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