Spectral Evolution in Blazars : The Case of CTA 102

Active Galactic Nuclei (AGN) are among the most powerful objects in the universe. In their centre they host a supermassive black hole (BH) with up to 1010 solar masses and an accretion disk is formed around them feeding the system. A fraction of the in-falling mater is ejected perpendicularly to the accretion disk forming the so-called jets. These relativistic flows are highly collimated and propagate up to kiloparsec distances from their central engine. The observed emission of AGN jets shows strong variability throughout the electro-magnetic spectrum which reflects variations in source intrinsic parameters such as the magnetic field and the rest-mass density. The variation in the emission of AGN jets can be best studied in their most powerful representatives, the blazars (AGN jets seen under a small viewing angle). The blazar CTA 102 underwent a historic radio outburst in April 2006 which provides a perfect laboratory for studying the spectral evolution. CTA 102 has been a target of single-dish and VLBI observations for several years. In this work we use both kind of observations to study and model the spectral evolution during the flare. We use the dense sampling of the single-dish observations to trace the evolution of the flare in the turnover-frequency and turnover flux density plane and modelled the results with a modified shock-in-jet model, assuming a travelling shock recollimation shock interaction. To test this hypothesis, we combine archival VLBI observations from the MOJAVE program (15 GHz) and Boston University Blazar Monitoring program (43 GHz) with our multi-frequency VLBI observations during the 2006 flare. The VLBI kinematic provides a unique view on the parsec-scale structure of CTA 102 over the last 15 years and reveals several stationary features. Our hypothesis of a shock-shock interaction as possible mechanism behind the 2006 is confirmed by a detailed spectral analysis of the multi-frequency VLBI observations. We use 2D relativistic hydrodynamic simulations (RHD) to bridge the sparse time sampling of the observations and to further investigate the non-linear process of travelling shock recollimation shock interaction. From the simulations we compute the non-thermal emission taking adiabatic and radiative losses into account. The synthetic single dish spectra and radio maps can reproduce the observed structure in the VLBI maps and variation in the single dish spectra during the flare. In addition, we present observable predictions for the interaction between a travelling shock and a recollimation shock

The Accelerating Jet of 3C 279

Analysis of the proper motions of the sub-parsec scale jet of the quasar 3C 279 at 15 GHz with the VLBA shows significant accelerations in four of nine superluminal features. Analysis of these motions is combined with the analysis of flux density light curves to constrain values of Lorentz factor and viewing angle (and their derivatives) for each component. The data for each of these components is consistent with significant changes to the Lorentz factor, viewing angle and azimuthal angle, suggesting jet bending with changes in speed. We see that for these observed components Lorentz factors are in the range Γ = 10−41, viewing angles are in the range ϑ = 0.1◦ −5.0◦ , and intrinsic (source frame) flux density is in the range, Fν , int = 1.5×10−9−1.5×10−5 Jy. Considering individual components, the Lorentz factors vary from Γ = 11 − 16 for C1, Γ = 31 − 41 for C5, Γ = 29 − 41 for C6 and Γ = 9 − 12 for C8, indicating that there is no single underlying flow speed to the jet and likely we are seeing pattern speeds from shocks in the jet. The viewing angles vary in time from 0.6◦ to 1.5◦ in the case of C1 (the least extreme example), vary from 0.5◦ to 5.0◦ in the case of C8 and vary from 0.1◦ to 0.9◦ for C5 (the last two being the most extreme examples). The intrinsic flux density varies by factors from 1.4 for C8 and 430 for C5. Theoretical analysis of the accelerations also indicates potential jet bending. In addition, for one component, C5, polarization measurements also set limits to the trajectory of the jet

Two-Temperature GRMHD Simulations of Black Hole Accretion Flows with Multiple Magnetic Loops

