These are the abstracts submitted in random order:
Freek Roelofs, Radboud University Nijmegen
Imaging the event horizon of Sagittarius A* from space
It has been proposed that Very Long Baseline Interferometry (VLBI) at sub-millimeter waves will allow us to image the shadow of the black hole in the center of our Milky Way, Sagittarius A* (Sgr A*), and thereby test basic predictions of the theory of general relativity. In this work, the imaging prospects of a new Space VLBI (SVLBI) mission concept have been investigated. An initial design study of the concept has been performed for the purpose of the Event Horizon Imager (EHI). The concept may be suitable for imaging Sgr A* at high frequencies (up to ~690 GHz), which has significant advantages over performing ground-based VLBI at 230 GHz. The setup investigated here consists of two satellites with either 3-meter or 6-meter dishes in polar or equatorial circular Medium-Earth Orbits (MEOs) with slightly different radii. This setup will result in a dense spiral-shaped uv-coverage with long baselines (up to ~60 G\lambda), allowing for extremely high-resolution and high-fidelity imaging of radio sources. We simulate observations of a general relativistic magnetohydrodynamics (GRMHD) model of Sgr A* for the proposed configuration and noise calculated from model system parameters. Even though the resulting signal-to-noise ratio on individual measurements may be low, gridding the uv-plane and averaging visibilities in the same grid cell accumulated over multiple months of observations will be sufficient to obtain a strong signal, even at the long baselines. Our simulations show that with this method, images of the black hole shadow of Sgr A* with a resolution of 4 uas (about 8% of the shadow diameter) could be reconstructed within a few months of observing time. The concept may be of high scientific value. The black hole shadow could be measured much more precisely than with ground-based VLBI, allowing for stronger tests of General Relativity and accretion models.
Ralph Eatough, MPIfR Bonn
Measurements of the magneto-ionic properties of the Galactic Centre with PSR J1745-2900
PSR J1745-2900 — a radio loud magnetar that lies close to Sgr A* — is a unique probe of the interstellar environment toward the Galactic Centre. Atypically for magnetars, it has remained active in the radio band for close to five years since its discovery. Because PSR J1745-2900 also displays a rapid proper motion, it has enabled measurements of the Faraday Rotation and Dispersion Measure along multiple lines of sight. In this talk I will present results on the Galactic Centre environment near PSR J1745-2900 from radio monitoring observations with a suite of European telescopes.
Robert Wharton, MPIfR Bonn
Searching for Galactic Center Pulsars with the Very Large Array
The Karl G. Jansky Very Large Array (VLA) is an extremely sensitive and versatile instrument that is particularly well suited for searching for pulsars in the Galactic center. I will give a brief overview of the different types of pulsar observing possible with the VLA and then discuss how we are using these new pulsar modes for Galactic center pulsar searches. Finally, I will give some preliminary results and discuss applications for next generation pulsar surveys.
Pablo Torne, IRAM
Update on BHC-related progress at the IRAM 30-m observatory
The IRAM 30-m telescope (a.k.a Pico Veleta) is an important facility for the Event Horizon Telescope array and for the surveys searching for pulsars closely orbiting Sgr A*. This telescope has undergone significant upgrades in the last year, in particular related to backends, finally reaching the specifications of the project: a 64-Gbps recording system on a IF of 5-9 GHz. Additionally, the BHC-funded dedicated pulsar backend has recently been upgraded to a large bandwidth of 8 GHz. With this latest equipment, the upcoming observations for the imaging of black holes and AGNs, and for searching and studying pulsars at millimetre wavelengths, will be carried out with a very high sensitivity. We will review these last technical upgrades, and discuss the status of the Galactic Centre pulsar survey carried out from the IRAM 30-m at wavelengths between 3 and 2 mm (~87 to 140 GHz).
Michael Kramer, MPIfR Bonn
BHC and Pulsar work within the EHT
This talk will introduce the activities around the BHC pulsar work envisioned for the EHT and how, apart from science questions, it can also lead to informed decisions in other areas.
Kuo Liu, MPIfR Bonn
Observing pulsars with a Phased ALMA
Observing pulsars with an interferometer is possible, when it can be operated as a phased array. While it is relatively easy to achieve phasing at low frequencies, keeping stable phasing at high radio frequencies requires regular readjustments which are typically inconsistent with pulsar observing modes. A particular case is ALMA, where the phasing is re-done every 10 sec. In order to investigate the impact on pulsar observations, the Vela pulsar has been observed in January 2017. I will present the results so far, i.e. integrated pulse profile of Vela obtained by folding the data with a known ephemeris. I will also present the polarisation properties of the detection at multiple frequency bands. Additional tests made with VLBI data on Sgr A* indicate some systematics that we did not seen in Vela. We present those and discuss their impact for future pulsar observations and other observing modes.
Shan-Shan Zhao, Radboud University Nijmegen
ipole + Themis: Fitting new ray-tracing code with EHT observations
Ipole is a new public ray-tracing code for covariant, polarized radiative transport. Themis is a framework for performing analyses of the EHT observations. By coupling ipole and Themis, we can use MCMC sampler to traverse the whole parameter space and find out the best fit model parameters by fitting EHT observations under Bayesian statistical frame. This talk will show the latest progress of the work.
Roger Deane, University of Pretoria
MeqSilhouette refactored: now with wide-user base capability”
I’ll update the collaboration on the significantly refactored version of MeqSilhouette, an effort that was motivated by the desire to empower the user. MeqSilhouette can now be run in several different ways other than from source, including Docker and in a Jupyter Notebook. I’ll demonstrate the usability, new features, the discuss the more immediate applications with respect to EHT synthetic data generation.
Luciano Rezzolla, Goethe University Frankfurt
BHC theory: a report from Frankfurt.
