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 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 /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

Atmospheric De-coherence From Moran & Dhawan 1995

VLBI Coherence Tcoh ~ 4sec Tcoh ~ 10sec ALMA Tcoh ~ 35sec

H-Maser/CSO Comparison  y (s 230GHz345GHz450GHz 5x x x Costa et al 1991

Cryogenic Sapphire Osc for VLBI UWA Metrology Group (Tobar et al)

A CSO VLBI Ref. locked to GPS CSO 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 = 60R s 1mas/yr = 0.25c Walker, Ly, Junor & Hardee 2008

Central Mass M ~ 4x10 6 M  Rsch = 10  as SgrA* Proper Motion V < 15km/s

X-ray/NIR Flares: An Indirect Size Time 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 ~ 

1.3mm Observations of SgrA* 4500km Fringe Spacing = 55  as : A Baseball on the Moon

Determining a Size (Caveat) Gammie et al 14 Rsch (  as)  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 & Loeb 230 GHz, ISM scattered

Closure Phases: Hawaii-CARMA-Chile Spin = 0.9 Hot-spot at ~ 6R g 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 (  ) < 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=12  as Use high frequency VLBI : resolution increases but scattering descreases.

Seeing Through the Scattering  OBS deviates from scattering for  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 results likely suffer from beam depolarization.

In situ standard testing