Flares in the Galactic Centre – I. Orbiting flux tubes in magnetically arrested black hole accretion discs

Recent observations of Sgr A* by the GRAVITY instrument have astrometrically tracked infrared (IR) flares at distances of ∼10 gravitational radii (rg). In this paper, we study a model for the flares based on 3D general relativistic magnetohydrodynamic (GRMHD) simulations of magnetically arrested accretion discs (MADs) that exhibit violent episodes of flux escape from the black hole magnetosphere. These events are attractive for flare modelling for several reasons: (i) the magnetically dominant regions can resist being disrupted via magnetorotational turbulence and shear; (ii) the orientation of the magnetic field is predominantly vertical as suggested by the GRAVITY data; and (iii) the magnetic reconnection associated with the flux eruptions could yield a self-consistent means of particle heating/acceleration during the flare events. In this analysis, we track erupted flux bundles and provide distributions of sizes, energies, and plasma parameter. In our simulations, the orbits tend to circularize at a range of radii from ∼5 to 40rg⁠. The magnetic energy contained within the flux bundles ranges up to ∼1040erg⁠, enough to power IR and X-ray flares. We find that the motion within the magnetically supported flow is substantially sub-Keplerian, in tension with the inferred period–radius relation of the three GRAVITY flares

Black hole parameter estimation with synthetic very long baseline interferometry data from the ground and from space

Context. The Event Horizon Telescope (EHT) has imaged the shadow of the supermassive black hole in M 87. A library of general relativistic magnetohydrodynamics (GMRHD) models was fit to the observational data, providing constraints on black hole parameters. Aims. We investigate how much better future experiments can realistically constrain these parameters and test theories of gravity. Methods. We generated realistic synthetic 230 GHz data from representative input models taken from a GRMHD image library for M 87, using the 2017, 2021, and an expanded EHT array. The synthetic data were run through an automated data reduction pipeline used by the EHT. Additionally, we simulated observations at 230, 557, and 690 GHz with the Event Horizon Imager (EHI) Space VLBI concept. Using one of the EHT parameter estimation pipelines, we fit the GRMHD library images to the synthetic data and investigated how the black hole parameter estimations are affected by different arrays and repeated observations. Results. Repeated observations play an important role in constraining black hole and accretion parameters as the varying source structure is averaged out. A modest expansion of the EHT already leads to stronger parameter constraints in our simulations. High-frequency observations from space with the EHI rule out all but ∼15% of the GRMHD models in our library, strongly constraining the magnetic flux and black hole spin. The 1σ constraints on the black hole mass improve by a factor of five with repeated high-frequency space array observations as compared to observations with the current ground array. If the black hole spin, magnetization, and electron temperature distribution can be independently constrained, the shadow size for a given black hole mass can be tested to ∼0.5% with the EHI space array, which allows tests of deviations from general relativity. With such a measurement, high-precision tests of the Kerr metric become within reach from observations of the Galactic Center black hole Sagittarius A*

Using space-VLBI to probe gravity around Sgr A*

CONTEXT: The Event Horizon Telescope (EHT) will soon provide the first high-resolution images of the Galactic Centre supermassive black hole candidate Sagittarius A* (Sgr A*), enabling us to probe gravity in the strong-field regime. In addition to studying the accretion process in extreme environments, the obtained data and reconstructed images could be used to investigate the underlying spacetime structure. In its current configuration, EHT is able to distinguish between a rotating Kerr black hole and a horizon-less object such as a boson star. Future developments can increase the ability of EHT to tell different spacetimes apart. AIMS: We investigate the capability of an advanced EHT concept, including an orbiting space antenna, to image and distinguish different spacetimes around Sgr A*. METHODS: We used general-relativistic magneto-hydrodynamical simulations of accreting compact objects (Kerr and dilaton black holes as well as boson stars) and computed their radiative signatures via general-relativistic radiative transfer. To facilitate a comparison with upcoming and future EHT observations, we produced realistic synthetic data including the source variability, diffractive, and refractive scattering while incorporating the observing array, including a space antenna. From the generated synthetic observations, we dynamically reconstructed black hole shadow images using regularised maximum entropy methods. We employed a genetic algorithm to optimise the orbit of the space antenna with respect to improved imaging capabilities and u − v-plane coverage of the combined array (ground array and space antenna) and developed a new method to probe the source variability in Fourier space. RESULTS: The inclusion of an orbiting space antenna improves the capability of EHT to distinguish the spin of Kerr black holes and dilaton black holes based on reconstructed radio images and complex visibilities

State-of-the-art energetic and morphological modelling of the launching site of the M87 jet

M87 has been the target of numerous astronomical observations across the electromagnetic spectrum, and very long baseline interferometry has resolved an edge-brightened jet1,2,3,4. However, the origin and formation of its jets remain unclear. In our current understanding, black holes (BH) are the driving engine of jet formation5, and indeed the recent Event Horizon Telescope observations revealed a ring-like structure in agreement with theoretical models of accretion onto a rotating Kerr BH6. In addition to the spin of the BH being a potential source of energy for the launching mechanism, magnetic fields are believed to play a key role in the formation of relativistic jets7,8. A priori, the spin, a⋆, of the BH in M87⋆ is unknown; however, when accounting for the estimates of the X-ray luminosity and jet power, values of |a_{*}| ≳ 0.5 appear favoured6. Besides the properties of the accretion flow and the BH spin, the radiation microphysics including the particle distribution (thermal6 and non-thermal^{9,10}) as well as the particle acceleration mechanism11 play a crucial role. We show that general relativistic magnetohydrodynamic simulations and general relativistic radiative transfer calculations can reproduce the broadband spectrum from the radio to the near-infrared regime and simultaneously match the observed collimation profile of M87, thus allowing us to set rough constraints on the dimensionless spin of M87* to be 0.5 ≲ a⋆ ≲ 1.0, with higher spins being possibly favoured

