Publications

Publications are shown in reverse chronological order. See also my ADS library.

Particle acceleration in kink-unstable jets
DOI: 10.3847/2041-8213/ab95a2
Publication date: June 2020
Authors: Davelaar, Jordy; Philippov, Alexander A.; Bromberg, Omer; Singh, Chandra B.

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Magnetized jets in GRBs and AGNs are thought to be efficient accelerators of particles, however the process responsible for the acceleration is still a matter of active debate. In this work, we study the kink-instability in non-rotating force-free jets using first-principle particle-in-cell simulations. We obtain similar overall evolution of the instability as found in MHD simulations. The instability first generates large scale current sheets, which at later times break up into small-scale turbulence. Reconnection in these sheets proceeds in the strong guide field regime, which results in a formation of steep power-laws in the particle spectra. Later evolution shows heating of the plasma which is driven by weak turbulence induced by the kink instability. These two processes energize particles due to a combination of ideal and non-ideal electric fields.

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Deep Horizon; a machine learning network that recovers accreting black hole parameters
DOI: 10.1051/0004-6361/201937014
Publication date: April 2020
Authors: van der Gucht, Jeffrey; Davelaar, Jordy; Hendriks, Luc; Porth, Oliver; Olivares, Hector; Mizuno, Yosuke; Fromm, Christian M.; Falcke, Heino

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The Event Horizon Telescope recently observed the first shadow of a black hole. Images like this can potentially be used to test or constrain theories of gravity and deepen the understanding in plasma physics at event horizon scales, which requires accurate parameter estimations. In this work, we present Deep Horizon, two convolutional deep neural networks that recover the physical parameters from the images of black hole shadows. We investigate the effects of a limited telescope resolution and of observations at different frequencies. We train two convolutional deep neural networks on a large image library of simulated mock data. The first network is a Bayesian deep neural regression network and is used to recover the viewing angle and the position angle, the mass accretion rate, the electron heating prescription and the black hole mass. The second network is a classification network that recovers the black hole spin . We find that with the current resolution of the Event Horizon Telescope, it is only possible to accurately recover a limited amount of parameters of a static image, namely the mass and mass accretion rate. Since potential future space-based observing missions will operate at frequencies above 230 GHz, we also investigated the applicability of our network at a frequency of 690 GHz. The expected resolution of space-based missions is higher than the current resolution of the Event Horizon Telescope and we show that Deep Horizon can accurately recover the parameters of simulated observations with a comparable resolution to such missions.

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Modeling non-thermal emission from the jet-launching region of M 87 with adaptive mesh refinement
DOI: 10.1051/0004-6361/201936150
Publication date: December 2019
Authors: J. Davelaar H. Olivares, O. Porth T. Bronzwaer, M. Janssen, F. Roelofs, Y. Mizuno, C.M. Fromm, H. Falcke, L. Rezzolla

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The galaxy M 87 harbors a kiloparsec-scale relativistic jet, whose origin coincides with a supermassive black hole. Observational mm-VLBI campaigns are capable of resolving the jet-launching region at the scale of the event horizon. In order to provide a context for interpreting these observations, realistic general-relativistic magnetohydrodynamical (GRMHD) models of the accretion flow are constructed. The characteristics of the observed spectral-energy distribution (SED) depend on the shape of the electrons’ energy-distribution function (eDF). The dependency on the eDF is omitted in the modeling of the first Event Horizon Telescope results. In this work, we aim to model the M 87 SED from radio up to NIR/optical frequencies using a thermal-relativistic Maxwell- Jüttner distribution, as well as a relativistic κ-distribution function. The electrons are injected based on sub-grid, particle-in-cell parametrizations for sub-relativistic reconnection. A GRMHD simulation in Cartesian-Kerr-Schild coordinates, using eight levels of adaptive mesh refinement (AMR), forms the basis of our model. To obtain spectra and images, the GRMHD data is post-processed with the ray-tracing code RAPTOR, which is capable of ray tracing through AMR GRMHD simulation data. We obtain radio spectra in both the thermal-jet and κ-jet models consistent with radio observations. Additionally, the κ-jet models also recover the NIR/optical emission. The models recover the observed source sizes and core shifts and obtain a jet power of ≈1e43 ergs/s. In the κ-jet models, both the accretion rates and jet powers are approximately two times lower than the thermal-jet model. The frequency cut-off observed at ν≈1e15 Hz is recovered when the accelerator size is 1e6 – 1e8 cm, this could potentially point to an upper limit for plasmoid sizes in the jet of M 87.

