Matthias Kadler

Context . Relativistic jets are among the most fascinating phenomena in the universe, and recent high-resolution very long baseline interferometric (VLBI) observations, including the Global mm-VLBI Array and the Event Horizon Telescope (EHT), are able to resolve their structure close to their launching site. These observations reveal strongly limb-brightened jet structures for Centaurus A (Cen A), M 87, and 3C 84. Aims . Thus, it was unclear which physical mechanism can generate the edge-brightened jet, and whether this structure is common for jets from low-luminosity active galactic nuclei (LLAGNs) seen under large viewing angles. Therefore, as a pilot study, we aimed to model the EHT observations of Cen A. Methods . We performed a 3D two-temperature general-relativistic magnetohydrodynamic (GRMHD) simulation of an accreting supermassive black hole (SMBH) and subsequent jet launching to study the plasma dynamics, and we computed the connected emission via general relativistic radiative transfer (GRRT) calculations considering possible anisotropies in the velocity distributions of the radiating electrons preferentially parallel to the magnetic field. In order to adjust our simulations to the EHT observations of Cen A, we carried out a Bayesian fitting in the Fourier plane. Results . We find that GRMHD simulations of magnetically arrested disks (MADs) combined with anisotropically emitting particle distributions along the direction of the magnetic field, parametrized by a value η = 0.07, are capable of mimicking the recent EHT observations of Cen A. In addition, we extracted a black hole (BH) mass of M _BH ≃ 6×10 ⁷ M _⊙ and a viewing angle of ϑ ≃ 72°. Our obtained model can reproduce key features of the EHT and Atacama Large Millimeter/submillimeter Array (ALMA) observations in total and polarized emission. Finally, we predict that the BH shadow in Cen A will be observable at a frequency of ∼3 THz and stress the importance of high-cadence and high-frequency ( ν > 350 GHz) observations to better constrain the particle acceleration, radiation microphysics, and accretion dynamics.

We report on a nine-month monitoring campaign of the blazar PG 1553+113, relying on three observations carried out in 2025 with the Imaging X-ray Polarimetry Explorer (IXPE) and supported by multiwavelength facilities. The source displayed pronounced variability across the electromagnetic spectrum, with X-ray flux changes by up to a factor of ∼5 and complex evolution of the optical polarization properties, including one of the largest (exceeding 150°) and fastest rotations in the electric vector position angle (EVPA) ever recorded. This swing of the EVPA was also accompanied by a temporary drop of the optical polarization degree to nearly zero. Significant X-ray polarization was observed during the third IXPE pointing, with a polarization degree Π _X = (18.4 ± 5.8)% and Ψ _X = 74° ± 9° in the 2–8 keV band, while only upper limits were obtained in the first two epochs. The optical data show that the second IXPE observation occurred shortly after a dramatic optical polarization event characterized by a rapid EVPA swing and strong depolarization. Two possible scenarios may explain the broadband polarimetric behavior: (i) the superposition of two emitting regions with nearly orthogonal magnetic field configurations and variable relative contributions, and (ii) the interaction of a single emitting region with a shock that temporarily reorders the magnetic field. In both cases, the data support a picture in which the X-ray and optical emissions arise from closely related but not strictly cospatial regions within a dynamically evolving, magnetically structured jet.

Multiwavelength polarimetry offers a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. Sources with synchrotron emission that spans a broad frequency range, from radio to X-rays, such as High Synchrotron Peak (HSP) type BL Lac objects, are particularly interesting. Previous measurements including radio, optical, and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical, and X-ray observations ( Swift , Nustar , and IXPE ) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232, we find a strong frequency dependence of the degree of polarization, with a ratio Π _X /Π _O ≃ 5.2. For RGB J0710+591, IXPE derives a 1 σ upper limit Π _X < 11.6%, comparable to the measured optical degree of polarization (average Π _O ∼ 12%). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that reproduces the observed data of both sources.

