General Relativity & Quantum Cosmology

Both the continuum-fitting and Fe-line methods of measuring black hole spin will be discussed and compared, with attention to sources of systematic error. Both methods rely on estimating the inner radius of the black hole's accretion disk and identifying it with the radius of the ISCO. The Fe-line method is extremely important because of its dominant role in measuring the spins of supermassive black holes, which is problematic for the continuum-fitting method. Meantime, both methods are applicable to stellar-mass black holes, and we will discuss current efforts to cross-check the spins of individual black holes. Finally, a comprehensive summary of spin results for both stellar-mass and supermassive black holes will be presented.

There are good reasons to believe that relativistic jets of AGN are powered by rotating black holes via the Blandford-Znajek mechanism. Although the original mathematical solution, which demonstrated the possibility of such energy extraction, was found 37 years ago, its physical nature still remains a subject of debate. I will give a brief review of some recent developments in this area with focus on the roles played by the EH and the Ergoregion in the Blandford-Znajek mechanism.

The radio community pioneered the use of closure phases to allow interferometric imaging even when fringe phases are compromised by atmospheric turbulence or unstable reference clocks. Eventually, better receivers and observing methods allowed phase referencing to provide direct measures of complex visibilities and eased the uncertainties using Fourier inversions required for imaging. At the same time, optical and infrared (O/IR) interferometers have been developed recently that can combine up to 6 telescopes simultaneously, at which point the inadequacies of classic imaging methods such as CLEAN were apparent. Here I report on dramatic progress within the O/IR interferometry to develop new image reconstruction techniques taking advantage of advances in ``compressed sensing'' theory and new approaches afforded by modern computing. Because the Event Horizon Telescope must rely on closure phases instead of direct Fourier phases, the new algorithms from the O/IR could be essential to extracting the most information from EHT observations and we demonstrate promising results using simulated EHT data.

The Event Horizon Telescope has measured compact emission in Sgr A* and M87 at resolutions comparable to their event horizons. Polarimetry with the EHT enables a powerful extension of this work, mapping magnetic field structures via the highly polarized synchrotron emission that is thought to dominate the compact emission. Sgr A* provides an especially attractive target for linear polarization studies with the EHT because it is unpolarized at the longer wavelengths where facility instruments are available. I will report on polarimetric results from our 2013 campaign, which demonstrate a sharp increase in the linear polarization fraction and variability of Sgr A* with increasing baseline. These data allow rich model-independent inferences about the polarization images and morphology and reveal that polarization is a sensitive probe of intrinsic variability.

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.