The current ability to test theories of gravity with black hole shadows

Our Galactic Centre, Sagittarius A*, is believed to harbour a supermassive black hole, as suggested by observations tracking individual orbiting stars1,2. Upcoming submillimetre very-long baseline interferometry images of Sagittarius A* carried out by the Event Horizon Telescope collaboration (EHTC)3,4 are expected to provide critical evidence for the existence of this supermassive black hole5,6. We assess our present ability to use EHTC images to determine whether 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 general-relativistic magnetohydrodynamical simulations and use general-relativistic radiative-transfer calculations to generate synthetic shadow images of a magnetized accretion flow onto a Kerr black hole. In addition, we perform these simulations and calculations for a dilaton black hole, which we take as a representative solution of an alternative theory of gravity. Adopting the very-long baseline interferometry configuration 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 black hole images as tests of general relativity

Using evolutionary algorithms to model relativistic jets: Application to NGC 1052

Context. High-resolution very long baseline interferometry (VLBI) observations of NGC 1052 show a two sided jet with several regions of enhanced emission and a clear emission gap between the two jets. This gap shrinks with increasing frequency and vanishes around ν ∼ 43 GHz. The observed structures are due to both the macroscopic fluid dynamics interacting with the surrounding ambient medium including an obscuring torus and the radiation microphysics. In order to model the observations of NGC 1052 via state-of-the art numerical simulations both the fluid-dynamical and emission processes have to be taken into account. Aims. In this paper we investigate the possible physical conditions in relativistic jets of NGC 1052 by directly modelling the observed emission and spectra via state-of-the-art special-relativistic hydrodynamic (SRHD) simulations and radiative transfer calculations. Methods. We performed SRHD simulations of over-pressured and pressure-matched jets using the special-relativistic hydrodynamics code Ratpenat. To investigate the physical conditions in the relativistic jet we coupled our radiative transfer code to evolutionary algorithms and performed simultaneous modelling of the observed jet structure and the broadband radio spectrum. During the calculation of the radiation we consider non-thermal emission from the jet and thermal absorption in the obscuring torus. In order to compare our model to VLBI observations we take into account the sparse sampling of the u-v plane, the array properties and the imaging algorithm. Results. We present for the first time an end-to-end pipeline for fitting numerical simulations to VLBI observations of relativistic jets taking into account the macro-physics including fluid dynamics and ambient medium configurations together with thermal and non-thermal emission and the properties of the observing array. The detailed analysis of our simulations shows that the structure and properties of the observed relativistic jets in NGC 1052 can be reconstructed by a slightly over-pressured jet (dk ∼ 1.5) embedded in a decreasing pressure ambient mediu

A Fast Na+/Ca2+-Based Action Potential in a Marine Diatom

BACKGROUND:Electrical impulses in animals play essential roles in co-ordinating an array of physiological functions including movement, secretion, environmental sensing and development. Underpinning many of these electrical signals is a fast Na+-based action potential that has been fully characterised only in cells associated with the neuromuscular systems of multicellular animals. Such rapid action potentials are thought to have evolved with the first metazoans, with cnidarians being the earliest representatives. The present study demonstrates that a unicellular protist, the marine diatom Odontella sinensis, can also generate a fast Na+/Ca2+ based action potential that has remarkably similar biophysical and pharmacological properties to invertebrates and vertebrate cardiac and skeletal muscle cells. METHODOLOGY/PRINCIPAL FINDINGS:The kinetic, ionic and pharmacological properties of the rapid diatom action potential were examined using single electrode current and voltage clamp techniques. Overall, the characteristics of the fast diatom currents most closely resemble those of vertebrate and invertebrate muscle Na+/Ca2+ currents. CONCLUSIONS/SIGNIFICANCE:This is the first demonstration of voltage-activated Na+ channels and the capacity to generate fast Na+-based action potentials in a unicellular photosynthetic organism. The biophysical and pharmacological characteristics together with the presence of a voltage activated Na+/Ca2+ channel homologue in the recently sequenced genome of the diatom Thalassiosira pseudonana, provides direct evidence supporting the hypothesis that this rapid signalling mechanism arose in ancestral unicellular eukaryotes and has been retained in at least two phylogenetically distant lineages of eukaryotes; opisthokonts and the stramenopiles. The functional role of the fast animal-like action potential in diatoms remains to be elucidated but is likely involved in rapid environmental sensing of these widespread and successful marine protists

Key Science Goals for the Next-Generation Event Horizon Telescope

The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide

G12 signaling through c-jun nh 2-terminal kinase promotes breast cancer cell invasion

10.1371/journal.pone.0026085PLoS ONE611