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Jason Dexter 8/27/2009

From the Event Horizon to Infinity: Connecting Simulations with Observations of Accreting Black Holes. Jason Dexter 8/27/2009. Accretion. Material falling onto a central object Gravitational binding energy radiation Any angular momentumdisk, spin+fieldsjets It’s everywhere: Stars

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Jason Dexter 8/27/2009

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  1. From the Event Horizon to Infinity:Connecting Simulations with Observations of Accreting Black Holes Jason Dexter 8/27/2009

  2. Accretion • Material falling onto a central object • Gravitational binding energyradiation • Any angular momentumdisk, spin+fieldsjets • It’s everywhere: • Stars • Planetary, debris disks • Compact Objects • (Super)novae • X-ray bursts • AGN, microquasars General Exam 8/27/2009

  3. Black Holes • a, M • Innermost stable circular orbit • Photon orbit General Exam 8/27/2009

  4. Astrophysical Black Holes • Types: • Stellar mass (100-101 Msun) • Supermassive (106-109 Msun) • IMBH? (103-106 Msun) • No hard surface • Energy lost to black hole • Inner accretion flow probes strong field GR • Astronomy↔Physics Non-accreting BH General Exam 8/27/2009

  5. Accretion Power • Black, but brightest persistent objects in universe • Ultrarelativistic jets • Black hole, galaxy evolution • AGN feedback M87 Jet (HST) General Exam 8/27/2009

  6. Accretion Disk Theory • Thin Disk Accretion (‘standard’, ‘alpha’) • Shakura & Sunyaev (1973), Novikov & Thorne (1973) • Cold & Bright (107 K, 105 Lsun) • AGN, “soft state” x-ray binaries • Advection Dominated Accretion (‘ADAF’,’RIAF’) • Ichimaru (1977), Narayan & Yi (1995), Yuan et al (2003) • Hot & Thick (1010 K) • Sgr A*, Low luminosity AGN, quiescent x-ray binaries Narayan & Quataert (2005) General Exam 8/27/2009

  7. The MRI • How does matter lose angular momentum? • Magnetized fluid with Keplarian rotation is unstable: “magnetorotational instability” • Velikhov (1959), Chandrasekhar (1961), Balbus & Hawley (1991) • Not viscosity, but transports angular momentum outaccretion! • Toy model -- assume ideal MHD: • Field tied to fluid elements • Tension force along field lines, “spring” General Exam 8/27/2009

  8. Toy Model of the MRI • Radially separated fluid elements differentially rotate. • “Spring” stretches, slows down inner element and accelerates outer. • Inner element loses angular momentum and falls inward. Outer element moves outward. • Differential rotation is enhanced and process repeats. Strong magnetic field growth, turbulence General Exam 8/27/2009

  9. GRMHD Simulations • More physics • 3D, global, fully relativistic • Produce MRI, turbulence, accretion • Difficult computationally • Short run times • No radiation • Need to compare to observations! De Villiers et al (2003) General Exam 8/27/2009

  10. Ray Tracing • Method for performing relativistic radiative transfer • Turn fluid variables in accretion flow into observed emission at infinity. • Radiative transfer equationPath integral • Two parts: • Calculate light trajectories. • Solve radiative transfer equation along ray  General Exam 8/27/2009

  11. Ray Tracing • Assume light rays are geodesics. (ω >> ωp, ωc) • Observer “camera” constants of motion • Trace backwards to ensure that all rays used make it to observer simultaneously. • Integrate along portions of rays intersecting flow. • IntensitiesImage, many frequenciesspectrum, many timeslight curve Schnittman et al (2006) General Exam 8/27/2009

  12. New Geodesics Code • Dexter & Agol (2009) : • New fast, accurate, analytic code to compute photon trajectories around spinning black holes. • Includes time, azimuth dependence. • Ideal for GRMHD! Luke Barnes Master’s Thesis General Exam 8/27/2009

  13. Toy Ray Tracing Problems: Thin Disk • Mapping of camera to equatorial plane • Image of Novikov & Thorne BH Schnittman & Bertschinger (2004); Dexter & Agol (2009) General Exam 8/27/2009

