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mm/submm VLBI of Sagittarius A*: An Event Horizon Telescope

This article discusses the observations and research conducted on Sagittarius A* using the Event Horizon Telescope (EHT), including topics such as accretion, radiation, and outflow. It explores the challenges faced in high-frequency VLBI observations and the determination of the size of Sagittarius A*.

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mm/submm VLBI of Sagittarius A*: An Event Horizon Telescope

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  1. mm/submm VLBI of Sagittarius A*:An Event Horizon Telescope Shep Doeleman MIT Haystack Observatory

  2. Centaurus A: Optical

  3. Centaurus A: Radio

  4. Chandra, VLA, VLT

  5. Accretion • Radiation • Outflow • GR in strong field regime • Accretion/jet models • Black hole parameters A. Fabian Chandra Web site

  6. Vstars ~104 km/s Central Mass M ~ 4x106 M Within 45 AU. Rsch = 10as

  7. Proper Motion Studies • Is SgrA* the central mass? • Luminosity consistent with stellar source. • Proper motion of SgrA* measured with VLA and VLBA : +/- 1km/s upper limits (Reid & Brunthaler 2004, Backer et al 1999) • If SgrA* in dynamical equilibrium with stars, then M>4x105 Msol. • So SgrA* is not Stellar and is coincident with dynamical center of Galaxy.

  8. 10000 20000 30000 Time offset (s) X-ray/NIR Flares: An Indirect Size VLT: Genzel et al 2003 ~17 min periodicity? Orbiting hot-spot at R=4 Rsch Baganoff et al 2001 Rise time < 300s Light crossing < 12 Rsch

  9. High Frequency VLBI Resolution: /D (cm) ~ 0.5 mas /D (1.3mm) ~ 30 as /D (0.8mm) ~ 20 as ISM Scattering: scat ~  Self Absorption: Intrinsic SgrA* size decreases with 

  10. VLBI Basics Earth Rotation Baseline Coverage Interferometer F T • Fourier Components (visibilities) Map

  11. 3mm and 7mm VLBI • 7mm (Bower et al 2004): VLBA • Scattering size 670as, Observed size 712as • Intrinsic size = 24 Rsch • 3mm (Shen et al 2005): VLBA • Scattering size 170as, Observed size 210as • Intrinsic size = 12.6 Rsch • Both in scattering dominated regime and insensitive to ISCO scale (60as for a=0). • Can’t use ‘facility’ VLBI instruments VLBA: ad hoc arrays using (smaller) mm/submm apertures. • High Frequency Challenges: Atmosphere, LO’s, weather, frequency reference stability. • Increased sensitivity is critical.

  12. Resolving Rsch-scale structures Falcke Melia Agol Spinning (a=1) Non-spinning (a=0) • SgrA* has the largest apparent Schwarzschild radius of any BH candidate. • Rsch = 10as • Shadow = 5.2 Rsch (non-spinning) = 4.5 Rsch (maximally spinning)

  13. New 4Gb/s VLBI System Digital Recorder (Mark5) Digital Backend (DBE) • Total cost $40-50K per station. • x16 in BW over current VLBA sustainable rates. • Equivalent to replacing VLBA with 50m antennas. • Planned VLBA/HSA 4Gb/s upgrade by early 2009: x4 in sensitivity over current VLBA sustainable rate.

  14. 1.3mm Observations of SgrA* 908km 4030km 4630km DAYS OBSERVING - April 2007

  15. mm/submm VLBI Collaboration MIT Haystack: Alan Rogers, Alan Whitney, Mike Titus, Dan Smythe, Brian Corey, Roger Cappallo, 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, Bob Vessot, Irwin Shapiro, Mark Gurwell, Ray Blundell, Bob Wilson James Clerk Maxwell Telescope: Remo Tilanus, Per Friberg UC Berkeley SSL: Dan Werthimer Caltech Submillimeter Observatory: Richard Chamberlain MPIfR: Thomas Krichbaum ASIAA: Makoto Inoue, Paul Ho

  16. SMT-CARMA SMT-JCMT Determining the size of SgrA* OBS = 43as (+14, -8) INT = 37as (+16, -10) JCMT-CARMA 1 Rsch = 10as Doeleman et al 2008

  17. Alternatives to a MBH Most condensations of smaller mass objects evaporate on short timescales. Remaining possibility: Boson Star R=Rsch + epsilon Depends on Boson mass M87? Mass BH = 3.2x109Msol Distance = 16Mpc Rsch=4 micro arc sec. Maoz 1998

  18. Does SgrA* have a surface? • A quiescent surface would radiate. • The radius R and dM/dt set the expected NIR flux. • NIR limits set dM/dt limits. • These are too low to power SgrA*. • If no surface then Event Horizon. A Surface or Event Horizon Broderick & Narayan 2006

