Talks

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.

Over the next few years the Event Horizon Telescope will greatly expand its capabilities from the current 8 Gbps at a few sites to 64 Gbps at a much larger number of sites. The good news is that this processing can proceed through 4 independent (16 Gbps) processing stages of 2 GHz of bandwidth. The bad news is that EHT stations come in multiple "flavors" each posing its own issues for correlation with DiFX (the current correlation option). This talk will discuss some of the issues and the road ahead.

The EHT data acquisition campaign in March 2015 will utilize new digital backends on many of the new and existing sites. We will briefly overview the state of the art for wideband digital backends for VLBI and discuss the several flavors that will be employed this coming year. Specifically, we will focus on details of the Roach2 Digital Backend (R2DBE), a new 16 Gbps wideband DBE unit that can be scaled to 64 Gbps, to be employed at single dish sites. The R2DBE digitizes 2.048 GHz of RF in each of two inputs and sends 8 Gbps single-channel data on each of two 10 GbE links to a Mark 6 data recorder with expansion chassis for a total of 16 Gbps. The system utilizes open source hardware, firmware, and software developed through the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER). Engineers at SAO have been members and developers for the CASPER group for several years, thus the challenge of rapidly prototyping a VLBI backend system on a limited budget and short time scale could be met by drawing on years of previous development and expertise. In four months the system was developed and demonstrated in 32 Gbps form for the South Pole Telescope, achieving fringes in a hybrid correlation between an R2DBE and an existing R1DBE. We present the tests and results achieved with the R2DBE 32 Gbps system for the South Pole, and upcoming developments in preparation for March 2015.