Conference

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.

The Large Millimeter Telescope (LMT) is a 50m diameter telescope at an altitude of 4600 m in the country of Mexico. It is a joint project between the University of Massachusetts, Amherst and the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE). At this time, the inner three rings of the telescope have been furnished with precision surface panels for an effective diameter of 32.5 m and effective surface rms better than 80 microns. With an active primary surface, the LMT can maintain this surface over a wide elevation range. While the outer rings will be procured and installed in the next 2 years, the telescope is into its third year of early science with two receivers, the Redshift Search Receiver (RSR) and AzTEC. I will provide a short description of the engineering and scientific status of the LMT. I will also summarize VLBI activities at the LMT to date, and present plans for a dual-polarization 1mm wavelength receiver for the LMT that will enable the telescope to participate in the EHT experiment by 2016.

The path from the current EHT to the full array capabilities has been mapped out over the past several years. Through the efforts of the EHT Technical Working Group and the full complement of experts involved in the project, we have a detailed plan and work breakdown to implement these improvements. I will describe the schedule for EHT expansion and enhancement from 2015 onward.

Convened in the summer of 2013, the EHT Technical Working Group (ETWG) was tasked with:1. Survey the capabilities at all EHT facilities 2. Establish a set of specifications for future EHT observations 3. Outline the technical developments needed to reach these goals 4. Based on prioritized science objectives, formulate a project roadmap that is grounded in technical feasibility with the resources available.This overview will report on the findings and conclusions arrived at by the ETWG and present specifications for future EHT observations at 230 and 345 GHz. I will also briefly discuss subsequent technical developments and the rollout of new equipment for the 2015 EHT observing campaign that will for the first time deploy recording at 16 Gb/s. With a relatively modest expansion over the next couple of years this mmVLBI system will be capable of 64 Gb/s for an aggregate bandwidth of 16 GHz i.e. 8x the bandwidth of existing EHT observations.

The supermassive black hole in the centre of the Milky Way, Sgr A*, is an ideal target for testing the properties of black holes. A number of experiments are being prepared or conducted, such as the monitoring of stellar orbits, the search for radio pulsars or the recording of an image of the shadow of a event horizon. The talk puts these efforts in context with other tests of general relativity and its alternatives. I will also compare the approaches in the Galactic centre, while concentrating in particular on the prospects of using pulsars as probes, also in combination with event horizon imaging.

In general relativity, astrophysical black holes are characterized uniquely in terms of their masses and spins and are described by the Kerr metric. The high sensitivity and resolution of the EHT will allow for unprecedented tests of the Kerr nature of black holes and, hence, of general relativity. I will present current and future limits on deviations from the Kerr metric from Sgr A* in the context of radiatively-inefficient accretion flow models. I will also show how largely model-independent constraints on such deviations can be obtained from the angular diameter of the photon ring of Sgr A* in combination with existing mass and distance measurements. Finally, I will describe a new general framework for the parametrization of deviations from the Kerr metric.