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TeraHertz Exploration and Zooming-in for Astrophysics (THEZA): ESA Voyage 2050 White Paper
arXiv: https://arxiv.org/abs/1908.08620
Publication: October 2019
Authors: Leonid I. Gurvits et al.

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The astrophysical agenda of the 21st century requires a very sharp view of celestial objects. High angular resolution studies are essential for fundamental studies of a broad variety of astrophysical phenomena ranging from relativistic physics of black holes, their gravitational and electromagnetic imprints, violent transient processes, including those producing detectable gravitational waves, birth and evolution of planetary systems. Over the past decades, radio astronomy made huge leap in achieving ground-breaking angular resolution measured in tens of microarcseconds (one tenth of nanoradian and better). Recently a global Event Horizon Telescope (EHT) collaboration obtained first direct images of the shadow of a super-massive black hole in the nucleus of the active galaxy M87. These observations were conducted at 230 GHz. The two first generation Space Very Long Baseline Interferometry (VLBI) missions, VSOP/HALCA led by the Japan Aerospace Exploration Agency (JAXA) and RadioAstron led by the Russia Roscosmos State Corporation and Russia Academy of Sciences, achieved the highest angular resolution at frequencies from 0.3 to 22 GHz in observations conducted in the period 1997 – 2019. The next step in advancing high angular resolution radio astronomy is in combining high frequency (millimeter and sub-millimeter wavelengths) and interferometric baselines exceeding the Earth diameter. The present THEZA White Paper describes a combination which would unify technology developments in giga-/tera-hertz instrumentation and space-borne radio astronomy. The current preprint version of the THEZA White Paper is slightly re-formatted and edited comparing to the official submitted version.

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Kink instability: evolution and energy dissipation in Relativistic Force-Free Non-Rotating Jets
DOI: 10.3847/1538-4357/ab3fa5
Publication date: October 2019
Authors: Omer Bromberg, Chandra B. Singh, Jordy Davelaar, Alexander A. Philippov

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We study the evolution of kink instability in a force-free, non-rotating plasma column of high magnetization. The main dissipation mechanism is identified as reconnection of magnetic field-lines with various intersection angles, driven by the compression of the growing kink lobes. We measure dissipation rates dUBϕ/dt≈−0.1UBϕ/τ, where τ is the linear growth time of the kink instability. This value is consistent with the expansion velocity of the kink mode, which drives the reconnection. The relaxed state is close to a force-free Taylor state. We constraint the energy of that state using considerations from linear stability analysis. Our results are important for understanding magnetic field dissipation in various extreme astrophysical objects, most notably in relativistic jets. We outline the evolution of the kink instability in such jets and derive constrains on the conditions that allow for the kink instability to grow in these systems.

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First M87 Event Horizon Telescope Results and the Role of ALMA
DOI: 10.18727/0722-6691/5150
Publication date: September 2019
Authors: C. Goddi et al.

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In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate super- massive black hole (SMBH) at the centre of the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image shows a ring of glowing plasma with a dark patch at the centre, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein’s theory of gravity, or general relativity, was made possible by assembling a global network of radio telescopes operating at millimetre wavelengths that for the first time included the Atacama Large Millimeter/submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical reality. The expansion of the array over the next few years will include new stations on different continents — and eventually satellites in space. This will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on the processes of black hole accretion and jet formation on event-horizon scales, thereby enabling more precise tests of general relativity in the truly strong field regime.

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Constrained transport and adaptive mesh refinement in the Black Hole Accretion Code
arXiv: 10.1051/0004-6361/201935559
Publication date: September 2019
Authors: H. Olivares, O. Porth, J. Davelaar, E.R. Most, C.M. Fromm, Y. Mizuno, Z. Younsi, L. Rezzolla

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Worldwide very long baseline radio interferometry arrays are expected to obtain horizon-scale images of supermassive black hole candidates as well as of relativistic jets in several nearby active galactic nuclei. This motivates the development of models for magnetohydrodynamic flows in strong gravitational fields. The Black Hole Accretion Code (BHAC) intends to aid with the modelling of such sources by means of general relativistic magnetohydrodynamical (GRMHD) simulations in arbitrary stationary spacetimes. New additions were required to guarantee an accurate evolution of the magnetic field when small and large scales are captured simultaneously. We discuss the adaptive mesh refinement (AMR) techniques employed in BHAC, essential to keep several problems computationally tractable, as well as staggered-mesh-based constrained transport (CT) algorithms to preserve the divergence-free constraint of the magnetic field, including a general class of prolongation operators for face-allocated variables compatible with them. Through several standard tests, we show that the choice of divergence-control method can produce qualitative differences in simulations of scientifically relevant accretion problems. We demonstrate the ability of AMR to reduce the computational costs of accretion simulations while sufficiently resolving turbulence from the magnetorotational instability. In particular, we describe a simulation of an accreting Kerr black hole in Cartesian coordinates using AMR to follow the propagation of a relativistic jet while self-consistently including the jet engine, a problem set up-for which the new AMR implementation is particularly advantageous. The CT methods and AMR strategies discussed here are being employed in the simulations performed with BHAC used in the generation of theoretical models for the Event Horizon Telescope Collaboration.

