1. Event Horizon Telescope Collaboration, K. Akiyama, and 348 colleagues (2019), First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole, The Astrophysical Journal, Vol. 875, p. https://ui.adsabs.harvard.edu/abs/2019ApJ…875L…1E

This paper summarizes the results of the Event Horizon Telescope Collaboration, presenting the famous black hole image in M87. I was the lead coordinator of this collaboration paper.

2. Falcke, H., F. Melia, and E. Agol (2000), Viewing the Shadow of the Black Hole at the Galactic Center, The Astrophysical Journal, Vol. 528, p. L13-L16, https://ui.adsabs.harvard.edu/abs/2000ApJ…528L..13F

In this paper, we showed that a black hole surrounded by an optically thin emission region will cast a dark shadow onto the observed picture. The size of the shadow is proportional to the mass and rather independent of the spin of the black hole. We predict that this shadow could be observed with (sub-)mm-VLBI in the Galactic Center. This was a key scientific motivation that eventually led to the Event Horizon Telescope. (for a few more shadows and explanation than what was possible to show in this letter also see: https://ui.adsabs.harvard.edu/abs/2000AIPC..522..317F/abstract)

3. Bower, G. C., H. Falcke, R. M. Herrnstein, J.-H. Zhao, W. M. Goss, and D. C. Backer (2004), Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging, Science, Vol. 304, p. 704-708, https://ui.adsabs.harvard.edu/abs/2004Sci…304..704B

This work measured the intrinsic size of Sgr A* with VLBI at cm and mm-waves and showed that the source was shrinking with increasing frequency and decreasing wavelength. This confirmed our theoretical predictions and showed that at wavelengths of 1 mm and shorter the emission should indeed come from event horizon scales.

4. Falcke, H., W. M. Goss, H. Matsuo, P. Teuben, J.-H. Zhao, and R. Zylka (1998), The Simultaneous Spectrum of Sagittarius A* from 20 Centimeters to 1 Millimeter and the Nature of the Millimeter Excess, The Astrophysical Journal, Vol. 499, p. 731-734, https://ui.adsabs.harvard.edu/abs/1998ApJ…499..731F

This was one of the first, if not the first, multi-wavelength campaign of the Galactic Center black hole. We identified that the mm-to-submm emission was synchrotron radiation coming from near the Event Horizon. We here proposed for the first time that submm-VLBI could be used to image the black hole against this background.

5. Melia, F. and H. Falcke (2001), The Supermassive Black Hole at the Galactic Center, Annual Review of Astronomy and Astrophysics, Vol. 39, p. 309-352, https://ui.adsabs.harvard.edu/abs/2001ARA&A..39..309M

This paper summarizes the theoretical and observational understanding of the Galactic Center black hole Sagittarius A* at the beginning of this century. A lot more has been learned, but some of the basics still stand today.

6. Eatough, R. P., Falcke, H., and 21 colleagues (2013), A strong magnetic field around the supermassive black hole at the centre of the Galaxy, Nature, Vol. 501, p. 391-394, https://ui.adsabs.harvard.edu/abs/2013Natur.501..391E

Here we present the detection of the first radio magnetar in the Galactic center. Its emission was used to infer the strength of the magnetic field the central black hole, Sgr A*, accretes from and also to derive the location of the “scattering screen”, which blurs our view of Sgr A*.

7. Janssen, M., and 10 colleagues (2019), rPICARD: A CASA-based calibration pipeline for VLBI data. Calibration and imaging of 7 mm VLBA observations of the AGN jet in M 87, Astronomy and Astrophysics, Vol. 626, p. https://ui.adsabs.harvard.edu/abs/2019A&A…626A..75J

8. Roelofs, F., and 208 colleagues (2020), SYMBA: An end-to-end VLBI synthetic data generation pipeline. Simulating Event Horizon Telescope observations of M 87, Astronomy and Astrophysics, Vol. 636, p. https://ui.adsabs.harvard.edu/abs/2020A&A…636A…5R

In these two papers, we provide the community with a complete software pipeline to calibrate, image and simulate Event Horizon Telescope data. To achieve this we integrated VLBI capabilities into the standard ALMA data reduction package CASA. With the SYMBA pipeline output from (GRMHD) computer simulations of black hole images can be run through a detailed VLBI-simulation and then automatically reduced like the real date.

Black holes – Astrophysics of the event horizon emission

1. Falcke, H., K. Mannheim, and P. L. Biermann (1993), The Galactic Center radio jet., Astronomy and Astrophysics, Vol. 278, p. L1-L4, https://ui.adsabs.harvard.edu/abs/1993A&A…278L…1F

This work models the radio emission of the Galactic Center black hole, Sgr A*, as emission from the jets. This predicted that higher frequencies should lead to smaller sizes and that the highest frequencies in the emission spectrum (at ≳230 GHz/1.3mm) should approach the event horizon region.

2. Yuan, F., S. Markoff, and H. Falcke (2002), A Jet-ADAF model for Sgr A*, Astronomy and Astrophysics, Vol. 383, p. 854-863, https://ui.adsabs.harvard.edu/abs/2002A&A…383..854Y

We had proposed that the radio emission of the black hole in the Galactic Center was produced by a milky relativistic outflow (jet), while the group surrounding Ramesh Narayan had suggested that the accretion flow onto Sgr A* was optically thin, radiatively inefficient, and addiction-dominated (ADAFs/RIAFs). We here show that a combination of a jet plus an ADAF can explain the observing signatures of this source well if the electron temperature in the jet is higher than in the accretion flow. This principle is now used in the (Rhigh>10) GRMHD models.

3. Mościbrodzka, M. and H. Falcke (2013), Coupled jet-disk model for Sagittarius A*: explaining the flat-spectrum radio core with GRMHD simulations of jets, Astronomy and Astrophysics, Vol. 559, p. https://ui.adsabs.harvard.edu/abs/2013A&A…559L…3M

We apply the idea of stronger electron heating in highly-magnetized jet-regions to general relativistic magnetohydrodynamic (GMHD) simulations of black holes and find that the characteristic flat radio spectrum and the radio size are naturally reproduced in these simulations.

4. Mościbrodzka, M., H. Falcke, and H. Shiokawa (2016), General relativistic magnetohydrodynamical simulations of the jet in M 87, Astronomy and Astrophysics, Vol. 586, p. https://ui.adsabs.harvard.edu/abs/2016A&A…586A..38M

We apply the same general relativistic magnetohydrodynamic (GMHD) simulations we used for Sgr A* to the black hole in M87 and predict its appearance at 230 GHz (1.3 mm wavelength). The resulting image agrees remarkably well with the one later found by the Event Horizon Telescope, taken one year later.

5. Mizuno, Y., Z. Younsi, C. M. Fromm, O. Porth, M. De Laurentis, H. Olivares, H. Falcke, M. Kramer, and L. Rezzolla (2018), The current ability to test theories of gravity with black hole shadows, Nature Astronomy, Vol. 2, p. 585-590, https://ui.adsabs.harvard.edu/abs/2018NatAs…2..585

One of the key ideas of our Black Hole Cam project was to be able to test alternative theories of Gravity and of black holes to data from the Event Horizon Telescope. Hence, we developed codes that are able to derive the dynamics of plasma flows and light bending in arbitrary metrics of spacetimes. This paper shows the first results where we self-consistently model accretion flows, jets, and light transport in non-Einsteinian theories. Some are not easy to disentangle from an image alone.

Black holes – the big picture

1. Falcke, H., E. Körding, and S. Markoff (2004), A scheme to unify low-power accreting black holes. Jet-dominated accretion flows and the radio/X-ray correlation, Astronomy and Astrophysics, Vol. 414, p. 895-903, https://ui.adsabs.harvard.edu/abs/2004A&A…414..895F

In this paper, we show that at low luminosity black holes look rather similar and the level of their radio and x-ray flux is determined by mass and accretion rate of the black hole. Hence, The emission of the Galactic Center, M87 or a low-power X-ray binary can be described by the same few parameters. This leads to the so-called “fundamental plane” of black holes that has been used widely to infer masses of black holes in the absence of other indicators

2. Falcke, H. and P. L. Biermann (1995), The jet-disk symbiosis. I. Radio to X-ray emission models for quasars., Astronomy and Astrophysics, Vol. 293, p. 665-682, https://ui.adsabs.harvard.edu/abs/1995A&A…293..665F

The first paper in a long series proposing that accretion flows and jets are intimately coupled systems, where the jet power is proportional to the accretion flow.

3. Markoff, S., H. Falcke, and R. Fender (2001), A jet model for the broadband spectrum of XTE J1118+480. Synchrotron emission from radio to X-rays in the Low/Hard spectral state, Astronomy and Astrophysics, Vol. 372, p. L25-L28, https://ui.adsabs.harvard.edu/abs/2001A&A…372L..25M

In this paper, we show that the X-ray emission in at least some X-ray binaries can be produced by synchrotron emission from the jet, rather than Comptonization in the accretion flow. This broke a long-standing paradigm.

4. Nagar, N. M., H. Falcke, and A. S. Wilson (2005), Radio sources in low-luminosity active galactic nuclei. IV. Radio luminosity function, importance of jet power, and radio properties of the complete Palomar sample, Astronomy and Astrophysics, Vol. 435, p. 521-543, https://ui.adsabs.harvard.edu/abs/2005A&A…435..521N

The last paper in a long series, studying the radio emission of low-luminosity black holes, of which Sgr A* and M87* are at the extreme ends. All share very similar radio properties, that are well explained by radio emission from a jet.

5. Falcke, H., A. S. Wilson, and C. Simpson (1998), Hubble Space Telescope and VLA Observations of Seyfert 2 Galaxies: The Relationship between Radio Ejecta and the Narrow-Line Region, The Astrophysical Journal, Vol. 502, p. 199-217, https://ui.adsabs.harvard.edu/abs/1998ApJ…502..199F

We present an analysis of Hubble-Space-Telescope observations of hot ionised gas (NLR) in Seyfert galaxies and compare it to high-resolution radio observations. The gas is illuminated and heated in a cone-like structure by a central source, presumably an accreting supermassive black hole. The radio structure is indicative of plumes from a radio jet and closely follows the emission line gas. We initially thought that the jet might shape the gas, but it might well be the other way round.