  14. Toy Ray Tracing Problems:Black Hole Shadow Bardeen (1973); Dexter & Agol (2009) Falcke, Melia & Agol (2000) General Exam 8/27/2009

  15. Sagittarius A* • Discovered as radio source by Balick & Brown (1974) • Mass, distance from stellar orbits (4x106 Msun at 8 kpc) • Extremely faint (102-3 Lsun) General Exam 8/27/2009

  16. Sgr A* • Best candidate for high-res VLBI imaging, but still tiny! (10-10 rad) • High resolution: ~λ/D • Sub-mm: scattering~λ2 • Doeleman et al, Nature, 2008: • Detections of Sgr A* at 1.3mm using an Arizona-Hawaii baseline. • Gaussian: size ~ 4 Rs General Exam 8/27/2009

  17. VLBI fits from a RIAF model Broderick et al (2008) General Exam 8/27/2009

  18. Emission from GRMHD • Units • Black hole mass sets length, time scales • Mass scale independent: free parameter scaled to produce observed flux and set accretion rate • Thermal synchrotron emission, absorption • Electron temperature? Yuan et al (2003) General Exam 8/27/2009

  19. VLBI fits from GRMHD Images and visibilities of a=0.9 simulation from Fragile et al (2007) Dexter, Agol & Fragile (2009); Doeleman et al (2008) i=10 degrees i=70 degrees 100 μas 10,000 km General Exam 8/27/2009

  20. Accretion Rate Constraint • From VLBI measurements alone • Independent of, consistent with constraints from polarimetry, spectral fitting • Strong spin, Te dependence? General Exam 8/27/2009

  21. Light Curves General Exam 8/27/2009

  22. Millimeter Flares Marrone et al (2008) Eckart et al (2008) General Exam 8/27/2009

  23. Sgr A* Summary • First time-dependent synchrotron images, light curves from 3D GRMHD • Excellent fits at all inclinations • If Sgr A* is face-on, may soon detect black hole shadow • New (model-dependent) method to constrain accretion rate • Magnetic turbulence can produce observed mm flares without magnetic reconnection General Exam 8/27/2009

  24. Limitations and Future Work • Non-conservative simulation • Equal ion/electron temperatures • Te(r) agrees with RIAF • Single spin, wavelength • Spin dependence of accretion rate constraint • Black hole mass constraint? • Polarization General Exam 8/27/2009

  25. Event Horizon Telescope From Shep Doeleman’s Decadal Survey Report on the EHT UV coverage (Phase I: black) Doeleman et al (2009) General Exam 8/27/2009

  26. Tilted Disks • “Tilted” GRMHD: Black hole spin axis not aligned with torus axis. • Solid body precession • Standing shocks, plunging streams. Fragile et al (2007), Fragile & Blaes (2008) General Exam 8/27/2009

  27. Tilted Disk Sgr A* Images • Low spin  Higher accretion rate to match observed flux  Optically thick flows • Tilted disks look funny • Need observational signatures! a=0.3, i=50 degrees a=0.7, i=0 degrees a=0.9, i=70 degrees General Exam 8/27/2009

  28. Inner Edge of Tilted Disks • Attempts to extract spin use thin disk spectra to locate rin, rina • Toy model: emissivity=density2, cut out fluid inside some radius General Exam 8/27/2009

  29. Summary • Ray tracing important for connecting state of the art simulations to observations! • New analytic geodesics code (Dexter & Agol 2009) • Fast, accurate, public • First synchrotron light curves, VLBI fits from GRMHD (Dexter, Agol & Fragile 2009) • May be on verge of directly observing “shadow” • Simulated flares agree with observations • Inner edge of tilted disks • May bias towards low spins General Exam 8/27/2009

  30. The Beautiful Future • Sgr A* • Expand VLBI analysis • Incorporate spectral constraints • Tilted Disks • Inner edge as a function of spin • QPOs? • Other systems • M87! • X-ray binaries, AGN McKinney & Blandford (2009) General Exam 8/27/2009

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