Signatures of strong gravity with GRAVITY
2010
The dynamics of stars and gas undoubtedly shows the existence of a 4 million solar mass black hole at the
center of the Milky Way: Sagittarius A* (SgrA*). Violent flare emission allows us to probe the immediate
environment of the central mass. Near-infrared polarimetry now shows signatures of strong gravity that are
statistically significant against randomly polarized red noise. Using these signatures we can derive spin and
inclination information of SgrA*. A combined synchrotron self Compton (SSC) and adiabatic expansion model
with source components peaking in the sub-mm domain can fully account for the observed flare flux densities
and the time delays towards the (sub-)mm flares that have been reported in some cases. We discuss the expected
centroid paths of the NIR images and summarize how the geometrical structure of the emitting region (i.e.
spot shape, presence of a torus or spiral-arm pattern etc.) affects this centroid tracks. While most of the
mentioned geometries are able to fit the observed fluxes, future NIR interferometry with GRAVITY at the
VLT will break some of the degeneracies between different emission models. In this contribution we summarize
several GRAVITY science cases for SgrA*. Our simulations propose that focusing GRAVITY observations on
the polarimetry mode could reveal a clear centroid track of the spot(s). A non-detection of centroid shifts cannot
rule out the multi-component model or spiral arms scenarios. However, a clear wander between alternating
centroid positions during the flares will prove the idea of bright long-lived spots occasionally orbiting the central
black hole.
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