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The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project
DOI: https://doi.org/10.3847/1538-4365/ab29fd
Publication date: August 2019
Authors: Oliver Porth et al.

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Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope, and high- precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD, and KORAL. Agreement among the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.

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First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
DOI: https://doi.org/10.3847/2041-8213/ab1141
Publication date: 10 April 2019
Authors: The Event Horizon Telescope Collaboration

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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.

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First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
DOI: https://doi.org/10.3847/2041-8213/ab0f43
Publication date: 10 April 2019
Authors: The Event Horizon Telescope Collaboration

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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.

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First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
DOI: https://doi.org/10.3847/2041-8213/ab0e85
Publication date: 10 April 2019
Authors: The Event Horizon Telescope Collaboration

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We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of ∼40 μas, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others’ work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions.

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First M87 Event Horizon Telescope Results. III. Data Processing and Calibration
DOI: https://doi.org/10.3847/2041-8213/ab0c57
Publication date: 10 April 2019
Authors: The Event Horizon Telescope Collaboration

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We present the calibration and reduction of Event Horizon Telescope (EHT) 1.3 mm radio wavelength observations of the supermassive black hole candidate at the center of the radio galaxy M87 and the quasar 3C 279, taken during the 2017 April 5-11 observing campaign. These global very long baseline interferometric observations include for the first time the highly sensitive Atacama Large Millimeter/submillimeter Array (ALMA); reaching an angular resolution of 25 μas, with characteristic sensitivity limits of ∼1 mJy on baselines to ALMA and ∼10 mJy on other baselines. The observations present challenges for existing data processing tools, arising from the rapid atmospheric phase fluctuations, wide recording bandwidth, and highly heterogeneous array. In response, we developed three independent pipelines for phase calibration and fringe detection, each tailored to the specific needs of the EHT. The final data products include calibrated total intensity amplitude and phase information. They are validated through a series of quality assurance tests that show consistency across pipelines and set limits on baseline systematic errors of 2% in amplitude and 1° in phase. The M87 data reveal the presence of two nulls in correlated flux density at ∼3.4 and ∼8.3 Gλ and temporal evolution in closure quantities, indicating intrinsic variability of compact structure on a timescale of days, or several light-crossing times for a few billion solar-mass black hole. These measurements provide the first opportunity to image horizon-scale structure in M87.

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First M87 Event Horizon Telescope Results. II. Array and Instrumentation
DOI: https://doi.org/10.3847/2041-8213/ab0c96
Publication date: 10 April 2019
Authors: The Event Horizon Telescope Collaboration

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The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ∼1.3 mm, EHT angular resolution (λ/D) is ∼25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s−1, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

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First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
DOI: https://doi.org/10.3847/2041-8213/ab0ec7
Publication date: 10 April 2019
Authors: The Event Horizon Telescope Collaboration

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When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42 ± 3 μas, which is circular and encompasses a central depression in brightness with a flux ratio ≳10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5 ± 0.7) × 1e9 M ☉. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

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Observing supermassive black holes in virtual reality
DOI: https://doi.org/10.1186/s40668-018-0023-7
Publication date: 19 November 2018
Authors: J. Davelaar, T. Bronzwaer, D. Kok, Z. Younsi, M. Mościbrodzka and H. Falcke

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We present a 360 (i.e., 4π steradian) general-relativistic ray-tracing and radiative transfer calculations of accreting supermassive black holes. We perform state-of-the-art three-dimensional general-relativistic magnetohydrodynamical simulations using the BHAC code, subsequently post-processing this data with the radiative transfer code RAPTOR. All relativistic and general-relativistic effects, such as Doppler boosting and gravitational redshift, as well as geometrical effects due to the local gravitational field and the observer’s changing position and state of motion, are therefore calculated self-consistently. Synthetic images at four astronomically-relevant observing frequencies are generated from the perspective of an observer with a full 360 view inside the accretion flow, who is advected with the flow as it evolves. As an example we calculated images based on recent best-fit models of observations of Sagittarius A*. These images are combined to generate a complete 360 Virtual Reality movie of the surrounding environment of the black hole and its event horizon. Our approach also enables the calculation of the local luminosity received at a given fluid element in the accretion flow, providing important applications in, e.g., radiation feedback calculations onto black hole accretion flows. In addition to scientific applications, the 360 Virtual Reality movies we present also represent a new medium through which to interactively communicate black hole physics to a wider audience, serving as a powerful educational tool.

