General Relativity & Quantum Cosmology

To improve the EHT?s sensitivity, resolution and imaging fidelity one might enhance station collecting area, add stations, or increase bandwidth. The latter improvement, bandwidth, is probably the most accessible. For many stations, including Submillimeter Array (SMA), the bandwidth is limited not by the receiver but by the digital signal processing. This talk will describe digital development at SMA. The upgraded SMA will contribute directly to the EHT, and technology under development has potential to benefit other EHT stations as well.

You've just finished running your code, and you're certain that you (and only you) know exactly what the region around a supermassive black hole looks like. You could lie back and wait for the accolades to roll in, but why not take an extra moment to make testable predictions that even the observers can understand? Synthetic data can help.

Scattering in the tenuous interstellar plasma blurs the image of Sgr A*. This effect decreases steeply with increasing frequency and becomes subdominant to the intrinsic emission structure at wavelengths close to a millimeter. I will discuss recent work that demonstrates how we can invert the blurring when properties of the scattering are known. With this technique, we can reconstruct the unscattered image of Sgr A* using EHT data. I will also show why some EHT observables -- such as closure phase and fractional polarization -- are largely immune to scattering. Finally, despite decades of study, there has been a recent flurry of progress in understanding the scattering properties, including studies of the Galactic Center magnetar and the discovery of refractive substructure in the scattering disk of Sgr A* at 1.3-cm wavelength. I will discuss these recent findings and their implications for imaging Sgr A* with the EHT

The angular resolution is the most fundamental to imaging the event-horizon-scale structure of supermassive black holes, and in order to realize the highest angular resolution ever, the EHT has been developing a world-wide (sub)mm VLBI array under the international collaborations.In order to boost the imaging capability of the EHT, we have been developing a new imaging method based on the technique so-called "sparse modeling", which allows us to directly solve the ill-posed Fourier-transform equations caused by incomplete sampling of visibilities.We show that the images obtained based on such an approach indeed provide an angular resolution a few times better than that of the standard imaging. We report on our simulations and also preliminary results of its application to real data, and discuss its potential in imaging black hole shadows with the EHT.

Sgr A* regularly flares in the X-ray and near-IR on ~hour timescales, and the EHT has already detected interday variability in 1.3 mm emission on long and short baselines. The addition of highly sensitive long baselines in 2015 will allow for the resolution of time variable structure on sub-minute timescales. This opportunity to observe dynamical process on event horizon scales comes with the challenge of sparse visibility coverage, but several strategies can recover rich information from the limited samples. I will review sources of variability for the emission near supermassive black holes from minute to year timescales, and discuss the prospects of EHT observations for understanding event-horizon-scale dynamics.