My research focuses on high-energy astrophysical processes within the framework of general relativity. My main interests include the study of compact objects such as neutron stars, quark stars, boson stars, hybrid stars with dark matter components, and black holes. Below are the most recent publications from my research. To watch a list of all my publications visit All my publications: (here)
The recent 230 GHz observations by the Event Horizon Telescope have resolved the innermost structure of the M87 galaxy, revealing a ring-like feature consistent with thermal synchrotron emission from a magnetized torus surrounding a rotating supermassive black hole. Moreover, Global Millimeter VLBI Array observations at 86 GHz have revealed a larger-scale, edge-brightened jet with clear signatures of nonthermal emission. The theoretical modelling of these observations involves advanced general-relativistic magnetohydrodynamic simulations of magnetized accretion disks around rotating black holes, together with the associated synchrotron emission, which is normally treated with simplified expressions for the electron temperature and assuming a purely thermal distribution. However, an important nonthermal component is expected to be present, making the thermal-emission model not only an approximation but also a source of degeneracy in the modelling. In view of this, we here present the first application of an ab initio approach to the electron temperature derived from microscopic simulations of turbulent collisionless plasmas. This novel method, which has no tunable coefficients and is fully specified by the thermodynamical and magnetic properties of the plasma, provides a better description of the jet morphology and width at 86 GHz, as well as of the broadband spectral emission. These findings highlight the importance of incorporating microscopic plasma physics in black hole imaging and emphasise the crucial role of magnetic reconnection in electron heating and acceleration processes.
Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. GRMHD models predict that the amplitude and location of the ring's brightness asymmetry fluctuate due to turbulence. We compare observations with simulations across varying black hole spin a*. For strongly magnetized models, data marginally disfavors |a*| ≲ 0.2, consistent with the Blandford–Znajek model. Future observations could improve spin constraints and distinguish jet-launching mechanisms and growth scenarios.
Sagittarius A* provides a unique laboratory to study accretion dynamics. Using ALMA data from the 2018 EHT campaign, we analyze full-polarization variability. We find low total intensity variability but higher polarization variability. Results suggest distinct emission regions and challenge standard synchrotron flare models, favoring continuous energy injection. These findings constrain physical mechanisms driving variability near supermassive black holes.
We use MCMC methods to obtain posterior distributions for S2 star orbital parameters under multiple black hole space-time models. Using observational data, we perform Bayesian comparisons. Results show no statistically significant preference among models, with most remaining indistinguishable within uncertainties.
We present the first 1.3 mm EHT observations of OJ 287, revealing polarization variability and superluminal motion. Results provide direct evidence of helical magnetic fields in jets and their interaction with plasma instabilities. This explains polarization variability and predicts future observational signatures.
We investigate the presence and spatial characteristics of the jet base emission in M87* at 230 GHz, enabled by the enhanced uv coverage in the 2021 Event Horizon Telescope (EHT) observations. The addition of the 12-m Kitt Peak Telescope and NOEMA provides two key intermediate-length baselines to SMT and the IRAM 30-m, giving sensitivity to emission structures at scales of ∼250 μ as and ∼2500 μ as (0.02 pc and 0.2 pc). Without these baselines, earlier EHT observations lacked the capability to constrain emission on large scales, where a "missing flux" of order ∼1 Jy is expected. To probe these scales, we analyzed closure phases, robust against station-based gain errors, and modeled the jet base emission using a simple Gaussian offset from the compact ring emission at separations >100 μ as. Our analysis reveals a Gaussian feature centered at ( Δ RA ≈320 μ as, Δ Dec ≈60 μ as), a projected separation of ≈5500 AU, with a flux density of only ∼60 mJy, implying that most of the missing flux in previous studies must arise from larger scales. Brighter emission at these scales is ruled out, and the data do not favor more complex models. This component aligns with the inferred direction of the large-scale jet and is consistent with emission from the jet base. While our findings indicate detectable jet base emission at 230 GHz, coverage from only two intermediate baselines limits reconstruction of its morphology. We therefore treat the recovered Gaussian as an upper limit on the jet base flux density. Future EHT observations with expanded intermediate-baseline coverage will be essential to constrain the structure and nature of this component.
Turbulence in curved space─times in general, and in the vicinity of black holes (BHs) in particular, represents a poorly understood phenomenon that is often analysed employing techniques developed for flat space─times. We here propose a novel approach to study turbulence in strong gravitational fields that is based on the computation of structure functions on generic manifolds and is thus applicable to arbitrary curved space─times. In particular, we introduce, for the first time, a formalism to compute the characteristic properties of turbulence, such as the second-order structure function or the power spectral density, in terms of proper lengths and volumes and not in terms of coordinate lengths and volumes, as customarily done. By applying the new approach to the turbulent rest-mass density field from simulations of magnetized disc accretion onto a Kerr BH, we inspect in a rigorous way turbulence in regions close to the event horizon, but also in the disc, the wind, and the jet. We demonstrate that the new approach can capture the typical behaviour of an inertial-range cascade and that differences up to 40−80% emerge in the vicinity of the event horizon with respect to the standard flat space─time approach. While these differences become smaller at larger distances, our study highlights that special care needs to be paid when analysing turbulence in strongly curved space─times.