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RAPTOR-I. Time-dependent radiative transfer in arbitrary spacetimes
DOI: https://doi.org/10.1051/0004-6361/201732149
Publication date: 15 May 2018
Authors: T. Bronzwaer, J. Davelaar, Z. Younsi, M. Mościbrodzka, H. Falcke, M. Kramer and L. Rezzolla

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Context. Observational efforts to image the immediate environment of a black hole at the scale of the event horizon benefit from the development of efficient imaging codes that are capable of producing synthetic data, which may be compared with observational data.
Aims. We aim to present RAPTOR, a new public code that produces accurate images, animations, and spectra of relativistic plasmas in strong gravity by numerically integrating the equations of motion of light rays and performing time-dependent radiative transfer calculations along the rays. The code is compatible with any analytical or numerical spacetime. It is hardware-agnostic and may be compiled and run both on GPUs and CPUs.
Methods. We describe the algorithms used in RAPTOR and test the code’s performance. We have performed a detailed comparison of RAPTOR output with that of other radiative-transfer codes and demonstrate convergence of the results. We then applied RAPTOR to study accretion models of supermassive black holes, performing time-dependent radiative transfer through general relativistic magneto-hydrodynamical (GRMHD) simulations and investigating the expected observational differences between the so-called fast-light and slow-light paradigms.
Results. Using RAPTOR to produce synthetic images and light curves of a GRMHD model of an accreting black hole, we find that the relative difference between fast-light and slow-light light curves is less than 5%. Using two distinct radiative-transfer codes to process the same data, we find integrated flux densities with a relative difference less than 0.01%.
Conclusions. For two-dimensional GRMHD models, such as those examined in this paper, the fast-light approximation suffices as long as errors of a few percent are acceptable. The convergence of the results of two different codes demonstrates that they are, at a minimum, consistent.

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General relativistic magnetohydrodynamical κ-jet models for Sagittarius A*
DOI: https://doi.org/10.1051/0004-6361/201732025
Publication date: 16 April 2018
Authors: J. Davelaar, M. Mościbrodzka, T. Bronzwaer and H. Falcke

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Context. The observed spectral energy distribution of an accreting supermassive black hole typically forms a power-law spectrum in the near infrared (NIR) and optical wavelengths, that may be interpreted as a signature of accelerated electrons along the jet. However, the details of acceleration remain uncertain.
Aim. In this paper, we study the radiative properties of jets produced in axisymmetric general relativistic magnetohydrodynamics (GRMHD) simulations of hot accretion flows onto underluminous supermassive black holes both numerically and semi-analytically, with the aim of investigating the differences between models with and without accelerated electrons inside the jet.
Methods. We assume that electrons are accelerated in the jet regions of our GRMHD simulation. To model them, we modify the electrons’ distribution function in the jet regions from a purely relativistic thermal distribution to a combination of a relativistic thermal distribution and the κ-distribution function (the κ-distribution function is itself a combination of a relativistic thermal and a non-thermal power-law distribution, and thus it describes accelerated electrons). Inside the disk, we assume a thermal distribution for the electrons. In order to resolve the particle acceleration regions in the GRMHD simulations, we use a coordinate grid that is optimized for modeling jets. We calculate jet spectra and synchrotron maps by using the ray tracing code RAPTOR, and compare the synthetic observations to observations of Sgr A*. Finally, we compare numerical models of jets to semi-analytical ones.
Results. We find that in the κ-jet models, the radio-emitting region size, radio flux, and spectral index in NIR/optical bands increase for decreasing values of the κ parameter, which corresponds to a larger amount of accelerated electrons. This is in agreement with analytical predictions. In our models, the size of the emission region depends roughly linearly on the observed wavelength λ, independently of the assumed distribution function. The model with κ = 3.5, ηacc = 5–10% (the percentage of electrons that are accelerated), and observing angle i = 30° fits the observed Sgr A* emission in the flaring state from the radio to the NIR/optical regimes, while κ = 3.5, ηacc < 1%, and observing angle i = 30° fit the upper limits in quiescence. At this point, our models (including the purely thermal ones) cannot reproduce the observed source sizes accurately, which is probably due to the assumption of axisymmetry in our GRMHD simulations. The κ-jet models naturally recover the observed nearly-flat radio spectrum of Sgr A* without invoking the somewhat artificial isothermal jet model that was suggested earlier.
Conclusions. From our model fits we conclude that between 5% and 10% of the electrons inside the jet of Sgr A* are accelerated into a κ distribution function when Sgr A* is flaring. In quiescence, we match the NIR upper limits when this percentage is <1%.