  19. Broderick & Loeb The minimum apparent size. Event Horizon Noble & Gammie

  20. SMT-CARMA SMT-JCMT Caveat: Very Interesting Structures 14 Rsch (140as) JCMT-CARMA Gammie et al

  21. But…April 2009 SgrA* Detections

  22. SgrA*: Luminosity • Leddington ~ 3.3x1044 erg/s • Accretion estimates : 1.6x10-5 Msol/yr to 3x10-6 Msol/yr, stellar loss ~3x10-3 Msol/yr • If 10% efficiency then Lacc ~ 1041 erg/s • Lx (Chandra) = 2x1033 erg/s (2-10 keV) • Lradio < 1036 erg/s • SgrA* is 10-8 Ledd and over 10-5 less than expected from accretion arguments. • RIAFs : Radiatively Inefficient Accretion Flows

  23. Model Correlated Flux Density Quiescent disk models Courtesy Gammie et al and Broderick&Loeb

  24. Constraining RIAF Models with VLBI Broderick, Fish, Doeleman & Loeb (2009)

  25. Adding ALMA Hawaii, CARMA, SMT Adding LMT Adding Telescopes

  26. 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.

  27. Hot Spot Model for SgrA* Flares

  28. Hot Spot Models (P=27min) 230 GHz, ISM scattered Models: Broderick & Loeb Spin=0.9, orbit = 2.5xISCO Spin=0, orbit = ISCO

  29. Hot Spot Model (a=0, i=30) SMTO-Hawaii-CARMA, 8Gb/s, 230GHz, 10sec points

  30. Closure Phases: Hawaii-CARMA-Chile Spin = 0.9 Hot-spot at ~ 6Rg Period = 27 min.

  31. Detecting Rsch Polarization Structures

  32. 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 leverage investments from ALMA and other development projects. • Collaboration: necessarily a international project but with well defined boundaries. • Strong element of training at intersection of submm science and interferometry. The Event Horizon Telescope

  33. Event Horizon Telescope Phase 1: 7 Telescopes Phase 2: 10 Telescopes Phase 3: 13 Telescopes

  34. Progression to an Image: 345GHz GR Model 7 Stations 13 Stations

  35. Adding Telescopes: uv coverage, flare coverage, closure quantities for real-time modeling. • 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 Technical Elements of EHT

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

  37. EHT Phases: Phase I: 7 station 8Gb/s array ALMA phasing, preliminary Rx/LO work, new frequency standards, new site studies, operations. 2010 -- 2014 Phase II: 10 station 32Gb/s dual-pol array Activate SEST, equip S.Pole, new frequency standards, install new 0.8/1.3mm dual-pol Rx, increase bandwidth of VLBI backends/recorders, relocate ATF dishes, operations. 2015 -- 2018 Phase III: 12 station array up to 64Gb/s install ATF dishes, array operations for 5 years. 2019 -- 2024

  38. VLBA Movie of M87 @ 43 GHz (7 mm)Walker et al. 2008 More luminous class of AGN with more massive central BH Eg M87, half the apparent size of SgrA* (1000 x more massive) Beam: 0.43x0.21 mas 0.2mas = 0.016pc = 60Rs 1mas/yr = 0.25c

  39. CARMA1-CARMA2 CARMA-ARO/SMT JCMT-ARO/SMT JCMT-CARMA Resolved component is > 300as Compact component is 35as FWHM, or ~ 4.5 Rsch. April 2009 M87 1.3mm VLBI Detections

  40. Mass of M87 Black Hole = 6.4x109 Msol • The angular size of the Schwarzschild radius is similar to SgrA*’s: 8as. • Apparent size of Black Hole ‘Shadow’ is 40as (5.2 Rsch for non spinning BH). • Apparent Size of the Innermost Stable Circular Orbit (ISCO) is 60as. • These are the relevant size scales for models of TeV photon generation and jet formation. • 1.3mm VLBI is now detecting compact structure on these size scales. • With higher sensitivity, more VLBI sites, and the phased array at SMA, this technique can image a radio loud AGN on Rsch scales. A Closeup of M87

  41. 1.3mm VLBI confirms ~4Rsch diameter for SgrA* • Non-Imaging VLBI can extract BH parameters. • submm VLBI is able to directly probe Event Horizon scales and trace time variable structure (complements GRAVITY, IXO). • Over the next decade: • A VLBI Event Horizon Telescope • will address fundamental questions of BH physics and space-time. Summary

  42. A Unique Opportunity“Right object, right technique, right time”

  43. ISCO and Black Hole Spin Orbital Period gives spin. Other (indirect) methods for detection of spin: -- QPO’s -- Modeling thin disks -- GR distortions of Fe lines -- IXO: spectral obs of individual hot-spots. -- GRAVITY: NIR obs of SgrA* centroid motion. C. Reynolds

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