Aims. We present a new neutrino-blazar multiwavelength-flare coincidence, observed in the blazar PKS 0215+015 that showed a strong multiwavelength outburst in positional and temporal coincidence with the IceCube neutrino track alert IC220225A similar to the case of TXS 0506+056. We investigate the immediate response of the radio jet to the major flare and a possible neutrino association. Methods. We performed target-of-opportunity observations of PKS 0215+015 with the Very Long Baseline Array (VLBA) at 15 GHz, 23 GHz, and 43 GHz in full polarization for six epochs with a monthly cadence following the neutrino event. We combined the VLBA observations with monitoring data from the Effelsberg 100-m telescope, the Australia Telescope Compact Array, and Fermi /LAT. Results. Based on our kinematic analysis, we identified a new rapid jet component with an apparent speed of ∼60−80 c, which was ejected around the arrival time of IC220225A. The fast component ejection is traced by a characteristic signature in polarization that suggests a shock–shock interaction with a quasi-stationary feature. By combining the VLBA results with radio variability data, we estimated a bulk Lorentz factor of Γ = 105 ± 56, and a jet viewing angle of ϑ = (1.47 ± 0.31)°. Conclusions. We note that the properties of the rapid component exceed previously reported maximum apparent jet speeds and Lorentz factors reported by continuous monitoring programs. This is likely only possible because we are observing an exceptional flaring event at high redshift ( z = 1.72), with a higher observing cadence than in typical monitoring programs. We suggest that neutrino production in PKS 0215+015 can happen through pγ -interactions with protons possibly accelerated within the fast moving feature. The target photon field could be external to the jet or explained by a multi-layered jet. The latter scenario would be in agreement with the presence of quasi-stationary features revealed in our analysis.

Context. Magnetically arrested accretion disks (MADs) in black-hole (BH) jet-launching simulations are very successful in modeling low-luminosity active galactic nuclei (AGNs) such as M87*. The Fishbone-Moncrief (FM) torus is well established for this purpose in numerical astrophysics. The extent of the magnetic vector potential inside the FM torus –which we dub the filling factor– has not been studied before in the case of MAD simulations. Aims. In detail, we stress the impact on the jet morphology, efficiency, and linear polarization imprints of the filling factor as a significant initial parameter of these simulations. Methods. We employed five 3D, general-relativistic magnetohydrodynamic (GRMHD) simulations initialized with large-scale tori, which are immersed in weak, poloidal magnetic fields. To study the impact of the spatial extent of the initial magnetic field, and hence the magnetic energy content in the torus, we scaled it with the filling factor with regard to the poloidal geometric area of the mass-density distribution. A common choice of the filling factor is complimented and investigated in terms of altered energetics and angular-momentum transport. Further, we investigated the polarized, radiative imprints of synchrotron emission on M87 at 86 GHz, comparing them with very long baseline interferometry (VLBI) observations. Therefore, we performed general-relativistic, polarized radiative transfer calculations on the GRMHD data, modeling thermal and nonthermal electron distributions. Results. Our simulations show that elevated filling factors significantly increase the electromagnetic (EM) energy contributions and outward angular-momentum transport in the jet due to the initially increased magnetic energy content in the torus. High magnetic fillings exhibit increased linear polarization fractions, agreeing with the observed ∼15% in M87*. We find the jet morphology more prone to disk vertical flux tubes generated by MAD events. We show that GRMHD simulations bracket the jet width measurements at the jet base in M87*. Conclusions. Increased magnetic filling of the FM torus produces jets that are noticeably brighter downstream compared to our reference models; hence, we also find high fillings well suited for extended GRMHD jet models of other low-luminosity AGNs.

