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Frequency Standards and VLBI: Observing an Event Horizon. Sheperd Doeleman MIT Haystack Observatory. mm/submm VLBI Collaboration. MIT Haystack : Alan Rogers, Alan Whitney, Mike Titus, Dan Smythe, Brian Corey, Roger Cappalo, Vincent Fish U. Arizona Steward Obs: Lucy Ziurys, Robert Freund

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frequency standards and vlbi observing an event horizon

Frequency Standards and VLBI: Observing an Event Horizon

Sheperd Doeleman

MIT Haystack Observatory

mm submm vlbi collaboration
mm/submm VLBI Collaboration

MIT Haystack: Alan Rogers, Alan Whitney, Mike Titus,

Dan Smythe, Brian Corey, Roger Cappalo, Vincent Fish

U. Arizona Steward Obs: Lucy Ziurys, Robert Freund

CARMA: Dick Plambeck, Douglas Bock, Geoff Bower

Harvard Smithsonian CfA: Jonathan Weintroub, Jim Moran,

Ken Young, Dan Marrone, David Phillips, Ed Mattison, Paul Yamaguchi

James Clerk Maxwell Telescope: Remo Tilanus, Per Friberg

UC Berkeley SSL: Dan Werthimer

Caltech Submillimeter Observatory: Richard Chamberlain

MPIfR: Thomas Krichbaum

JHU - Applied Physics Labs: Greg Weaver

Honeywell: Irv Diegel

the vlbi technique

The VLBI Technique

/D (cm) ~ 0.5 mas

/D (1.3mm) ~ 30 as

vlbi basics
VLBI Basics



Baseline Coverage



  • Map must be real valued
  • Usually most of map is blank
  • Visibilities Map
  • Sparsely Sampled
atmospheric de coherence

Atmospheric De-coherence

From Moran & Dhawan 1995

vlbi coherence

VLBI Coherence

Tcoh ~ 4sec

Tcoh ~ 10sec


~ 35sec

h maser cso comparison

H-Maser/CSO Comparison

Costa et al 1991

cryogenic sapphire osc for vlbi

Cryogenic Sapphire Osc for VLBI

UWA Metrology Group (Tobar et al)

the vlba 43 ghz m87 movie first 11 observations
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 Mass

M ~ 4x106 M

Rsch = 10as


Proper Motion

V < 15km/s

x ray nir flares an indirect size



Time offset (s)

X-ray/NIR Flares: An Indirect Size

VLT: Genzel et al 2003

~17 min periodicity?

Baganoff et al 2001

Rise time ~300s

Light crossing = 12 Rsch

what we really want the shadow
What we really want: the ‘Shadow’

Falcke et al

free fall





0.6mm VLBI

GR Code

1.3mm VLBI

SgrA* has the largest apparent Schwarzschild radius of any BH candidate. BUT… SgrA* scattered ~ 

1 3mm observations of sgra
1.3mm Observations of SgrA*


Fringe Spacing = 55as : A Baseball on the Moon

alternatives to a mbh
Alternatives to a MBH

Evaporation and


Maoz 1998

1 3mm vlbi
<= 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
Hot Spot Models (P=27min)

230 GHz, ISM scattered

Models: Broderick & Loeb

Spin=0.9, orbit = 2.5xISCO

Spin=0, orbit = ISCO

closure phases hawaii carma chile
Closure Phases: Hawaii-CARMA-Chile

Spin = 0.9

Hot-spot at ~ 6Rg

Period = 27 min.

hot spot model a 0 i 30
Hot Spot Model (a=0, i=30)

SMTO-Hawaii-CARMA, 8Gb/s, 230GHz, 10sec points

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

VLBI Fringes

Atmospheric Turbulence

GHz : Ionosphere

>1 GHz: Troposphere

scattering towards sgra
Scattering towards SgrA*
  • Scattering size ~ l2
  • Intrinsic Structure masked by scattering : need high frequencies.
  • Lack of observed scintillation of SgrA* at 0.8mm sets lower size

limit : 2Rsch=12mas

  • Use high frequency

VLBI : resolution

increases but scattering


seeing through the scattering
Seeing Through the Scattering

OBS deviates

from scattering

for cm

INT  SCAT

for mm

INT 

mm submm vlbi plans

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 results likely suffer from beam depolarization.