Metrics and Motivations for Earth-Space VLBI: Time-Resolving Sgr A* with the Event Horizon Telescope. (arXiv:1906.08828v1 [astro-ph.IM])

Metrics and Motivations for Earth-Space VLBI: Time-Resolving Sgr A* with the Event Horizon Telescope. (arXiv:1906.08828v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Palumbo_D/0/1/0/all/0/1">Daniel C. M. Palumbo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Doeleman_S/0/1/0/all/0/1">Sheperd S. Doeleman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Johnson_M/0/1/0/all/0/1">Michael D. Johnson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bouman_K/0/1/0/all/0/1">Katherine L. Bouman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chael_A/0/1/0/all/0/1">Andrew A. Chael</a>

Very-long-baseline interferometry (VLBI) at frequencies above 230 GHz with
Earth-diameter baselines gives spatial resolution finer than the ${sim}50
mu$as “shadow” of the supermassive black hole at the Galactic Center,
Sagittarius A* (Sgr A*). Imaging static and dynamical structure near the
“shadow” provides a test of general relativity and may allow measurement of
black hole parameters. However, traditional Earth-rotation synthesis is
inapplicable for sources (such as Sgr A*) with intra-day variability.
Expansions of ground-based arrays to include space-VLBI stations may enable
imaging capability on time scales comparable to the prograde innermost stable
circular orbit (ISCO) of Sgr A*, which is predicted to be 4-30 minutes,
depending on black hole spin. We examine the basic requirements for space-VLBI,
and we develop tools for simulating observations with orbiting stations. We
also develop a metric to quantify the imaging capabilities of an array
irrespective of detailed image morphology or reconstruction method. We validate
this metric on example reconstructions of simulations of Sgr A* at 230 and 345
GHz, and use these results to motivate expanding the Event Horizon Telescope
(EHT) to include small dishes in Low Earth Orbit (LEO). We demonstrate that
high-sensitivity sites such as the Atacama Large Millimeter/Submillimeter Array
(ALMA) make it viable to add small orbiters to existing ground arrays, as
space-ALMA baselines would have sensitivity comparable to ground-based non-ALMA
baselines. We show that LEO-enhanced arrays sample half of the
diffraction-limited Fourier plane of Sgr A* in less than 30 minutes, enabling
reconstructions of near-horizon structure with normalized root-mean-square
error $lesssim0.3$ on sub-ISCO timescales.

Very-long-baseline interferometry (VLBI) at frequencies above 230 GHz with
Earth-diameter baselines gives spatial resolution finer than the ${sim}50
mu$as “shadow” of the supermassive black hole at the Galactic Center,
Sagittarius A* (Sgr A*). Imaging static and dynamical structure near the
“shadow” provides a test of general relativity and may allow measurement of
black hole parameters. However, traditional Earth-rotation synthesis is
inapplicable for sources (such as Sgr A*) with intra-day variability.
Expansions of ground-based arrays to include space-VLBI stations may enable
imaging capability on time scales comparable to the prograde innermost stable
circular orbit (ISCO) of Sgr A*, which is predicted to be 4-30 minutes,
depending on black hole spin. We examine the basic requirements for space-VLBI,
and we develop tools for simulating observations with orbiting stations. We
also develop a metric to quantify the imaging capabilities of an array
irrespective of detailed image morphology or reconstruction method. We validate
this metric on example reconstructions of simulations of Sgr A* at 230 and 345
GHz, and use these results to motivate expanding the Event Horizon Telescope
(EHT) to include small dishes in Low Earth Orbit (LEO). We demonstrate that
high-sensitivity sites such as the Atacama Large Millimeter/Submillimeter Array
(ALMA) make it viable to add small orbiters to existing ground arrays, as
space-ALMA baselines would have sensitivity comparable to ground-based non-ALMA
baselines. We show that LEO-enhanced arrays sample half of the
diffraction-limited Fourier plane of Sgr A* in less than 30 minutes, enabling
reconstructions of near-horizon structure with normalized root-mean-square
error $lesssim0.3$ on sub-ISCO timescales.

http://arxiv.org/icons/sfx.gif