EHT Session

8 August 2024, 14:00 – 17:00
Venue: Nerina & Protea CTICC 2

Sub-session 1: Recent Advances in EHT Research

Chair: Roger Dean

14:00 – 14:24 Sera Markoff The Event Horizon Telescope: from first black hole images to addressing fundamental questions in (astro)physics
14:24 – 14:36 Georgios F. Paraschos Casting a long shadow: M87* one year later
14:36 – 14:48 Ioannis Myserlis The first event-horizon linear and circular polarization view of SgrA*
14:48 – 15:00 Anne-Kathrin Baczko The putative centre of NGC 1052
15:00 – 15:12 Venkatessh Ramakrishnan Broadband MWL properties of M87 during 2018 EHT campaign
15:12 – 15:24 Dominic Chang Bayesian Blackhole Photogrammetry
15:24 – 15:36 Yosuke Mizuno Numerical Modeling of Black Hole Shadow
15:36 – 15:48 Break

Sub-session 2: Upcoming Developments and Opportunities in EHT

Chair: Venkatessh Ramakrishnan

15:48 – 16:12 Anton Zensus Next steps for the EHT Collaboration
16:12 – 16:24 Shep Doelman Next-generation designs for the EHT
16:24 – 16:36 Jonathan Weintroub Extending the Event Horizon Telescope to Space: The Black Hole Explorer
16:36 – 16:48 Sergio Dzib Quijano Frequency Phase Transfer on future EHT observations
16:48 – 17:00 Roger Deane Past and potential future South African contributions towards the EHT

SOC

Juan Carlos Algaba (Co-Chair)
Geoffrey Bower (Co-Chair)
Yuzhu Cui
Roger Deane
Mariafelicia de Laurentis
Michael Lindqvist
Alexandra Tetarenko
Thalia Traianou (Co-Chair)

Titles and Abstracts

Speaker: Sera Markoff (API/GRAPPA, University of Amsterdam and Visiting Professor, University of Namibia)
Title: The Event Horizon Telescope: from first black hole images to addressing fundamental questions in (astro)physics

Abstract: It has been over 5 years since the Event Horizon Telescope Collaboration (EHTC) made its first groundbreaking image of the supermassive black hole in M87, followed by the image of our Milky Way’s own black hole Sgr A* a few years later. These results provide the first direct visual evidence for the existence of these long-predicted, yet exotic, objects, but represent only the beginning of this story. The EHTC continues to surprise with a wide variety of studies, from probing the nature of spacetime and particle acceleration processes, to resolving the launch sites and inner structure of powerful active galactic nuclei jets. In this talk, I will present an overview of the EHT’s achievements and set the stage for the rest of this special focus session.  


Speaker: G. F. Paraschos (Max Planck Institute for Radio Astronomy)
Title: Casting a long shadow: M87* one year later

Abstract: The event horizon of supermassive black holes (SMBHs) constitutes the best testbed to explore the limits of humanity’s understanding of physical laws.

With the advent of the first pictures of SMBHs by the Event Horizon Telescope (EHT) collaboration, taken in 2017, direct tests of general relativity were finally made possible. Here we present new results from our 2018 observing campaign of the SMBH at the center of galaxy M87. The improved EHT array in 2018, with increased bandwidth and the addition of the Greenland Telescope, produced images consistent with the ring-like structure seen in the 2017 observations. The 2018 images show a bright, asymmetric ring with a diameter of ~43 μas and a deep central flux depression, in agreement with the expected signature of strong gravitational lensing around a black hole. Importantly, the position angle of the brightness asymmetry shifted by ~30 degrees compared to 2017, providing new insights into the dynamics of the accreting material around the black hole. Detailed modelling of the 2018 data confirms the preference for ring-like geometric models, and yields a black hole mass estimate of (6.5 ± 0.7) × 10^9 solar masses, consistent with our previous measurements. These results solidify the EHT’s ability to robustly image the event horizon-scale structure of supermassive black holes.