We have performed a series of two-dimensional two-temperature general relativistic magnetohydrodynamic simulations of magnetized accretion flows initiated from tori with different sizes and poloidal magnetic loop polarities. In these two temperature simulations, we trace the process of heating electrons through turbulence and reconnection, most of the time these electrons are trapped in plasmoids. We found that the accretion process strongly depends on the size of the magnetic loops. The accretion flows never reach the magnetically arrested (MAD) regime in small loop cases. Interaction between magnetic field with different polarities dissipates and decreases the efficiency of magneto-rotational instability. The dependency on the wavelength of the loops places a lower limit on the loop size. In the large loop cases, after reaching a quasi-steady phase, a transition from Standard And Normal Evolution (SANE) flow to MAD flow is observed. The transition of the accretion state and the transition time depends on the initial loop wavelength. The formation of plasmoids strongly depends on the size of the magnetic loops. The frequent magnetic reconnection between the magnetic loops is responsible for the formation of most of the plasmoids. For some plasmoids, Kelvin-Helmholtz and tearing instabilities are coexisting, showing another channel of plasmoid formation. The simulations present that electrons in the plasmoids are well-heated up by turbulent and magnetic reconnection. Different properties of plasmoid formation in different magnetic field configurations provide new insights for the understanding of flaring activity and electron thermodynamics in Sgr A*.Comment: 18 pages, 25 figures, accepted for publication in MNRA

The Current Ability to Test Theories of Gravity with Black Hole Shadows

Our Galactic Center, Sagittarius A* (Sgr A*), is believed to harbour a supermassive black hole (BH), as suggested by observations tracking individual orbiting stars. Upcoming sub-millimetre very-long-baseline-interferometry (VLBI) images of Sgr A* carried out by the Event-Horizon-Telescope Collaboration (EHTC) are expected to provide critical evidence for the existence of this supermassive BH. We assess our present ability to use EHTC images to determine if they correspond to a Kerr BH as predicted by Einstein's theory of general relativity (GR) or to a BH in alternative theories of gravity. To this end, we perform general-relativistic magnetohydrodynamical (GRMHD) simulations and use general-relativistic radiative transfer (GRRT) calculations to generate synthetic shadow images of a magnetised accretion flow onto a Kerr BH. 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. Adopting the VLBI configuration from the 2017 EHTC campaign, we find that it could be extremely difficult to distinguish between BHs from different theories of gravity, thus highlighting that great caution is needed when interpreting BH images as tests of GR.Comment: Published in Nature Astronomy on 16.04.18 (including supplementary information); simulations at https://blackholecam.org/telling_bhs_apart

How to tell an accreting boson star from a black hole

The capability of the Event Horizon Telescope (EHT) to image the nearest supermassive black hole candidates at horizon-scale resolutions offers a novel means to study gravity in its strongest regimes and to test different models for these objects. Here, we study the observational appearance at 230 GHz of a surfaceless black hole mimicker, namely a non-rotating boson star, in a scenario consistent with the properties of the accretion flow onto Sgr A*. To this end, we perform general relativistic magnetohydrodynamic simulations followed by general relativistic radiative transfer calculations in the boson star space-time. Synthetic reconstructed images considering realistic astronomical observing conditions show that, despite qualitative similarities, the differences in the appearance of a black hole -- either rotating or not -- and a boson star of the type considered here are large enough to be detectable. These differences arise from dynamical effects directly related to the absence of an event horizon, in particular, the accumulation of matter in the form of a small torus or a spheroidal cloud in the interior of the boson star, and the absence of an evacuated high-magnetization funnel in the polar regions. The mechanism behind these effects is general enough to apply to other horizonless and surfaceless black hole mimickers, strengthening confidence in the ability of the EHT to identify such objects via radio observations.Comment: 16 pages, 12 figures. Published in MNRAS. Adding more information in the form of appendices, and a new simulation of a different boson star model. The conclusions do not chang

3D magnetised jet break-out from neutron-star binary merger ejecta: afterglow emission from the jet and the ejecta

