Post on 29-Jan-2016
transcript
Frequency Standards and VLBI: Observing an Event Horizon
Sheperd Doeleman
MIT Haystack Observatory
mm/submm VLBI CollaborationMIT Haystack: Alan Rogers, Alan Whitney, Mike Titus,
Dan Smythe, Brian Corey, Roger Cappalo, Vincent FishU. Arizona Steward Obs: Lucy Ziurys, Robert Freund CARMA: Dick Plambeck, Douglas Bock, Geoff BowerHarvard Smithsonian CfA: Jonathan Weintroub, Jim Moran,
Ken Young, Dan Marrone, David Phillips, Ed Mattison, Paul Yamaguchi
James Clerk Maxwell Telescope: Remo Tilanus, Per FribergUC Berkeley SSL: Dan WerthimerCaltech Submillimeter Observatory: Richard ChamberlainMPIfR: Thomas KrichbaumJHU - Applied Physics Labs: Greg WeaverHoneywell: Irv Diegel
The VLBI Technique
/D (cm) ~ 0.5 mas
/D (1.3mm) ~ 30 as
VLBI Basics
Interferometer Baseline Coverage
F T
Earth Rotation
• Visibilities Map
• Sparsely Sampled
• Map must be real valued
• Usually most of map is blank
Averaging over Time and Frequency
Frequency
Time
dφdν
=delay
dφdt
= fringerate
Atmospheric De-coherence
From Moran & Dhawan 1995
VLBI Coherence
Tcoh ~ 4sec
Tcoh ~ 10sec
ALMA
Tcoh ~ 35sec
H-Maser/CSO Comparison
y( s 230GHz 345GHz 450GHz
5x10-14 0.55 0.3 0.14
2x10-14 0.91 0.83 0.73
1x10-14 0.98 0.95 0.92
φ =2πντσ y (τ )
C =e−φ2
Costa et al 1991
Cryogenic Sapphire Osc for VLBI
UWA Metrology Group (Tobar et al)
A CSO VLBI Ref. locked to GPSCSO
CSO Control
Centaurus A: Optical
Centaurus A: Radio
The VLBA 43 GHz M87 Movie First 11 Observations
Beam: 0.43x0.21 mas 0.2mas = 0.016pc = 60Rs 1mas/yr = 0.25c
Walker, Ly, Junor & Hardee 2008
Central MassM ~ 4x106 M
Rsch = 10as
SgrA*Proper MotionV < 15km/s
X-ray/NIR Flares: An Indirect Size
10000 3000020000Time offset (s)
Baganoff et al 2001
Rise time ~300s Light crossing = 12 Rsch
VLT: Genzel et al 2003~17 min periodicity?
What we really want: the ‘Shadow’
GR Code 0.6mm VLBI 1.3mm VLBI
rotating
non-rotating
free fall
orbiting
Falcke et al
SgrA* has the largest apparent Schwarzschild radius of any BH candidate. BUT… SgrA* scattered ~ 2
1.3mm Observations of SgrA*
4500km
Fringe Spacing = 55as : A Baseball on the Moon
Determining a Size (Caveat)
Gammie et al14
Rsc
h (
a
s)
FWHM = 3.7 Rsch
Alternatives to a MBH
Maoz 1998
Evaporation and Condensation
The minimum apparent size.Broderick & Loeb
Noble & Gammie
Event Horizon
<= 1.3mm-VLBI• Number of antennas is limited.
• More sensitive to weather.
• More sensitive to phase noise in electronics and H-maser.
• Time hard to get on mm-wave telescopes.
• Calibration difficult: use closure relations
Hot Spot Models (P=27min)
Spin=0, orbit = ISCO Spin=0.9, orbit = 2.5xISCO
Models: Broderick & Loeb230 GHz, ISM scattered
Closure Phases: Hawaii-CARMA-Chile
Spin = 0.9Hot-spot at ~ 6Rg
Period = 27 min.
Hot Spot Model (a=0, i=30)
SMTO-Hawaii-CARMA, 8Gb/s, 230GHz, 10sec points
Summary• 1.3mm VLBI confirms ~4Rsch diameter for SgrA*• Implies that SgrA* is offset from Black Hole.• submm VLBI is able to directly probe Event
Horizon scales and trace time variable structure.
• Move to 345/450GHz requires frequency standards with y() < 10-14 at 10s.
• Exploring H-Maser alternatives and modifying H-masers for short-term stability.
• Imaging/Modeling Event Horizon possible within ~5 years: new telescopes in Chile.
• Spare frequency standards?
VLBI Fringes
Atmospheric Turbulence
GHz : Ionosphere
>1 GHz: Troposphere
Scattering towards SgrA*• Scattering size ~ 2
• Intrinsic Structure masked by scattering : need high frequencies.
• Lack of observed scintillation of SgrA* at 0.8mm sets lower size
limit : 2Rsch=12as
• Use high frequency VLBI : resolution increases but scattering descreases.
Seeing Through the Scattering
OBS deviatesfrom scatteringfor cm
INT SCAT
for mm
INT
mm/submm VLBI plans
• Phase up apertures on Mauna Kea and CARMA to increase SNR (x10).• Observe again on SMT-HI-CARMA triangle.• Within 2 years add 4th antenna (Chile or LMT).• Move to 345GHz and dual polarization.
Connected element polarimetry resultslikely suffer from beam depolarization.
In situ standard testing