Abstract
ABSTRACT With the advent of the Event Horizon Telescope (EHT), a millimeter/submillimeter very long baseline interferometer (VLBI), it has become possible to image a handful of black holes with sub-horizon resolutions. However, these images do not translate into microarcsecond absolute positions due to the lack of absolute phase information when an external phase reference is not used. Due to the short atmospheric coherence time at these wavelengths, nodding between the source and phase reference is impractical. However, here we suggest an alternative scheme which makes use of the fact that many of the VLBI stations within the EHT are arrays in their own right. With this we show that it should be possible to absolutely position the supermassive black holes at the centers of the Milky Way (Sgr A*) and M87 relative to nearby objects with precisions of roughly 1 μas. This is sufficient to detect the perturbations to Sgr A*'s position resulting from interactions with the stars and stellar-mass black holes in the Galactic cusp on year timescales, and severely constrain the astrophysically relevant parameter space for an orbiting intermediate-mass black hole, implicated in some mechanisms for producing the young massive stars in the Galactic center. For M87, it allows the registering of millimeter images, in which the black hole may be identified by its silhouette against nearby emission, and existing larger-scale radio images, eliminating present ambiguities in the nature of the radio core and inclination, opening angle, and source of the radio jet.
Highlights
The Event Horizon Telescope (EHT), a proposed Earth-sized array of existing millimeter and sub-millimeter observatories, promises to provide microarcsecond imaging resolutions via very-long baseline interferometry (VLBI) (Doeleman et al 2009a)
With the advent of the Event Horizon Telescope (EHT), a millimeter/sub-millimeter very-long baseline interferometer (VLBI), it has become possible to image a handful of black holes with sub-horizon resolutions
Turbulent, magnetized plasmas flows are a generic component of both accretion and jet models for Sgr A*’s emission, and it is not surprising that transient features arise near the black hole with dynamical times comparable to the orbital timescale at the innermost stable circular orbit (ISCO)
Summary
The Event Horizon Telescope (EHT), a proposed Earth-sized array of existing millimeter and sub-millimeter observatories, promises to provide microarcsecond imaging resolutions via very-long baseline interferometry (VLBI) (Doeleman et al 2009a). Phasing together the multiple telescopes on Mauna Kea (SMA, JCMT & CSO; Weintroub 2008) and many antenna at CARMA will improve this further, increasing sensitivity by a factor of 3 at some sites, making it possible to detect sources considerably dimmer than Sgr A* and M87 This will make it possible to contemplate doing astrometry on microarcsecond scales using the EHT. In the case of the Galactic Center, 7 mm-VLBI observations using the VeryLong Baseline Array have already paid substantial scientific dividends: placing tight constraints upon the motion of Sgr A* (Reid et al 1999a; Reid & Brunthaler 2004; Reid et al 2008), providing independent parallax measurements of our distance from the Galactic center (Reid et al 2009), and registering the radio and infrared reference frames of the central 0.1 pc (Menten et al 1997; Reid et al 2003, 2007) All of these make specific use of background quasars to locate sources within the Galactic center to sub-milliarcsecond precisions.
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