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Scientists and lay people alike are intrigued by black holes. To the scientist, they provide the ultimate test of Einstein's theory of General Relativity (GR) and underlie exotic phenomena such as particle jets moving at nearly light speed. To the public, black holes are mysterious points of no return that capture the imagination. Both views reflect a fascination with the question: What happens at the edge of a black hole?

A Black Hole Imager[3](BHI) would answer this question by directly resolving the event horizons of black holes (BHs). Viewing BHs with spatially resolved spectroscopy and timing enables direct comparison of matter's behavior in deep gravitational wells to predictions of GR. Thus a BHI would obtain the first direct look at a region of space that differs fundamentally from any in human experience. Thus BHI combines the rigor of a physics experiment with the excitement of exploring bizarre aspects of matter in the tortured space-time fabric surrounding BHs.

While it may seem paradoxical to image an object from which light cannot escape, a BH can be seen in silhouette against the hot material spiraling toward the event horizon. In X-rays, light from the accretion disk is directly viewed as it bends around the BH, and so one can see the actual distortion of space-time by the intense, ultimate gravitational field.

Spectral features emitted by material spiraling in toward the BH add color to the picture. There is evidence of broad X-ray iron lines,[18][41] where redshifts and blueshifts from the strong gravity and relativistic beaming effects close to the event horizon are observed. These features will provide a third dimension that can be used to directly map the space-time metric in the vicinity of a BH.

The X-ray band is natural for BH imaging. BH X-ray sources have extraordinarily high surface brightness; they produce X-ray emission lines from the near vicinity of the BH event horizons; and X-rays have extremely short wavelengths, making them suitable for high angular resolution imaging. The best targets (see Table 1) are nearby Active Galactic Nuclei (AGN) and the supermassive BH (SMBH) at the heart of our own Galaxy. While they are closer by a factor of 103, stellar mass BHs have 106-107 times smaller event horizons than SMBHs---thus SMBHs are appropriate systems to observe with the BHI.

The strong gravity of the BH distorts the space around it and creates a lens that magnifies the BH shadow by a factor of ~2.[19] For example, the M87 AGN is believed to harbor a 3-billion solar mass BH with a magnified event horizon angular diameter of 8 to 16 microarcsec (μas). The 4-million solar mass BH at the Galactic Center will have an angular extent of up to 50 μas. In bright, nearby AGN, which host BHs of 10 to 100 million solar masses, the event horizon shadows will subtend angles of 0.1 to 1.0 μas.

In the X-ray/γ-ray bands, there are several known approaches to build a BHI. The short wavelength of the radiation means that the required instrument aperature is ~1000 times smaller than that in the visible. Examples for 0.1 μas are 6000m at 0.5 keV, 500m at 6 keV and 30 m at 100 keV. Key components of architectures involving grazing and normal incidence optics as components of an interferoemter as well as refractive optics at higher energies have already been demonstrated in the laboratory but further technology is needed to optimize the strategy to build a BHI. BHI will also require advacements in technology related to formation flying and precision alignment---both of which are required by other vision missions.

The three approaches under consideration are to:

In the winter of 2009, the BHI team made the following white paper submissions to the National Academies "(217) 978-7693"



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