LOFAR AND COSMIC RAYS

1. van Haarlem, M. P., and 200 colleagues (2013), LOFAR: The LOw-Frequency ARray, Astronomy and Astrophysics, Vol. 556, p. https://ui.adsabs.harvard.edu/abs/2013A&A…556A…2V

This paper summarizes the LOFAR Radio telescope, which is a network of low-frequency radio antennas, studying lightning and cosmic rays in the atmosphere, surveying the sky for black holes, and looking for faint radio signals from the big-bang afterglow in a baby universe. I was an International project scientist for LOFAR and later chair of its board. As such, I was deeply involved in its technical specification, design, prototyping, development of its scientific program and development of the scientific community.

2. Falcke, H., and 75 colleagues (2005), Detection and imaging of atmospheric radio flashes from cosmic ray air showers, Nature, Vol. 435, p. 313-316, https://ui.adsabs.harvard.edu/abs/2005Natur.435..313F

This paper presents pioneering detections of radio emission from cosmic rays hitting the atmosphere with a digital low-frequency radio telescope. The paper has led to a resurgence of the radio-detection technique for cosmic rays, that is now built into LOFAR and is also part of a major upgrade of the AUGER observatory – the world’s largest cosmic ray detector.

3. Buitink, S., and 103 colleagues (2016), A large light-mass component of cosmic rays at 10^17-10^17.5 electronvolts from radio observations, Nature, Vol. 531, p. 70-73, https://ui.adsabs.harvard.edu/abs/2016Natur.531…70B

In our second Nature paper on the radio detection of Cosmic Rays we now use LOFAR data and compare it to detailed computer simulations. The complex emission pattern on the ground is well fitted by the theory and can be used to derive the composition of cosmic ray particles around the “2nd knee” in the cosmic ray spectrum, suggesting they are of Galactic origin.

OTHER FUN STUFF

1. Brunthaler, A., M. J. Reid, H. Falcke, L. J. Greenhill, and C. Henkel (2005), The Geometric Distance and Proper Motion of the Triangulum Galaxy (M33), Science, Vol. 307, p. 1440-1443, https://ui.adsabs.harvard.edu/abs/2005Sci…307.1440B

We repeated the (in)famous van Maanen experiment and directly detected proper motion and rotation of the Galaxy M33. Comparing the rotational velocity of the galaxy on the sky with the one inferred from Doppler measurements for the radial velocity component, we can derive a geometric distance of a few Million light years.

2. Falcke, H. and L. Rezzolla (2014), Fast radio bursts: the last sign of supramassive neutron stars, Astronomy and Astrophysics, Vol. 562, p. https://ui.adsabs.harvard.edu/abs/2014A&A…562A.137F

Here we proposed a model to explain fast radio burst (FRBs) with a model where a supramassive, rotationally supported neutron star slows down and then collapses to a black hole. The expulsion of the magnetosphere can lead to semi-coherent prompt radio emission. While some FRBs repeat and cannot be explained that way, it is quite possible, if not likely, that such a mechanism exists in the universe and maybe explains a subset of FRBs.

3. Hare, B. M., and 64 colleagues (2019), Needle-like structures discovered on positively charged lightning branches, Nature, Vol. 568, p. 360-363, https://ui.adsabs.harvard.edu/abs/2019Natur.568..360H

We use techniques derived to detect cosmic rays with LOFAR to image lightning discharges. This provides an unprecedented spatial and temporal resolution that allows us to make detailed three-dimensional movies of the lightning build-up and shows, e.g., the shedding of ionized skins (needles) around ionization channels where lightning will strike multiple times.

SPACE MISSIONS

1. Jester, S. and H. Falcke (2009), Science with a lunar low-frequency array: From the dark ages of the Universe to nearby exoplanets, New Astronomy Reviews, Vol. 53, p. 1-26, https://ui.adsabs.harvard.edu/abs/2009NewAR..53….1J

Here we describe the basic principles and science cases of a low-frequency array of antennas and make the case of space-based mission, in particular on the lunar far side. The most prominent application will be the search for the dark ages of the universe, the phase after the big bang when the universe was filled with hydrogen and no stars had formed yet. This paper is part of a series of studies we have conducted to investigate lunar low-frequency mission and has led to the Netherlands Chines Low-Frequency Explorer (NCLE), a low-frequency antenna onboard the Chines QueQiao lunar communication relay satellite.

2. Roelofs, F., and 10 colleagues (2019), Simulations of imaging the event horizon of Sagittarius A* from space, Astronomy and Astrophysics, Vol. 625, p. https://ui.adsabs.harvard.edu/abs/2019A&A…625A.124R

We present an innovative concept for a space-VLBI mission, the Event Horizon Imager (EHI). Three smaller sub-millimeter-wave dishes orbit the earth on slightly different, adjustable orbits. They are connected by high data rate laser links and acts as one giant telescope with almost perfect uv-coverage. This allows one to make razor sharp high-definition images of black holes more than an order of magnitude better than from the ground with the EHT.