Resolving Black Holes with Millimeter VLBI

Transcription

1 Resolving Black Holes with Millimeter VLBI Vincent L. Fish MIT Haystack Observatory and the Event Horizon Telescope collaboration Model courtesy C. Gammie

2 Bringing resolution to black holes There is lots of interesting physics to be done with black holes: Accretion physics Outflow/jet collimation Tests of general relativity Sgr A* is a great laboratory for testing GR, but we need to understand the astrophysics of the material we see Our understanding of black holes is limited by the lack of resolution The goal of the Event Horizon Telescope collaboration is to study the black holes with the largest apparent angular sizes (especially Sgr A*, M87) at high angular resolution

3 Outline Background on the Galactic Center BH Sgr A* Current millimeter VLBI observations of Sgr A* and M87 Scientific implications of long-baseline detections Overview of present capabilities of the mm VLBI array Progression of the Event Horizon Telescope Future scientific possibilities (testing GR)

4 Evidence that Sgr A* hosts a black hole Stellar orbits

5 Evidence that Sgr A* hosts a black hole Stellar orbits Density Maoz 1998

6 Evidence that Sgr A* hosts a black hole Stellar orbits Density Lack of motion Reid & Brunthaler 2004

7 Evidence that Sgr A* hosts a black hole Stellar orbits Density Lack of motion X-ray flares Baganoff et al. 2001

8 Evidence that Sgr A* hosts a black hole Stellar orbits Density Lack of motion X-ray flares NIR flares? Genzel et al. 2003

9 Evidence that Sgr A* hosts a black hole Stellar orbits Density Lack of motion X-ray flares NIR flares? Conclusion: There is a very dense, very massive object in the center of the Galaxy

10 What might Sgr A* look like? Emission likely comes from a disk and/or a jet Emission is variable on short timescales, suggesting source structure variation on spatial scales of a few RSch courtesy A. Broderick We need to be able to resolve Sgr A* courtesy C. Gammie cartoon courtesy Chandra

11 Features of optically thin disk emission in GR Doppler boosting/deboosting Low-inclination disk models look big (would be resolved out on long baselines) High-inclination disk models produce a bright crescent of emission that is much more compact Photon orbit Gravitational redshift Secondary/tertiary images Back side of disk lensed into field of view Innermost stable circular orbit 6 GM/c 2 for a=0 (no spin) Approaching Receding 1 GM/c 2 for a=1 (maximal spin) but lensed to appear bigger (RSch = 2 GM/c 2 ) 80 Models courtesy A. Broderick

12 GR MHD model Natural variability Features similar to RIAF model: photon orbit Doppler boosting etc.

13 The apparent sizes of black holes Sgr A*: Mass ~ 4.3 x 10 6 Msun (Gillessen et al. 2009) Distance ~ 8.0 to 8.4 kpc RSch ~ 0.08 AU = 10 μas Sgr A* has the largest apparent event horizon from Earth RSch = 2GM/c 2 scales linearly with mass Stellar-mass black holes appear much smaller (factor of 10 6 less massive, but nowhere near factor of 10 6 closer) Next largest: M87 These angular scales are very small, but it is possible to achieve them now from the surface of the Earth

14 The need for millimeter VLBI Resolution: at 230 GHz, Hawaii-Arizona ~ 60 μas, Hawaii-Chile ~ 30 μas Interstellar scattering: goes as λ 2 Doeleman et al. 2008

15 The need for millimeter VLBI Resolution: at 230 GHz, Hawaii-Arizona ~ 60 μas, Hawaii-Chile ~ 30 μas Interstellar scattering: goes as λ 2 Atmospheric opacity courtesy CSO Atmospheric Transmission Interactive Plotter

16 The need for millimeter VLBI Resolution: at 230 GHz, Hawaii-Arizona ~ 60 μas, Hawaii-Chile ~ 30 μas Interstellar scattering: goes as λ 2 Atmospheric opacity Sweet spot is 230 or 345 GHz window Note that resolution is tens of microarcseconds, corresponding to a few Schwarzschild radii (unlensed) At present, no other technique can achieve this level of angular resolution

17 Very Long Baseline Interferometry Visibility is a Fourier component of sky image (amplitude and phase) Longer baseline = finer angular resolution We cannot yet image Sgr A* (not enough baselines) Nevertheless, observables (e.g., visibility amplitude, closure phase) give information about small structure

18 The current state of mm VLBI observations Observations in 2007, 2009, and km 4030km CARMA 4630km JCMT (+ SMA + CSO) In 2010 also observed with ASTE in Chile (but no detections yet) SMT Used with permission from University of Arizona, T. W. Folkers, photographer

19 Size of emitting region in Sgr A* 2007 observations: Detected SMT-CARMA and SMT-JCMT 2009 observations (3 epochs): Detected JCMT-CARMA also Assuming circular Gaussian: Measured size 43 μas (+14, -8) Deconvolved size 37 μas (+16, -10) ~ 3.7 RSch Sizes are consistent across all 4 epochs

20 Minimum apparent size The emission from Sgr A* cannot be centered on the black hole Close to the black hole, strong lensing affects apparent size from the viewpoint of a distant observer General relativity Implication: the emission at 230 GHz must be (partially) optically thin and offset from the center of the black hole Newtonian

21 Density Putting 4 x 10 6 Msun into 37 μas at 8 kpc yields a lower limit on density of several x Msun pc -3 Any collection of ordinary matter would collapse or disperse on a very short time scale Alternate theories exist, but a black hole is the most mundane possibility for the mass of Sgr A* Maoz 1998

22 Existence of an event horizon In the absence of an event horizon, near infrared fluxes and VLBI sizes place a lower limit on the efficiency of conversion of gravitational binding energy: > 99.6% There must be an event horizon Broderick et al. 2009

23 Constraining model parameters If the emitting region in Sgr A* can be described by a disk, current detections place limits on model parameters (e.g., spin of black hole, inclination of axis, orientation on sky)

24 Constraining model parameters If the emitting region in Sgr A* can be described by a disk, current detections place limits on model parameters (e.g., spin of black hole, inclination of axis, orientation on sky) RIAF simulations strongly disfavor low-inclination models (i.e., Sgr A* is not face-on) GR MHD simulations give same result (Mościbrodzka et al. 2009) Inclination Position angle Spin Broderick et al. 2009

25 Constraining model parameters In 2009, we detected Sgr A* on Hawaii-California baseline also Even better constraints Spin Inclination Broderick et al. 2011

26 Constraining model parameters In 2009, we detected Sgr A* on Hawaii-California baseline also Even better constraints on black hole and disk parameters Spin Inclination Position Angle

27 M87 M87 hosts a supermassive black hole with a jet Angular scale (rsch) similar to Sgr A* Walker et al. 2008, 43 GHz

28 Brief summary of current results Millimeter VLBI is already producing interesting science Sgr A* contains a black hole - Density/coalescence arguments - Existence of an event horizon There is very compact emission - This emission must be offset from center of black hole We can place constraints on emission morphology - Very likely not a face-on disk M87 has very compact emission as well Increasing sensitivity and array coverage will lead to even more interesting science

29 Sensitivity and Atmospheric Coherence SNR limited by atmospheric coherence Timescale for coherent integration is 1-30 sec (typically <10) Can segment data (coherently) and incoherently average segments, but... SNR for incoherent averaging asymptotes to t ¼ (not t ½ ) It is important to get signal as quickly as possible

30 Atmospheric coherence Atmospheric turbulence smears out delay rate spectrum Nearly identical scans on the same calibrator on two different days Good coherence: SNR 90 Bad coherence: SNR 25 Necessary to segment data and average incoherently Different source Initial probability of false detection: 40%

31 Technology Wideband digital backends and Mark 5B+ recorders Current capability: 4 Gbit s -1 sustained rate Near future: 16 Gbit s -1 Goal: 32 Gbit s -1 sustained, full polarization, phased arrays

32 CfA/SMA group working to produce phased-array processor for Mauna Kea (JCMT+CSO+SMA) Phased arrays Processor can easily be adapted to CARMA, Plateau de Bure International team (including Mareki Honma) has proposed to phase ALMA

33 Event Horizon Telescope Red: Phase I Yellow: Phases II and III? some telescopes exist (SPT, SEST, Haystack) new sites (ATF dishes?) as seen from Sgr A*

34 The Event Horizon Telescope Phase I telescope locations Hawaii (phased JCMT + CSO + SMA) Arizona (ARO/SMT) California (phased CARMA)

35 The Event Horizon Telescope Phase I telescope locations Hawaii (phased JCMT + CSO + SMA) Arizona (ARO/SMT) California (phased CARMA) Chile (ASTE, APEX, and/or ALMA)