Speaker: I. Myserlis (Institut de Radioastronomie Millimétrique (IRAM)):
Title: The first event-horizon linear and circular polarization view of SgrA*

Abstract: Earlier this year, the Event Horizon Telescope (EHT) Collaboration presented the first linear and circular polarization images in the immediate surroundings of SgrA*, the supermassive black hole at the heart of our Galaxy. Our analysis revealed high levels of resolved linear polarization, a prominent spiral electric vector position angle (EVPA) pattern around the emission ring and hints for an east-west dipolar pattern in the resolved circular polarization. Along with our earlier linear and circular polarization analysis of M87* on event-horizon scales, these results suggest that magnetic fields are dynamically important for all black holes and how they interact with their environment. Using both simple analytic models and general relativistic magnetohydrodynamic (GRMHD) simulations, we interpret the polarization structure of SgrA* and the observed levels of rotation measure (RM) in the context of models with either internal or external Faraday rotation and discuss their implications in terms of the motion of accreting material as well as the spin and inclination of SgrA*. Our results pave the way for putting more stringent constraints on the black hole and its accretion flow using future 345 GHz observations and dynamical imaging of SgrA*.


Speaker: Anne-Katrin Bazcko (Chalmers University of Technology)
Title: The putative centre of NGC 1052

Abstract: Besides decades of observational studies the formation and collimation of relativistic jets in Active Galactic Nuclei (AGN) is not yet fully understood. Since recently, the vast improvements of VLBI in terms of sensitivity and resolution allow a direct comparison of observations of the jet collimation and acceleration region with predictions from GRMHD simulations. We have investigated this region in the bipolar jet in the nearby AGN NGC 1052, which allows us to also study whether the jets evolve symmetrically. I will present our result from combining multiple radio interferometric observations with the VLBA, the GMVA, and the EHT taken during one week in spring 2017. These observations detected NGC 1052 for the first time with the EHT. Resolving the size of the central region in-between both jet bases, we provide new constraints on the jet collimation profile down to 250 Schwarzschild radii as well as on the magnetic field strength. Our results are consistent with previous estimates at lower frequencies, providing an upper limit of the magnetic field strength of around 10^4 Gauss at 1 Schwarzschild radii and supporting two changes of the jet collimation profile within the inner 10^5 Schwarzschild radii.


Speaker: Venkatessh Ramakrishnan (University of Turku)
Title: Broadband MWL properties of M87 during 2018 EHT campaign

Abstract: The nearby elliptical galaxy M87 contains one of the only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to gamma-ray energies) took part in the second M87 EHT campaign. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high energy (VHE) gamma-rays, as well as details of the individual observations and light curves. We also conduct phenomenological modelling to investigate the basic source properties. We present the first VHE gamma-ray flare from M87 detected since 2010. The flux above 350 GeV has more than doubled within a period of ~36 hours. We find that the X-ray flux is  enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE gamma-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and emphasises the need for combined image and spectral modelling.


Speaker: Dominic Chang (Center for Astrophysics | Harvard & Smithsonian)
Title: Bayesian Blackhole Photogrammetry

Abstract: We propose a simple, analytic dual-cone accretion model for horizon scale images of the cores of Low-Luminosity Active Galactic Nuclei (LLAGN), including those observed by the Event Horizon Telescope (EHT). Our underlying model is of synchrotron emission from an axisymmetric, magnetized plasma, which is constrained to flow within two oppositely oriented cones that are aligned with the black hole’s spin axis. We show that this model can accurately reproduce images for a variety of time-averaged general relativistic magnetohydrodynamic (GRMHD) simulations, that it accurately recovers both the black hole and emission parameters from these simulations, and that it is sufficiently efficient to be used to measure these parameters in a Bayesian inference framework with radio interferometric data. We show that non-trivial topologies in the source image can result in non-trivial multi-modal solutions when applied to observations from a sparse array, such as the EHT 2017 observations of M87*. The presence of these degeneracies underscores the importance of employing Bayesian techniques that adequately sample the posterior space for the interpretation of EHT measurements. We fit our model to the EHT observations of M87* and find a 95% Highest Posterior Density Interval (HPDI) for the mass-to-distance ratio of θg∈(2.84,3.75)μas, and give an inclination of θo∈(11 degrees,24 degrees). These new measurements are consistent with mass measurements from the EHT and stellar dynamical estimates (e.g., Gebhardt et al. 2011; EHTC et al. 2019a,b; Liepold et al. 2023), and with the spin axis inclination inferred from properties of the M87* jet (e.g., Walker et al. 2018).


Speaker: Yosuke Mizuno (Tsung-Dao Lee Institute, Shanghai Jiao Tong University)
Title: Numerical Modeling of Black Hole Shadow

Abstract: Recently the Event Horizon Telescope has presented a ring-like emission structure of M87 and our galactic center Sgr A* at 1.3mm with unprecedented angular resolutions. These images are consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. In this talk, I would like to overview the theoretical interpretation of EHT results of M87, and present the recent progress of theoretical modeling by numerical simulations of magnetized accretion flows onto a black hole. I also discuss the numerical modeling of relativistic jets and comparison with the observations.


