Strong Gravity

I summarize several physical processes that can produce efficient particle acceleration in radiatively inefficient accretion flows. I then describe the implications for non-thermal emission and EHT observations of Sgr A*.

Long wavelength measurements provide sensitive probes of the intrinsic structure of Sgr A* and of the scattering properties of the line-of-sight interstellar medium. At this wavelength, scattering dominates the apparent size of the source but careful closure amplitude techniques can provide highly accurate structural information. We present new results from the VLBA at 7mm wavelength that for the first time reveal two-dimensional intrinsic structure while also demonstrating the stability of the intrinsic size during periods of significant activity at NIR and X-ray wavelengths. These results also demonstrate the stability of the scattering medium over time. New observations of the Galactic Center pulsar PSR J1745-2900 show that the scattering properties of Sgr A* are spatially coherent over an angular scale of at least a few arc seconds. Analysis of the angular and temporal broadening data for the pulsar place the scattering medium at a distance of kiloparsecs away from the Galactic Center, resolving a significant mystery regarding the scattering medium.

Sgr A* is one of the few black holes whose capture radius is just resolvable with the <0.5” spatial resolution of the Chandra X-ray Observatory. Motivated by this potential we proposed for the deepest ever view of our Galactic center in 2012, via a Chandra-HETGS “X-ray Visionary Project”. Involving over 60 members of the Galactic center community, we obtained ~35 days of observations within a single year, for the first time also using the spectral gratings. This campaign doubled the photon count compared to the 12 years of observations before, and involved simultaneous observations from radio through gamma-ray frequencies. We are still analyzing these data sets, in combination with ongoing monitoring campaigns since, which together provide key constraints for the modeling of Sgr A* and environs necessary for the Event Horizon Telescope. I will present the highlights of this campaign so far, with a focus on results that can alter/define our ideas about the accretion geometry and plasma conditions near the black hole.

Over the last 20 years, advances in high angular resolution imaging technology has enabled the motions of individual stars to be tracked at the Galactic Center. This has provided the best evidence to date not only for the presence of a supermassive black hole at the center of our Galaxy, but for the existence of black holes in general. These high resolution measurements have also revealed an environment surrounding the black hole that is quite unexpected in a number of ways, challenging our understanding of the physical processes between black holes and their surround stars and gas. As the only galactic nucleus in which individual stellar orbits can be measured, the Galactic Center is now offering new insights into the fundamental physics of black holes, with unique tests of Einstein's theory of General Relativity on the horizon, and the astrophysics processes between black holes and their host galaxies that are thought to shape the co-evolution of central black holes and their host galaxies. I will review the work done by the UCLA Galactic Center Group and others on measuring stellar orbits, with an emphasis on (1) our recent model for G2 as a binary star that has been driven to merge through three-body interactions with the central black hole, (2) precision measurements of the distance to the black hole (R0), which is a fundamental constant for many astrophysical studies (e.g., galactic structure), and the mass of the black hole (Mbh), which as a ratio with R0 (Mbh/R0) is a key ingredient for interpreting future EHT measurements of the black hole's shadow, and (3) upcoming tests of General Relativity that are within reach through precision measurements of stellar orbits.

We discuss how polarized emission can place constraints on the properties of accretion flows and jets in the strong gravity regime for systems like SgrA* and M87 being observed by the Event Horizon Telescope.

Black holes are the engines that power quasars and active galactic nuclei throughout cosmic time. The masses of black holes in nearby galaxies define clear correlations with the properties of their host galaxies. These results suggest that black holes, while a thousand times lighter than the galaxy, grow alongside their hosts during its cosmic evolution. I will discuss the growth of black holes, and the establishment of the connection between galaxies and black holes.