We present the third data release of the LOFAR Two-metre Sky Survey (LoTSS-DR3). The survey images cover 88% of the northern sky and were created from 12 950 h of data (18.6 PB) accumulated over 10.5 years. Producing the images took 20 million core hours of processing through direction-independent and direction-dependent calibration pipelines that correct for instrumental effects as well as spatially and temporally varying ionospheric distortions. In our 120–168 MHz continuum mosaic images with an angular resolution of 6″ (9″ below declination 10°) we catalogue 13 667 877 sources, formed from 16 943 656 Gaussian components. The scatter in the astrometric precision approximately follows the expected noise-like behaviour but with an additional systematic component of at least 0.24″ that is likely due to calibration imperfections. The random flux density scale error is 6%, while the systematic offset was previously shown to be within 2%. The median sensitivity of our mosaics is 92 μJy beam ⁻¹ , improving to 68 μJy beam ⁻¹ at high observing elevations, but degrading to 183 μJy beam ⁻¹ at the celestial equator due to station area projection effects. Completeness simulations, accounting for realistic source models, time- and bandwidth-smearing effects, and astrometric errors, indicate that we detect more than 95% of compact sources with integrated flux densities exceeding 9 times the local root mean square (RMS) noise. However, the recovered source counts in a particular integrated flux density bin do not match the injected counts until flux densities exceed 45 times the local RMS noise. The Euclidean-normalised differential source counts derived from the survey constrain the radio source population over five orders of magnitude and are in good agreement with previous deep and wide-area surveys. All data products are publicly available, including catalogues, individual-field Stokes I , Q , U , and V images, mosaicked Stokes I images, and uv data with associated direction-dependent calibration solutions.

The γ -ray–loud blazar TXS 2013+370, a powerful multiwavelength emitter at z = 0.859, underwent an exceptional gigaelectron volt outburst in late 2020 to early 2021. In this work, we present full polarization VLBI imaging at 22, 43, and 86 GHz (11 February 2021) together with contemporaneous single-dish monitoring (OVRO 15 GHz; SMA 226 GHz) and Fermi –LAT light curves to localize the high-energy dissipation site and probe the magnetic field of the inner jet. The images enabled us to study the jet structure and field topology on sub-parsec scales, revealing a compact near-core knot at r ≃ 40 − 60 μas along with the gigaelectron volt (GeV) flare and a flat core-dominated spectrum ( α ≳ −0.5). The core has strong linear polarization and exhibits a ∼50° electric vector polarization angle rotation at 86 GHz. The pixel-based and integrated fits we employed yielded a high, uniform rotation measure, RM = (7.8 ± 0.2)×10 ⁴ rad m ⁻² , consistent with an external Faraday screen. Performing a cross-correlation of Fermi –LAT and 15 GHz light curves revealed a highly significant peak, with the γ rays leading by Δ t = (102 ± 12) d. Adopting β _app = 4.2 ± 0.5 and θ = 4.1° ±0.2° implies a de-projected separation of Δ r _{γ  − 15} = (2.71 ± 0.47) pc and locates the GeV emission between the jet apex and ∼0.42 pc (in the 1 σ range) downstream. Our results do not pinpoint the emission site; rather, they support two valid scenarios. The γ -ray production occurs within the broad-line region (∼0.07 pc), where external-Compton scatters optical/UV photons to γ -rays, and beyond the broad-line region, reaching ∼0.42 pc (1 σ ) within the inner parsecs, where external-Compton scattering of dusty-torus infrared photons dominates. Both scenarios are compatible in the allowed range of emission distances, while opacity-driven core shifts modulate the observed radio– γ delay without requiring large relocations of the dissipation zone.

In 2023 November, the Fermi Large Area Telescope detected a γ-ray flare from the high-redshift blazar GB6 B1428+4217 (z = 4.715). We initiated a multiwavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100 m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observations, including possible ionized absorption features or signatures of bulk Comptonization of thermal electrons, which are also detected during the flaring episode. Simultaneous optical data revealed a polarization fraction of ∼8% in the R band, confirming that synchrotron emission dominated over thermal emission from the accretion disk. The hard X-ray flux was enhanced during the flare. Modeling of the broadband spectral energy distribution suggests that the high-energy component is dominated by Compton scattering by external seed photons from the accretion disk. The origin of the flare is consistent with the injection of a hard-spectrum electron population in the emission region. With a γ-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated spectral energy distribution and extreme luminosity are in line with expectations from the blazar sequence. High-redshift flares like this are critical for understanding jet physics in the early Universe and may improve detection prospects with future missions such as the Compton Spectrometer and Imager.

Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets.