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Astro2020 Science White Paper on Horizon Scales

Thematic Areas: and Fundamental Physics, Formation and Evolution of Compact Objects, Evolution, Multi-Messenger and Astrophysics

Principal Author: Name: Sheperd S. Doeleman, EHT Director Institution: Center for Astrophysics | Harvard and Smithsonian Email: [email protected]

Co-authors: Kazunori Akiyama1,2,3, Lindy Blackburn4, Katherine L. Bouman4, Geoffrey C. Bower5, Avery E. Broderick6, Andrew Chael4,7, Vincent L. Fish2, Michael D. Johnson4,7, Thomas P. Krichbaum8, Colin J. Lonsdale2, Daniel Palumbo4,7, Dominic W. Pesce4,7, Alexander W. Raymond4,7, Jonathan Weintroub4, Maciek Wielgus4,7

1 National Radio Astronomy Observatory 2 Massachusetts Institute of Technology Haystack Observatory 3 National Astronomical Observatory of Japan 4 Center for Astrophysics Harvard & Smithsonian | 5 Institute of Astronomy & Astrophysics, Academia Sinica 6 Perimeter Institute for Theoretical Physics 7 Black Hole Initiative, Harvard University 8 Max--Institut für Radioastronomie

White Paper Description: High-resolution imaging of supermassive black holes can test and elucidate processes of accretion/jet formation on scales of the . Enhancements achievable within the decade would provide high-fidelity, time-resolved observations of Sgr A*, M87, and other black holes, enabling breakthroughs in black hole astrophysics.

1 1 Introduction

It is generally accepted that accretion onto supermassive black holes powers active galactic nuclei (AGN), producing luminosity that can outshine the combined starlight from entire [36]. Relativistic jets produced by some of these central engines extend for tens of thousands of light years, often well beyond the extent of the host galaxy [7]. The accretion flows feeding these black holes are expected to become optically thin at millimeter wavelengths [6], allowing emission in this waveband to serve as a probe of physical processes, dynamics, and General Relativistic effects that hold near the event horizon. High frequency very-long-baseline interferometry (VLBI) on Earth-sized baselines can resolve the immediate vicinity of nearby event horizons. The detection of horizon-scale structure (defined here as the black hole photon orbit) in Sgr A* 6 [16], the 4 10 M black hole at the [25, 26, 28], confirmed that imaging a∼ black× hole was possible in principle. The Event Horizon Telescope (EHT) project formed at the outset of the Astro2010 review [18] to implement a program of development aimed at building a global 1.3 mm VLBI network to observe Sgr A* and M87, the much 9 larger 3 6 10 M black hole in the Virgo A galaxy [24, 45]. Over the ensuing decade, continued∼ observations− × of both Sgr A* and M87 [16, 20, 17, 1, 33, 37], have made more detailed measurements of these sources through expansion to new VLBI sites and full polarimetric observations. In April 2017, the EHT made first observations with a full array capable of imaging (Figure 1), and data analysis is proceeding with the expectation that results will be released this year.

Figure 1: A map of the EHT. Stations active in 2017 and 2018 are shown with connecting lines and labeled in yellow, sites in commission are labeled in red, and legacy sites are labeled in gray. Nearly-redundant baselines are overlaying each other, i.e. to ALMA/APEX and SMA/JCMT.

For the purposes of this white paper, we will assume that such horizon-scale imaging is

2 15 EHT 2017 (230 GHz) Sgr A∗ 15 EHT 2017 (230 GHz) M87

10 10 as as µ µ

15 15 5 5 ) ) λ λ 0 0 v (G v (G

5 5 − −

10 10 − −

15 +future stations +future stations 15 +future stations +future stations − (230 GHz) (230+345 GHz) − (230 GHz) (230+345 GHz) 15 10 5 0 5 10 15 15 10 5 0 5 10 15 u (Gλ) − − − u (Gλ) − − −

Figure 2: EHT baseline coverage for Sgr A* (left) and M87 (right). Each plot shows the April 2017 1.3 mm EHT coverage (black), the 1.3 mm coverage anticipated for the future (black and red), and potential added coverage at 0.87 mm (blue). possible and describe what might be achieved scientifically over the coming decade with anticipated enhancement of the EHT capabilities. Higher resolution (e.g., through space-VLBI or expansion to 0.87 mm wavelength), higher sensitivity through larger recorded bandwidth, and addition of new VLBI sites leading to faster accumulation of Fourier spacings will bring new science possibilities into range. Real-time movies of accretion and jet launching may become possible [32, 8], linking physics processes near the black hole with relativistic outflows, and new tests of General Relativity that focus on matter orbits rather than light bending can be made.

2 Testing GR

Detecting a black hole shadow in M87 or Sgr A* [4, 38, 19, 44, 10] would demonstrate that the central massive objects in these galaxies lie within their photon orbits—a mere 1.5 times the event horizon radius for a non-spinning black hole—providing strong evidence for an event horizon [11, 13]. For Sgr A*, the mass-to-distance ratio is accurately measured from stellar orbits [25, 26, 27], predicting a shadow diameter of 48-52 µas depending upon the spin and inclination of the black hole. Thus, measuring the shadow shape and diameter provides a null hypothesis test of GR [43]. The combination of temporally and spatially resolved movies of black holes, possible in principle for both Sgr A* and M87 [e.g., 32, 8, 42], will enable tracking of luminous matter orbits (Figure 3). These orbits can be used to map the black hole spacetime, leading directly to estimation of black hole spin and tests of the “no hair” theorem [31, 30, 29, 12].

3 fsotbslnsta ol esniiet xeddeiso rmtejto scales network on dense jet a the lacks from it emission M87, extended in to imaging hole sensitive to black be matched the would is > near that for capability ring baselines extends EHT photon short and current the of hole the in black While emission central 41]. bright the [34, the near years originates light that of jet thousands radio a harbors M87 Formation Jet minutes 8]. 27 [32, of minutes 3 period of potential the timescales a the on of with demonstrating A* expansion A* reconstructions, Sgr Sgr assuming in Corresponding orbiting [9] flares Bottom: study A* spot” to 15]. “hot Sgr from a from “B” of flare [Model images simulated Simulated a Top: of EHT. Reconstruction 3: Figure tde n a epdsigihahoai esn ffcsfo nrni lsaphysics. plasma intrinsic from measure effects rotation lensing and achromatic index distinguish spectral help up may space-VLBI and open Multi-frequency the studies also image 35]. would to [22, wavelengths necessary holes shadow longer black resolution the including the how angular near extending of increased structures Further study the filamentary jets. detailed provide narrow relativistic would enable jet their space will the power into jet from and EHT and emission launch hole extended black reveal the the should throughout of and images magnitude, These of large order region base. existing an launching to by anchored the range baselines in short dynamic at dynamics observations of or addition with hole structure The combined black jet in apertures, range. the the is dynamic as to array insufficient such these 2017 to features link EHT due horizon-scale cannot The bright it [14]. to but jet access shadow, and providing flow of magnetohydrodynamic accretion capable relativistic M87 principle general the 3D of a event simulation of the (GRMHD) reconstructions to shows close 4 jet Figure the powers and launches hole black the horizon. how reveal to sufficient ∼

100 Future EHT Truth 0 µ 50 as min 0 ∆ osqety mgspoue sn h urn H r nieyt be to unlikely are EHT current the using produced images Consequently, . µ as 8 min 5 ∆ rgtesTmeaue(10 Temperature Brightness λ 0min 10 ∆ 0 = 4 16 . 87 m(iue2,wl mrv h imaging the improve will 2), (Figure mm 9 5min 15 ∆ K) 24 0min 20 ∆ 32 Reconstruction Figure 4: Left: GRMHD snapshot from a simulation of M87 [14]. Main panel is log scale; inset is linear scale. Right: Reconstruction from synthetic data assuming a future EHT array that combines both 1.3 and 0.87 mm observations. This reconstruction reveals both the circular 40 µas ring surrounding the black hole shadow and its connection to the jet. ∼ 4 Black Hole Accretion

Black hole accretion almost certainly powers the brightest objects in the universe, from to γ-ray bursts. But black hole accretion flows are still poorly constrained [5, 46], and observations have not yet been able to image the innermost accretion structure. EHT observations can resolve the instabilities that drive disk turbulence [2, 3], with particularly rich time-domain information from Sgr A*, whose innermost stable circular orbit (ISCO) period is between 4 and 30 minutes, depending on the black hole spin [15, 21, 23, 40]. The expanded coverage of future EHT arrays would enable movie reconstructions of Sgr A* over individual observing epochs. Moreover, in addition to monitoring accretion dynamics, studies can be made of the evolution of Sgr A* flares, which occur approximately daily at all wavelengths from radio to x-rays [39, 47]. Recently, the near-infrared interferometer GRAVITY has provided tantalizing evidence for orbits of ‘hot-spots’ in the accretion flow of Sgr A* [27]. Figure 3 shows individual frames of a movie reconstructed from simulated data of such an orbiting hot spot.

5 Resolving Other Supermassive Black Holes

The two prime targets of the EHT, Sgr A* and M87, are the only known black hole sources for which terrestrial VLBI at 1 mm can resolve the shadow and horizon-scale structure around the black hole. These∼ two sources have significant differences in black hole mass, accretion and jet power, and host galaxy type. Nearby low-luminosity AGN could fill these gaps with improvements in angular resolution.

5 3C273 10

Cen A Ground Truth with MEO/GEO Satellites OJ287 Sgr A*

M87 1 345 GHz nominal resolution (ground) 230 GHz nominal resolution (ground)

M104

NGC 1052 345 GHz nominal resolution (ground +230 GEO) GHz nominal resolution (ground + GEO) IC 1459 M81 M84 IC 4296

230 GHz0.1 flux density (Jy) 1010 M NGC 4261 8 10 M NGC 1399

106 M 5 µas Ground EHT resolution 0.1 1 10 100 Shadow diameter (µas)

Figure 5: (Left panel) Typical 230 GHz flux density versus predicted black hole shadow diameter for a number of current and potential future EHT targets; the size of each point scales with the estimated SMBH mass. The primary science targets, Sgr A* and M87, are plotted in green. Nominal resolutions for a variety of possible EHT array configurations are indicated using vertical lines. (Right panel) Simulated image reconstruction of the M104 black hole shadow with satellites in MEO and GEO orbits [22]. The yellow line indicates a typical ground-array resolution ( 20 µas), while the white ellipse shows the resolution of an array with MEO/GEO satelites.∼ Improvement in angular resolution with space-borne elements would resolve horizon-scale structures in nearby radio-loud LLAGN.

Adding satellites in medium or geosynchronous Earth orbits (MEO and GEO, respectively) would significantly expand the range of sources that can be probed at horizon-scale resolution [22]. As shown in Figure 5, the black hole shadow may be resolved in nearby radio-loud sources such as the Sombrero Galaxy (M104, NGC4594), IC1459 and M84 (NGC4374), which would fill the significant gap between Sgr A* and M87. Horizon-scale observations of these new sources will provide further unique clues to understand the nature of the black hole accretion and jet genesis. For instance, it is unknown whether the stark difference in jet power is due to differences in the black hole spin, the accretion rate, or other properties of the accretion flow.

6 Summary

Over the coming year, the first EHT results will clarify the state of the art in black hole imaging on horizon scales, bringing into focus the full science potential of this new field. Expected enhancements to the EHT would enable time-resolved videos of black hole jet launching and accretion, with potential significant expansion of black hole physics in Sgr A*, M87 and other sources that require high angular resolution.

6 References

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