We report three epochs of polarized images of M87* at 230 GHz using data from the Event Horizon Telescope (EHT) taken in 2017, 2018, and 2021. The baseline coverage of the 2021 observations is significantly improved through the addition of two new EHT stations: the 12 m Kitt Peak Telescope and the Northern Extended Millimetre Array (NOEMA). All observations result in images dominated by a bright, asymmetric ring with a persistent diameter of 43.9 ± 0.6 μas, consistent with expectations for lensed synchrotron emission encircling the apparent shadow of a supermassive black hole. We find that the total intensity and linear polarization of M87* vary significantly across the three epochs. Specifically, the azimuthal brightness distribution of the total intensity images varies from year to year, as expected for a stochastic accretion flow. However, despite a gamma-ray flare erupting in M87 quasi-contemporaneously to the 2018 observations, the 2018 and 2021 images look remarkably similar. The resolved linear polarization fractions in 2018 and 2021 peak at ∼5%, compared to ∼15% in 2017. The spiral polarization pattern on the ring also varies from year to year, including a change in the electric vector position angle helicity in 2021 that could reflect changes in the magnetized accretion flow or an external Faraday screen. The improved 2021 coverage also provides the first EHT constraints on jet emission outside the ring, on scales of ≲1 mas. Overall, these observations provide strong proof of the reliability of the EHT images and probe the dynamic properties of the horizon-scale accretion flow surrounding M87*.
Simulations of relativistic plasmas traditionally focus on the dynamics of two-species mixtures of charged particles under the influence of external magnetic fields and those generated by particle currents. However, the extreme conditions of astrophysical plasmas near compact objects, such as black holes and neutron stars, are often characterized by mixtures of electrons, protons, and positrons, whose dynamics can differ significantly, because of the considerable mass contrast. We present the first two-dimensional particle-in-cell simulations of relativistic turbulence and magnetic reconnection in a three-species plasma, varying the relative abundances of electrons, protons, and positrons, while employing realistic mass ratios to achieve unprecedented accuracy. We find that turbulence leads to the formation of magnetic islands, current sheets, and plasmoids. Reconnection occurs between these structures, with plasma composition playing a key role in determining the number of reconnection sites and their energy conversion efficiency. In particular, as the proton fraction increases, very small-scale features of the turbulence are washed out, while global dissipative effects are amplified. Finally, using a novel generalization of Ohm's law for a relativistic multispecies plasma, we find that the reconnection rate is primarily governed by the electric fields associated with the divergence of the positron and electron pressure tensors. These results provide new insights into dissipation and particle acceleration in turbulent relativistic plasmas, such as those near black holes and neutron stars, and can be used to interpret their high-energy emission and phenomenology.
We investigate the origin of the elliptical ring structure observed in the images of the supermassive black hole M87*, aiming to disentangle contributions from gravitational, astrophysical, and imaging effects. Leveraging the enhanced capabilities of the Event Horizon Telescope (EHT)'s 2018 array, including improved (u,v)-coverage from the Greenland Telescope, we measured the ring's ellipticity using five independent imaging methods, obtaining a consistent average value of τ = 0.08‑0.02+0.03 with a position angle of ξ = 50.1‑7.6+6.2 degrees. To interpret this measurement, we compared it to general relativistic magnetohydrodynamic (GRMHD) simulations spanning a wide range of physical parameters including the thermal or nonthermal electron distribution function, spins, and ion-to-electron temperature ratios in both low- and high-density regions. We find no statistically significant correlation between spin and ellipticity in GRMHD images. Instead, we identify a correlation between ellipticity and the fraction of non-ring emission, particularly in nonthermal models and models with higher jet emission. These results indicate that the ellipticity measured from the M87* emission structure is consistent with that expected from simulations of turbulent accretion flows around black holes, where it is dominated by astrophysical effects rather than gravitational ones. Future high-resolution imaging, including space very long baseline interferometry and long-term monitoring, will be essential to isolate gravitational signatures from astrophysical effects.
Aims. We investigated the polarization and Faraday properties of Messier 87 (M87) and seven other radio-loud active galactic nuclei (AGNs) at λ0.87 mm (345 GHz) using the Atacama Large Millimeter/submillimeter Array (ALMA). Our goal was to characterize the linear polarization (LP) fractions, measure Faraday rotation measures (RMs), and examine the magnetic field structures in the emission regions of these AGNs. Methods. We conducted full-polarization observations as part of the ALMA Band 7 very long baseline interferometry (VLBI) commissioning during the April 2021 Event Horizon Telescope (EHT) campaign. We analyzed the LP fractions and RMs to assess the nature of Faraday screens and magnetic fields in the submillimeter emission regions. Results. We find LP fractions between 1% and 17% and RMs exceeding 105 rad m‑2, which are 1–2 orders of magnitude higher than typically observed at longer wavelengths (λ>3 mm). This suggests denser Faraday screens or stronger magnetic fields. Additionally, we present the first submillimeter polarized images of the M87 jet and the observed AGNs, revealing RM gradients and sign reversals in the M87 jet indicative of a kiloparsec-scale helical magnetic field structure. Conclusions. Our results provide essential constraints for calibrating, analyzing, and interpreting VLBI data from the EHT at 345 GHz, representing a critical step toward submillimeter VLBI imaging.