The potential of millimeter wavelength very long baseline interferometry (VLBI) to detect and constrain time-variable structures that could give rise to observed SgrA* flaring is surveyed. The focus is on a model in which an orbiting hot spot is embedded in an accretion disk. Such orbiting hot-spots are lensed into asymmetric shapes by the intense black hole gravity, resulting in variable VLBI closure phases (the sum of interferometric phase around a closed triangle of VLBI baselines). Tracking these phases can be used to search for periodicity associated with orbits near the event horizon.

Doeleman, Fish, Broderick, Loeb, Rogers, ApJ, v695, p.59-74 (2009), "Detecting Flaring Structures in Sagittarius A* with High-Frequency VLBI"


Tracking orbits around a black hole: Left: models of an orbiting hot spot from a GR model (top) and after accounting for scattering through the interstellar medium (bottom). Middle: closure phases on the Hawaii-California-Chile baseline with only one ALMA antenna in Chile. Right: closure phases on the same triangle, but with 50 ALMA antennas phased to act as one VLBI element. The signal-to-noise is so high in the right panel that we can monitor structural changes on 10 second time scales.

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Massachusetts Institute of Technology