The Event Horizon Telescope

The Event Horizon TelescopeVincent L. Fish

MIT Haystack Observatory

Model courtesy C. Gammie

The Event Horizon TelescopeThe Event Horizon Telescope

Image Credit: APEX, IRAM, G. Narayanan, J. McMahon, JCMT/JAC, S. Hostler, D. Harvey, ESO/C. Malin

The Event Horizon TelescopeVLBI array at 1.3 mm / 0.87 mm observing supermassive black holes

Primary goals Test General Relativity Understand accretion and outflow around black holes Make pretty pictures

Primary targets Sgr A* (our own Galaxy) M87 Active Galactic Nuclei with jets

Interesting sources for jet physics serve double-duty as calibrators

What might Sgr A* look like?Emission likely comes from a disk and/or a jet

Emission is variable on short timescales, suggesting source structure variation on spatial scales of a few RSch

We need to be able to resolve Sgr A*

cartoon courtesy Chandracourtesy C. Gammie

courtesy A. Broderick

The apparent sizes of black holesSgr A*:

Mass ~ 4.3 x 106 Msun (Gillessen et al. 2009)Distance ~ 8.0 to 8.4 kpcRSch ~ 0.08 AU = 10 μas

Sgr A* has the largest apparent event horizon from EarthNext largest: M87

These angular scales are very small,but it is possible to achieve them nowfrom the surface of the Earth

courtesy R.C. Walker

Size of Sgr A*

Size of Sgr A* implies that emission is offset from center of black hole

Modelsleft: A. Broderick

right: Moscibrodzka+ 2014

Measured size: 3.7 RSch (descattered)

VLF+ 2011

Asymmetry

EHT data tell us that emission in Sgr A* is asymmetric

Some evidence for variability---changes in source structure

Polarization

Very high linear polarization on long baselines in Sgr A* (much higher than on short baselines)

There are ordered magnetic fields near the inner edge of the accretion flow

Johnson+ 2015

Baseline Coverage

New stations provide substantial gain in (u,v) coverage

Published Data 2017

Baseline Coverage

New stations provide substantial gain in (u,v) coverage

This will enable imaging…

…but we need to use better algorithms than CLEAN

Lu+ 2014 using simulated EHT data of M87

A Brief Timeline of VLBI at 1 mm• mid 1990s: IRAM PV, PdB (0.224 Gb/s)• early 2000s: series of experiments, mixed results

Bandwidth Increases

Analog Filter Bank Tape Recorder

Not-so-ancient history

Bandwidth Increases

Digital Backend Digital Recorder

Today: 4-16 Gb/s EHT Goal: 64 Gb/s

A Brief Timeline of VLBI at 1 mm• mid 1990s: IRAM PV, PdB (0.224 Gb/s)• early 2000s: series of experiments, mixed results• 2007: JCMT, CARMA, SMT (4 Gb/s)

A Brief Timeline of VLBI at 1 mm• mid 1990s: IRAM PV, PdB (0.224 Gb/s)• early 2000s: series of experiments, mixed results• 2007: JCMT, CARMA, SMT (4 Gb/s)• 2009: JCMT, 2 CARMA, SMT (also phased SMA tests)

A Brief Timeline of VLBI at 1 mm• mid 1990s: IRAM PV, PdB (0.224 Gb/s)• early 2000s: series of experiments, mixed results• 2007: JCMT, CARMA, SMT (4 Gb/s)• 2009: JCMT, 2 CARMA, SMT (also phased SMA tests)• 2011: phased SMA/JCMT/CSO, phased+ref CARMA, SMT

A Brief Timeline of VLBI at 1 mm• mid 1990s: IRAM PV, PdB (0.224 Gb/s)• early 2000s: series of experiments, mixed results• 2007: JCMT, CARMA, SMT (4 Gb/s)• 2009: JCMT, 2 CARMA, SMT (also phased SMA tests)• 2011: phased SMA/JCMT/CSO, phased+ref CARMA, SMT• 2012: first dual-pol experiment (8 Gb/s); first fringes to APEX

A Brief Timeline of VLBI at 1 mm• mid 1990s: IRAM PV, PdB (0.224 Gb/s)• early 2000s: series of experiments, mixed results• 2007: JCMT, CARMA, SMT (4 Gb/s)• 2009: JCMT, 2 CARMA, SMT (also phased SMA tests)• 2011: phased SMA/JCMT/CSO, phased+ref CARMA, SMT• 2012: first dual-pol experiment (8 Gb/s); first fringes to APEX• 2013: first global experiment

The Event Horizon Telescope

View from Sgr A* View from M87

A Brief Timeline of VLBI at 1 mm• mid 1990s: IRAM PV, PdB (0.224 Gb/s)• early 2000s: series of experiments, mixed results• 2007: JCMT, CARMA, SMT (4 Gb/s)• 2009: JCMT, 2 CARMA, SMT (also phased SMA tests)• 2011: phased SMA/JCMT/CSO, phased+ref CARMA, SMT• 2012: first dual-pol experiment (8 Gb/s); first fringes to APEX• 2013: first global experiment• 2015: first fringes to SPT and ALMA,

global 16 Gb/s observations with LMT, SWARM

Haystack Correlator(s)

New EHT Correlator40Gb Ethernet

Older Geodesy CorrelatorInfiniband


global 16 Gb/s observations with LMT, SWARM• 2016: 32 Gb/s observing


global 16 Gb/s observations with LMT, SWARM• 2016: 32 Gb/s observing• 2017: ALMA

ALMA

Excellent telescopes in a very dry place (Atacama desert, Chile)

We have developed a phased-array processor to sum signals from over 60 antennas, equivalent to an 85-m dish


global 16 Gb/s observations with LMT, SWARM• 2016: 32 Gb/s observing• 2017: ALMA• 2018: 64 Gb/s observing, Kitt Peak, Greenland Telescope• Future: Haystack, Owens Valley, LLAMA, ... ?

SummaryThe Event Horizon Telescope is studying the environment immediately around supermassive black holes

Thick accretion disks and jets light up very close to the event horizon, show signs of asymmetry predicted by general relativity, and are threaded by ordered magnetic fields

Technological enhancements have enabled these breakthroughs

Phased ALMA will make the EHT a true imaging array

Documents

The Event Horizon Telescope