R2DBE: A Wideband Digital Backend for the Event Horizon Telescope

Citation:

Laura Vertatschitsch, Rurik Primiani, André Young, Jonathan Weintroub, Geoffrey B. Crew, Stephen R. McWhirter, Christopher Beaudoin, Sheperd Doeleman, and Lindy Blackburn. 2015. “R2DBE: A Wideband Digital Backend for the Event Horizon Telescope.” Publications of the Astronomical Society of the Pacific, 127, Pp. 1226-1239.

Abstract:

The Event Horizon Telescope (EHT) is an earth-size aperture synthesisradio astronomy array capable of making high-resolution measurements ofsubmillimeter emission near the event horizon of supermassive blackholes. The EHT uses existing standalone submillimeter radio telescopeswhich are retrofitted to serve as VLBI stations. Current instrumentdevelopment goals include increasing the number of stations in the arrayand increasing their sensitivity. We have developed a 4 GHz bandwidthdigital backend (DBE) unit, based on the CASPER (Collaboration forAstronomy Signal Processing and Electronics Research) open source ROACH2(Reconfigurable Open Architecture Computing Hardware) platform. TheROACH2 digital backend, which we call the R2DBE, has dual channels eachsampling at a rate of 4096 MSps (megasamples-per-second), a factor of 4improvement over the previous generation system. Recording 2-bits persample, the bandwidth is equivalently stated as 16 gigabits-per-second(Gbps). This paper includes system design of the R2DBE, discusseslaboratory test results of the system using correlated noise input, andpresents field test results. The R2DBE was distributed to seven sites inearly 2015, enabling the EHT campaign in 2015 March to collect data with2 GHz bandwidth in each polarization. The 16 gigabit-per-second (Gbps)R2DBE can be scaled to create a 64 Gbps system using four R2DBEs inparallel. Thus, it enables a clear path to the EHT's goal of 4 GHzdual-polarization and dual-sideband across the array.

Recent Publications

The Event Horizon Telescope Collaboration. 4/10/2019. “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole.” ApJL, 875, L6, Pp. 1-44. Publisher's Version Abstract

We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42 ± 3 μas and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc2 = 3.8 ± 0.4 μas. Folding in a distance measurement of ${16.8}_{-0.7}^{+0.8}\,\mathrm{Mpc}$ gives a black hole mass of $M=6.5\pm 0.2{| }_{\mathrm{stat}}\pm 0.7{| }_{\mathrm{sys}}\times {10}^{9}\hspace{2pt}{M}_{\odot }$ . This measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.

The Event Horizon Telescope Collaboration. 4/10/2019. “First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring.” ApJL, 875, L5, Pp. 1-31. Publisher's Version Abstract

The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.

Sheperd S. Doeleman

Smithsonian Astrophysicist and Harvard Senior Research Fellow

R2DBE: A Wideband Digital Backend for the Event Horizon Telescope

Citation:

Abstract:

Recent Publications