I will report on the theoretical efforts carried out in Frankfurt within the scientific scopes of BHC. In particular, I will briefly review those topics that will be discussed more in detail in subsequent talks, namely, simulations of dilaton black holes, parameterised test-particle dynamics, and various code developments.
Yosuke Mizuno, Goethe University Frankfurt
On the Present Ability to Test Theories of Gravity via BH Shadows
Upcoming sub-millimetre VLBI images of Sgr A* carried out by the EHT Collaboration are expected to provide critical evidence for the existence of this supermassive black hole. In this work we assess our present ability to use EHTC images to determine if they correspond to a Kerr black hole as predicted by Einstein’s theory of general relativity or to a black hole in alternative theories of gravity. To this end, we perform GRMHD simulations and use GRRT calculations to generate synthetic shadow images of a magnetised accretion flow onto a Kerr black hole. In addition, and for the first time, we perform GRMHD simulations and GRRT calculations for a dilaton BH, which we take as a representative solution of an alternative theory of gravity. Taking into account the configuration of the VLBI observing array from the 2017 EHTC campaign, we find that it could be extremely difficult to distinguish between black holes from different theories of gravity, thus highlighting that great caution is needed when interpreting BH images as tests of general relativity.
Ziri Younsi, Goethe University Frankfurt
Progress on Radiative Transfer around Black Holes
In this talk, I will give an overview of the progress made in Frankfurt in the past 12 month with respect to general-relativistic radiative transfer calculations. I will discuss the recent developments of the radiative transfer code, BHOSS, as applied to perform high-accuracy GRRT calculations of electromagnetic radiation from astrophysical plasmas in BH environments, as well as some of our future intended directions of scientific application, in particular with respect to Sgr A*.
Roman Gold, Goethe University Frankfurt
THEMIS – A parameter estimation framework for the EHT: Overview & first results
I will present recent work in developing THEMIS, a modular, extensible parameter estimation framework for the Event Horizon Telescope. Specifically, I will demonstrate the validity of this new tool by reproducing known results of fitting Gaussian, crescent and RIAF models to old Event Horizon Telescope data. I will highlight some new capabilities such as fitting simultaneously visibility magnitude and closure phase data, yielding new non-trivial results even with the old data set. Finally, I will give an outlook of what the next steps in this general framework could be.
Christian Fromm, Goethe University Frankfurt
Radiative signatures of parsec-scale jets
Using state-of-the-art relativistic magneto-hydrodynamical simulations and ray-tracing algorithms we model the dynamics of jets along with their radiation microphysics, investigating the impact of magnetisation on the jet dynamics and the observed emission. During the post-processing procedure we account for the properties of the observing array (sparse uv-plane) and the imaging algorithm, enabling a more direct comparison between simulations and ground or space-based VLBI observations.
Hector Olivares, Goethe University Frankfurt
Imaging highly relativistic non-black hole space-times: the case of boson stars
General relativistic magneto-hydrodynamic (GRMHD) simulations and radiative transfer (GRRT) calculations are powerful tools to study a variety of accretion scenarios potentially observable by the Event Horizon Telescope, some of them involving extensions to General Relativity. In this talk, we first describe new additions to the GRMHD code BHAC, namely Upwind Constrained Transport algorithms for the evolution of the magnetic field and adaptive mesh refinement procedures compatible with them, which allow a significant speed-up while satisfying the solenoidal constraint of the magnetic field at machine precision. Then, we discuss an application of this code to simulate accretion onto a boson star, an object appearing as a consequence of some dark matter models and models of particle physics derived from string theory. The mass and compactness of some boson stars, as well as their lack of a hard surface makes them similar to supermassive black holes, despite the absence of an event horizon. By comparing the dynamics of plasma accreting from a magnetized torus onto a Schwarzschild black hole and onto a non rotating boson star and analysing GRRT images obtained from radiation transport calculations using the code BHOSS, we discuss the possibility of using shadow images to distinguish a black hole from a boson star, and more generally, to infer the presence of an event horizon.
Mariafelicia De Laurentis, Goethe University Frankfurt
Test-particle dynamics in general spherically symmetric black hole spacetimes
To date, the most precise tests of general relativity have been achieved through pulsar timing, albeit in the weak-field regime. Since pulsars are some of the most precise and stable “clocks” in the Universe, present observational efforts are focused on detecting pulsars in the vicinity of supermassive black holes (most notably in our Galactic Centre), enabling pulsar timing to be used as an extremely precise probe of strong-field gravity. In this paper a mathematical framework to describe test-particle dynamics in general black hole spacetimes is presented, and subsequently used to study a binary system comprising a pulsar orbiting a black hole. In particular, taking into account the parameterization of a general spherically symmetric black hole metric, general analytic expressions for both the advance of the periastron and for the orbital period of a massive test particle are derived. Furthermore, these expressions are applied to four representative cases of solutions arising in both general relativity and in alternative theories of gravity. Finally, this framework is applied to the Galactic Centre S-stars and four distinct pulsar toy models. It is shown that by adopting a fully general-relativistic description of test-particle motion which is independent of any particular theory of gravity, observations of pulsars can help impose better constraints on alternative theories of gravity than is presently possible.
Oliver Porth, Goethe University Frankfurt
Simulations of black hole accretion
In this contribution I will provide an update on ongoing development and current projects with the BlackHoleAccretionCode. BHAC is an oct-tree adaptive mesh refinement general relativistic magnetohydrodynamics code which now features staggered mesh constrained transport and thus conserves divergence of the magnetic field to machine precision. Recent additions are the resistive physics module, the updated particles module as well as the ability to incorporate nuclear equations of state. First science cases have been published or submitted. Finally, I will summarise the GRMHD code comparison project of the EventHorizonTelescope collaboration.