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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

Averaging over time and frequency

Averaging over Time and Frequency



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)

A cso vlbi ref locked to gps


A CSO VLBI Ref. locked to GPS

CSO Control

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

Determining a size caveat

14 Rsch (140as)

Gammie et al

Determining a Size (Caveat)

FWHM = 3.7 Rsch

Alternatives to a mbh
Alternatives to a MBH

Evaporation and


Maoz 1998

The minimum apparent size

Broderick & Loeb

The minimum apparent size.

Event Horizon

Noble & Gammie

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.