36 The Event Horizon Telescope Phase I telescope locations Hawaii (phased JCMT + CSO + SMA) Arizona (ARO/SMT) California (phased CARMA) Chile (ASTE, APEX, and/or ALMA) Pico Veleta (IRAM 30 m) Plateau de Bure (phased)

37 The Event Horizon Telescope Phase I telescope locations Hawaii (phased JCMT + CSO + SMA) Arizona (ARO/SMT) California (phased CARMA) Chile (ASTE, APEX, and/or ALMA) Pico Veleta (IRAM 30 m) Plateau de Bure (phased) Mexico (LMT)

38 The Event Horizon Telescope Phase I telescope locations Hawaii (phased JCMT + CSO + SMA) Arizona (ARO/SMT) California (phased CARMA) Chile (ASTE, APEX, and/or ALMA) Pico Veleta (IRAM 30 m) Plateau de Bure (phased) Mexico (LMT) (u,v) coverage is important because we don t know a priori what the interesting spatial scales and orientations are

39 Tracking rapid source structure changes Sgr A* is known to be variable on timescale of a few minutes Innermost stable circular orbital period ranges from ~4 min (maximally rotating) to ~30 min (Schwarzschild)

40 Closure quantities Tracking structural changes on rapid timescales at millimeter wavelengths (where the coherence time is short) will require robust, non-imaging observables Closure phase and amplitude are independent of most complex gain, clock, and tropospheric delay errors

41 Tracking rapid source structure changes Sgr A* is known to be variable on timescale of a few minutes Innermost stable circular orbital period ranges from ~4 min (maximally rotating) to ~30 min (Schwarzschild) Closure phases will be able to detect source structure changes Periodicity could give clue to spin

42 Closure phases including Chile In some models (especially high spin and/or inclination), long baselines are needed to see large closure phases Sensitivity is very important; a single dish may not be enough Phased ALMA a = 0.9, hot spot at 6 RG

43 Imaging Eventual goal will be to produce an image of Sgr A* (and M87) This will require both sensitivity and good (u,v) coverage Model 7 stations 13 stations 345 GHz, interstellar scattering included

44 Testing general relativity: Shadow size Black hole shadow or silhouette predicted from any optically-thin emission The size of the shadow is only very weakly dependent on the spin of the black hole dshadow ~ 10.4 GM/c 2 (Schwarzschild black hole) 9.0 GM/c 2 (maximally spinning black hole) To within a few percent, we know: the predicted shadow diameter in GM/c 2 the mass of Sgr A* the distance to Sgr A* GM/c 2 in μas Testable prediction of GR: size of shadow to within less than 10 μas Falcke et al. (2001)

45 Testing general relativity: The no-hair theorem Black holes have three hairs: mass, spin, and charge Improbable that black holes have (much) charge, so only 2 hairs Mass (M): monopole moment Spin (a): dipole moment In the Kerr metric, all higher order terms are set by M and a Glampedakis & Babak (2006) and Johannsen & Psaltis (2010) investigate divergence from Kerr in the quadrupole term Q = -M(a 2 + εm 2 ) Non-Kerr solutions would show deviations in shape of photon orbit

46 Testing general relativity: The no-hair theorem Figures courtesy T. Johannsen & D. Psaltis ε = 0 ε = 0.5 a = 0 a = 0.4

47 Testing general relativity: The no-hair theorem Phased ALMA will be especially sensitive to the spatial scales on which we might see deviations from GR

48 Conclusions Millimeter VLBI observations have constrained the sizes of emitting regions in Sgr A* and M87 to be in the GR regime These observations are already producing interesting science (existence of event horizon, emission offset from black hole, constraints on model parameters...) Sensitivity upgrades and the inclusion of additional telescopes in future observations will be able to detect signatures of changing source structures in Sgr A* Future expansion of the Event Horizon Telescope collaboration may allow for very high resolution images of Sgr A* and M87, enabling tests of general relativity in the vicinity of a black hole

49 The Event Horizon Telescope collaboration MIT Haystack NRAO Harvard-Smithsonian CfA / SMA U. Arizona / ARO CARMA JCMT Caltech / CSO ASIAA NAOJ MPIfR-Bonn IRAM / Pico Veleta + Plateau de Bure UC Berkeley UMass Amherst / LMT & others