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Faraday rotation in GRMHD simulations of the jet launching zone of M87
DOI: https://doi.org/10.1093/mnras/stx587
Publication date: 28 March 2017
Authors: M. Mościbrodzka, J. Dexter, J. Davelaar, H. Falcke

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Abstract: Non-very long baseline interferometry (VLBI) measurements of Faraday rotation at millimetre wavelengths have been used to constrain mass accretion rates (⁠M˙⁠) on to supermassive black holes in the centre of the Milky Way and in the centre of M87. We constructed general relativistic magnetohydrodynamics models for these sources that qualitatively well describe their spectra and radio/mm images invoking a coupled jet–disc system. Using general relativistic polarized radiative transfer, we now also model the observed mm rotation measure (RM) of M87. The models are tied to the observed radio flux; however, the electron temperature and accretion rate are degenerate parameters and are allowed to vary. For the inferred low viewing angles of the M87 jet, the RM is low even as the black hole accretion rate increases by a factor of ≃100. In jet-dominated models, the observed linear polarization is produced in the forward jet, while the dense accretion disc depolarizes the bulk of the near-horizon scale emission that originates in the counter jet. In the jet-dominated models, with increasing accretion rate and increasing Faraday optical depth, one is progressively sensitive only to polarized emission in the forward jet, keeping the measured RM relatively constant. The jet-dominated model reproduces a low net-polarization of ≃1 per cent and RMs in agreement with observed values due to Faraday depolarization, however, with M˙ much larger than the previously inferred limit of 9 × 10−4  M yr−1. All jet-dominated models produce much higher RMs for inclination angles i ≳ 30°, where the line of sight passes through the accretion flow, thereby providing independent constraints on the viewing geometry of the M87 jet.

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BlackHoleCam: Fundamental physics of the galactic center
DOI: https://doi.org/10.1142/S0218271817300014
Publication date: 18 August 2016
Authors: C. Goddi et al.

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Abstract: Einstein’s General theory of relativity (GR) successfully describes gravity. Although GR has been accurately tested in weak gravitational fields, it remains largely untested in the general strong field cases. One of the most fundamental predictions of GR is the existence of black holes (BHs). After the recent direct detection of gravitational waves by LIGO, there is now near conclusive evidence for the existence of stellar-mass BHs. In spite of this exciting discovery, there is not yet direct evidence of the existence of BHs using astronomical observations in the electromagnetic spectrum. Are BHs observable astrophysical objects? Does GR hold in its most extreme limit or are alternatives needed? The prime target to address these fundamental questions is in the center of our own Milky Way, which hosts the closest and best-constrained supermassive BH candidate in the universe, Sagittarius A* (Sgr A*). Three different types of experiments hold the promise to test GR in a strong-field regime using observations of Sgr A* with new-generation instruments. The first experiment consists of making a standard astronomical image of the synchrotron emission from the relativistic plasma accreting onto Sgr A*. This emission forms a “shadow” around the event horizon cast against the background, whose predicted size (∼50μas) can now be resolved by upcoming very long baseline radio interferometry experiments at mm-waves such as the event horizon telescope (EHT). The second experiment aims to monitor stars orbiting Sgr A* with the next-generation near-infrared (NIR) interferometer GRAVITY at the very large telescope (VLT). The third experiment aims to detect and study a radio pulsar in tight orbit about Sgr A* using radio telescopes (including the Atacama large millimeter array or ALMA). The BlackHoleCam project exploits the synergy between these three different techniques and contributes directly to them at different levels. These efforts will eventually enable us to measure fundamental BH parameters (mass, spin, and quadrupole moment) with sufficiently high precision to provide fundamental tests of GR (e.g. testing the no-hair theorem) and probe the spacetime around a BH in any metric theory of gravity. Here, we review our current knowledge of the physical properties of Sgr A* as well as the current status of such experimental efforts towards imaging the event horizon, measuring stellar orbits, and timing pulsars around Sgr A*. We conclude that the Galactic center provides a unique fundamental-physics laboratory for experimental tests of BH accretion and theories of gravity in their most extreme limits.

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