabled by mm-VLBI
Tuesday, May 20, 14
![Page 3: Ultra-High Angular Resolution VLBI - Science Website · 2014-07-30 · Ultra-High Angular Resolution VLBI enabled by mm-VLBI Tuesday, May 20, 14. The quest for high resolution VLBI](https://reader030.vdocument.in/reader030/viewer/2022041019/5ece2721cebd7c0f84041042/html5/thumbnails/3.jpg)
The quest for high resolution VLBI
typical resolution (ground-based):λ/D (cm) ∼ 0.5 mas
for a given frequency (e.g., maser sources), resolution can be improved only by increasing D (space VLBI)
space VLBIshorter λ
future: space (sub)mm-VLBI
{
Both are challenging, but feasible
Tuesday, May 20, 14
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Marscher et al.
Self-absorption:look “deeper”
Advantages provided by mm-VLBI
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Size
Wavelength
Sgr A*
Scattering in the ISMϴscat ∝ λ2
scatt
ering
law
Doeleman et al. 2008
Advantages provided by mm-VLBI
Self-absorption:look “deeper”
Tuesday, May 20, 14
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Faraday rotation:𝜒 ∝ λ2
Advantages provided by mm-VLBI
Scattering in the ISMϴscat ∝ λ2
Self-absorption:look “deeper”
Tuesday, May 20, 14
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EHT Sites● Mauna Kea, Hawaii: SMA (~8 x 6-m, single polarization) JCMT (15-m, single polarization) ● Mount Graham, Arizona: SMT (10-m, dual polarization) ● Inyo Mountains, California: CARMA (5 x 10-m + 3 x 6-m, dual polarization;10-m, dual polarization, reference) ● Sierra Negra, Mexico: LMT (50-m)
● Atacama desert, Chile, APEX, (12-m)● Atacama desert, ALMA, (85-m)
● Pico Veleta (Sierra Nevada, Spain, 30-m)● Plateau de Bure (France, 35-m)
● South Pole Telescope (10-m)● Greenland Telescope (12-m)
The EHT as viewed from Sgr A*
The Event Horizon Telescope: (a global (sub)mm-VLBI array)
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Not all black holes are created equal:• Sgr A*: 4 million M☉ BH, Rsch = 10 μas
• M87: ∼6.6 billion M☉ BH, Rsch = 7.5 μas
a=0.998 a=0
size = 9/2 * Rsch size = sqrt(27)*Rsch
(Bardeen 1973, Falcke, Agol & Melia 2000)
EHT provides well-matched resolution!
~ 30-20 µas
(near) Future goal: black hole shadow imaging
Tuesday, May 20, 14
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(Lu et al. 2014, ApJ, in press)
model reconstruction
MEM, Bayesian approachVarying Loading Radii
Minimum requirements:
1. The counter jet has to be sufficiently bright for the black hole to cast a jet against (Rload ≼11 M)
2. The phased ALMA has to be included in the array with bandwidth × coherence time ≿ 4GHz × 12 s at 230 GHz (more stringent requirement at 345GHz)
{
Imaging the BH shadow in M87
Tuesday, May 20, 14
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(Lu et al. 2014, ApJ, in press)
model reconstruction
MEM, Bayesian approachVarying Loading Radii
Minimum requirements:
1. The counter jet has to be sufficiently bright for the black hole to cast a jet against (Rload ≼11 M)
2. The phased ALMA has to be included in the array with bandwidth × coherence time ≿ 4GHz × 12 s at 230 GHz (more stringent requirement at 345GHz)
{
Imaging the BH shadow in M87
Tuesday, May 20, 14
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model scattered reconstruction
other applications:low frequency VLBA images of Sgr A*or, other scatter-broadened sources(?)
The effects of scattering can be mitigated by correcting the visibilities before reconstructing the image
reconstruction of corrected visibilities
Fish
et
al. i
n pr
ep
Imaging the BH shadow in Sgr A*(overcome scattering broadening)
230 GHz
RIAF + Ray Tracing
Tuesday, May 20, 14
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SMT-CARMA
SMT-JCMT
About 4 Schwarzschildradii across
1.3 mm emission offset from the BH
JCMT-CARMA
Doeleman et al. 2008, Nature
SgrA* has the largest apparent event horizon of any black hole in the Universe
Horizon-scale structure in Sgr A*
Tuesday, May 20, 14
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Resolving jet-launching structure in M87
Doeleman et al. 2012, Science
size of jet base
set by ISCO?
M87 measured size = 5.5 Rsch
prograde
retrograde
1 sigma
cred
it:Sk
y &
Tel
esco
pe
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Resolving structure in Sgr A*
consistent sign (daily average) over many days over the course of multiple years [compare: characteristic timescale GM/c^3 ~ 20s]
Fish et al. in prep
no point symmetry: elliptical Gaussian, uniform ring, two-sided jet in the sky plane etc.
Non-zero closure phase detected Median closure phase (+6.3 deg) on the California-Hawaii-Arizona triangle
sign of day-to-day variability
call for physically motivated models
Tuesday, May 20, 14
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EHT polarimetry calibration
before calibration
Johnson et al.
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EHT polarimetry calibration
after calibration
Johnson et al.
Fractional Polarization due to instrumentation is removedmodest and slow varying polarization in BL Lac
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low polarization on short baselines (beam depolarization?)
high polarization on long baselinesfine-scale structures are polarized
EHT polarimetry: 3C279
N
E
0.5 pc
3C 279, EHT@230GHz
John
son
et a
l.
Lu et al. 2013
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Probing inner structure of AGN jets: an example
-0.4-0.200.20.40.6Right ascension (mas)
0
0.5
1
1.5
2
2.5
Rel
ativ
e d
ecli
nat
ion
(m
as)
2004.112005.392006.522007.272008.542009.32
3.4
° /yr
NRAO 530
Lu, Krichbaum & Zensus, 2011
investigate Tb at the jet base
multi-epoch data to study jet acceleration & “precession”?(may need to combine low frequency data)
preliminary2011
2012
NRAO 530
3C 279
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Horizon-scale structures in Sgr A* and M 87 detected
Imaging black hole shadow in Sgr A* and M87 demonstrated (within reach in next few years)
Polarimetry as a new tool to probe B field structure in the vicinity of nearby black holes
New data point towards “complex” and extremely compact structures in Sgr A*
Study AGN jet formation and propagation on sub-pc scales (horizon scales for M87)
Summary
Tuesday, May 20, 14