Speaker: Anton Zensus (Max Planck Institute for Radio Astronomy)
Title: Next Steps for the EHT Collaboration 

Abstract: The Event Horizon Telescope Collaboration is rooted in the collaboration of institutions operating the observatories and correlation facilities used for the experiments. I will give a top-level overview of the EHT’s mid-range science plan and technical roadmap that guide developments for the near future, from the perspective of the facilities. Alongside these planned developments, I will summarize the set of organizational changes the EHTC has made to enable and empower sustainable scientific collaboration for participating researchers at all career levels. Finally, I will address how the EHT could develop from a focused project to an open user facility, such as the Global Millimeter VLBI Array.


Speaker: Shepherd Doelman (Center for Astrophysics | Harvard & Smithsonian)
Title: Next-generation designs for the EHT

Abstract: The next-generation EHT is a program to design and implement a transformative enhancement to the Event Horizon Telescope (EHT). Where prior EHT work has delivered still images of M87 and SgrA* on horizon scales, and near-term developments will probe time variability of structure, a next-generation array will capture the detailed motion of luminous matter as it spirals in to the event horizon, realizing the goal of black hole “cinema.” Through increases in sensitivity, resolution, and spectral range, the ngEHT will expand on the EHT’s mission by uniquely addressing fundamental questions in black hole research: tests of gravity, the nature of accretion and the mechanism of jet launch, and the evolution of black hole parameters across cosmic time.  By establishing new VLBI sites, broadening instrument bandwidth, and observing in multiple frequencies, the ngEHT upgrade will realize nearly an order of magnitude increase in the amount of data collected.  This talk will summarize the technical designs, architecture and science capabilities that will result by 2030.


Speaker:  Jonathan Weintroub (Center for Astrophysics | Harvard & Smithsonian)
Title: Extending the Event Horizon Telescope to Space:  The Black Hole Explorer  (Presented on behalf of the Black Hole Explorer Team)

Abstract: The Black Hole Explorer (BHEX) is a space mission aiming to discover and measure a black hole’s photon rings.  BHEX captures light that has circled a black hole from half an orbit to multiple complete orbits.  A series of ever sharper photon rings, produced from light that has orbited the black hole increasingly many times before escaping, lie within the Event Horizon Telescope (EHT) ring images but are blended together at the resolution of the EHT.  Discerning these photon rings requires extremely fine angular resolution only achieved by extending the EHT array into space.  The physical properties of the black hole are imprinted on the size and shape of the photon ring, creating a new window into relativistic physics.  This talk summarizes the primary BHEX science goals and the instrumentation being developed to make the mission a reality within the next decade. 

https://www.blackholeexplorer.org


Speaker: Sergio Dzib Quijano (Max Planck Institute for Radio Astronomy)
Title: Frequency Phase Transfer on future EHT observations

Abstract: Frequency Phase Transfer (FPT) enabled by multiband receivers in Very Long Baseline Interferometry (VLBI) operations has a transformative potential when observing at frequencies above 22 GHz. The technique has successfully been implemented by the Korean VLBI Network (KVN), and promises significant advancements in sensitivity and dynamic range for VLBI imaging, particularly at 86 GHz. The FPT technique could also enhance future EHT observations, especially because of the higher frequency observations; 230 and 345 GHz. This talk will present current developments and results of FPT VLBI observations and plans to incorporate the technique into EHT observations.


Speaker: Roger Deane (University of the Witwatersrand / University of Pretoria)
Title: Past and potential future South African contributions towards the EHT

Abstract: Geodetic and Astronomical Very Long Baseline Interferometry (VLBI) have a proud history in South Africa, with the HartRAO 26 m playing an invaluable role in the bid to host the SKA. The South African investment in cutting-edge radio interferometers (MeerKAT, MeerKAT+, SKA-MID) has been carefully targeted to utilise unique geographic strengths as an astronomical site inside a much broader national strategic initiative. This includes a significant human capacity development programme, resulting in an emerging prominence in radio astronomy across scientific, software, and hardware domains, as well as a deepening engagement with African and global partners.  In this talk, I will share how this national strategy has already led to direct and indirect contributions to the Event Horizon Telescope and look toward future possibilities as South Africa continues to grow and diversify its scientific and technical proficiency in radio astronomy, including high-frequency VLBI.