An Event Horizon Telescope:(sub)mm VLBI of Sgr A*

Shep Doeleman

MIT Haystack Observatory

SgrA*: Best Case for a SMBH• Stellar orbits within 45 AU: M~4x106 Msol • Proper motions < 1km/s: M>10^5 Msol

(Backer & Sramek, Reid & Brunthaler)• Submm/NIR/X-ray flares variability.

VLT: Genzel et al 2003

Baganoff et al 2001

Ghez et al 2005

Resolving Rsch-scale structures

Spinning (a=1) Non-spinning (a=0)

FalckeMeliaAgol

• SgrA* has the largest apparent Schwarzschild radius of any BH candidate.

• Rsch = 10as• Shadow = 5.2 Rsch (non-spinning)

= 4.5 Rsch (maximally spinning)

High Frequency VLBI

Resolution:/D (cm) ~ 0.5 mas/D (1.3mm) ~ 30 as/D (0.8mm) ~ 20 as

ISM Scattering:scat ~

Can’t use VLBA

Short Challenges:Weather, freq. stnds, electronics noise

Increased Sensitivity

Wideband VLBI Systems

• Total cost $40-50K per station.• x16 in BW over current VLBA sustainable rates.• Equivalent to replacing VLBA with 50m antennas.• Used for VLBA upgrade: x4 in sensitivity over current VLBA

sustainable rate.

Digital Backend (DBE) Digital Recorder (Mark5)

mm/submm VLBI CollaborationMIT Haystack: Alan Rogers, Alan Whitney, Mike Titus,

Dan Smythe, Brian Corey, Roger Cappallo, 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, Bob Vessot, Irwin Shapiro, Mark Gurwell, Ray Blundell, Bob Wilson

James Clerk Maxwell Telescope: Remo Tilanus, Per FribergUC Berkeley SSL: Dan WerthimerCaltech Submillimeter Observatory: Richard ChamberlainMPIfR: Thomas KrichbaumASIAA: Makoto Inoue, Paul Ho

1.3mm Observations of SgrA*

4630km

DAYS OBSERVING - April 2007

4030km

908km

Determining the size of SgrA*

SMT-CARMA

SMT-JCMT

OBS = 43as (+14, -8)

INT = 37as (+16, -10)

OBS = θ INT2 +θSCAT

2

Doeleman et al 2008

1 Rsch = 10as

ρ =1023M e pc−3

JCMT-CARMA

The minimum apparent size.Broderick & Loeb

Noble & Gammie

Event Horizon

Ra =3 3 Rsch R≤1.5Rsch

R / 1−Rsch / R R>1.5Rsch

⎧⎨⎪

⎩⎪

Caveat: Very Interesting Structures

Gammie et al

14 R

sch

(1

as)

SMT-CARMA

SMT-JCMT

JCMT-CARMA

But…April 2009 SgrA* Detections

Adding ALMA

Adding Telescopes: RIAF example

Adding LMTHawaii, CARMA, SMT

Time Variable Structures

• Variabilty in NIR, x-ray, submm, radio.

• Probe of metrics near BH, and of BH spin.

• Violates Earth Rotation aperture synthesis.

• Use ‘good’ closure observables to probe structure as function of time.

Hot Spot Model for SgrA* Flares

QuickTime™ and aFLIC Animation decompressorare needed to see this picture.

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.

Detecting Rsch Polarization Structures

The Event Horizon Telescope

• An international collaborative project assemble a global submm-VLBI array for observing and resolving Event Horizons.

• Key observations have removed scientific uncertainty (i.e. will we see anything?).

• Technical Elements: low risk extensions of ongoing efforts and leveraging investments from ALMA and other development projects.

• Collaboration: necessarily a international project using many facilities.

• Strong element of training at intersection of submm science and interferometry.

Adding Telescopes

Phase 1: 7 Telescopes Phase 2: 10 TelescopesPhase 3: 13 Telescopes

Technical Elements of EHT• ALMA quality, dual pol rx and LO for all sites.• Testing hardware for in-situ verification.• Central correlation facility.• VLBI backends/recorders at rates up to 64Gb/s• Phased Array processors (ALMA, PdeBure, CARMA, Hawaii)• Low noise freq. references.• Logistics/Observations/Project Management

Hawaii Phased Array Success(CSO+JCMT+SMA)-CARMA

Progression to an Image: 345GHz

GR Model 7 Stations 13 Stations

Summary• 1.3mm VLBI confirms ~4Rsch diameter for SgrA*• Non-Imaging VLBI can extract BH parameters.• Imaging soon possible with EHT developments.• Increased sensitivity enables submm VLBI to

directly probe Event Horizon scales and trace time variable structure (complements GRAVITY, IXO).

• Over the next decade: A VLBI Event Horizon Telescopewill address fundamental questions of BH physics and space-time.

A Unique Opportunity“Right object, right technique,

right time”