We perform three-dimensional (3D) general-relativistic magnetohydrodynamic simulations to model the jet break-out from the ejecta expected to be produced in a binary neutron-star merger. The structure of the relativistic outflow from the 3D simulation confirms our previous results from 2D simulations, namely, that a relativistic magnetized outflow breaking out from the merger ejecta exhibits a hollow core of $\theta_{\rm core}\approx4^{\circ}$, an opening angle of $\theta_{\rm jet}\gtrsim10^{\circ}$, and is accompanied by a wind of ejected matter that will contribute to the kilonova emission. We also compute the non-thermal afterglow emission of the relativistic outflow and fit it to the panchromatic afterglow from GRB170817A, together with the superluminal motion reported from VLBI observations. In this way, we deduce an observer angle of $\theta_{\rm obs}= 35.7^{\circ \,\,+1.8}_{\phantom{\circ \,\,}-2.2}$. We further compute the afterglow emission from the ejected matter and constrain the parameter space for a scenario in which the matter responsible for the thermal kilonova emission will also lead to a non-thermal emission yet to be observed.Comment: MNRAS accepted, updated versio

On the opening angle of magnetised jets from neutron-star mergers: the case of GRB170817A

The observations of GW170817/GRB170817A have confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst. In the standard picture of a short gamma-ray burst, a collimated highly relativistic outflow is launched after merger and it successfully breaks out from the surrounding ejected matter. Using initial conditions inspired from numerical-relativity binary neutron-star merger simulations, we have performed general-relativistic hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations in which the jet is launched and propagates self-consistently. The complete set of simulations suggests that: (i) MHD jets have an intrinsic energy and velocity polar structure with a ``hollow core'' subtending an angle $\theta_{\rm core}\approx4^{\circ}-5^{\circ}$ and an opening angle of $\theta_{\rm jet}\gtrsim10^{\circ}$; (ii) MHD jets eject significant amounts of matter and two orders of magnitude more than HD jets; (iii) the energy stratification in MHD jets naturally yields the power-law energy scaling $E(>\Gamma\beta)\propto(\Gamma\beta)^{-4.5}$; (iv) MHD jets provide fits to the afterglow data from GRB170817A that are comparatively better than those of the HD jets and without free parameters; (v) finally, both of the best-fit HD/MHD models suggest an observation angle $\theta_{\rm obs} \simeq 21^{\circ}$ for GRB170817A.Comment: 9 pages, 5 figures, submitte

Comparison of the ion-to-electron temperature ratio prescription: GRMHD simulations with electron thermodynamics

The Event Horizon Telescope (EHT) collaboration, an Earth-size sub-millimetre radio interferometer, recently captured the first images of the central supermassive black hole in M87. These images were interpreted as gravitationally-lensed synchrotron emission from hot plasma orbiting around the black hole. In the accretion flows around low-luminosity active galactic nuclei such as M87, electrons and ions are not in thermal equilibrium. Therefore, the electron temperature, which is important for the thermal synchrotron radiation at EHT frequencies of 230 GHz, is not independently determined. In this work, we investigate the commonly used parameterised ion-to-electron temperature ratio prescription, the so-called R-$\beta$ model, considering images at 230 GHz by comparing with electron-heating prescriptions obtained from general-relativistic magnetohydrodynamical (GRMHD) simulations of magnetised accretion flows in a Magnetically Arrested Disc (MAD) regime with different recipes for the electron thermodynamics. When comparing images at 230 GHz, we find a very good match between images produced with the R-$\beta$ prescription and those produced with the turbulent- and magnetic reconnection- heating prescriptions. Indeed, this match is on average even better than that obtained when comparing the set of images built with the R-$\beta$ prescription with either a randomly chosen image or with a time-averaged one. From this comparative study of different physical aspects, which include the image, visibilities, broadband spectra, and light curves, we conclude that, within the context of images at 230 GHz relative to MAD accretion flows around supermassive black holes, the commonly-used and simple R-$\beta$ model is able to reproduce well the various and more complex electron-heating prescriptions considered here.Comment: 18 pages, 22 figures, accepted for publication in MNRA

Constraints on black-hole charges with the 2017 EHT observations of M87∗

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87∗ shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87∗ is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes