Lena C. Debbrecht

Relativistic jets launched by active galactic nuclei are fundamental for understanding the physics of accreting supermassive black holes and their immediate environments, but the origin of these jets remains an open question. The relativistic jet of blazar NRAO 150 has complex morphology that evolves on short timescales because the projection effects are strong. This enabled us to perform a detailed kinematic analysis. We used data by the Very Long Baseline Array and the European Very Long Baseline Interferometry Network from 2010 until 2019 at 43 GHz to understand the formation and launching processes of the jet in NRAO 150. We studied the γ -ray and radio light curves together with total intensity and linear polarisation information to probe the connection between flaring events, γ -ray emission, and the ejection of new jet features. Furthermore, we investigated the magnetic field configuration in the innermost jet region as captured in polarised light to gain insights about its configuration before, during, and after a γ -ray flare. Our results indicate a close temporal link between the γ -ray flaring activity and the ejection of new very long baseline interferometry ( VLBI) jet components, suggesting that the high-energy emission is produced downstream of the VLBI core. The combined kinematic and polarimetric evidence further indicates a toroidal magnetic field in the inner jet. This highlights the key role of magnetic fields in governing the jet dynamics and high-energy emission in NRAO 150.

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.

Relativistic jets launched by active galactic nuclei are fundamental for understanding the physics of accreting supermassive black holes and their immediate environment, but the mechanisms that drive the jet launching remain uncertain. We investigate the sub-parsec jet of 3C 84 using multi-epoch multi-frequency very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array at 22 and 43 GHz. We analysed the evolution of the spectral index gradient in the core region to relate the observed structure to physical interpretations and to distinguish between competing jet-launching models. Furthermore, we examined the effect of the ambient medium and magnetic field configuration on the jet morphology and dynamics over time and explored their connection to a coinciding γ -ray flare. Our spectral analysis reveals significant changes across three epochs, indicating dynamic activity between filamentary structures on sub-parsec scales, evolving magnetic fields, and a complex interaction with the surrounding medium, all of which shape the innermost jet and might affect its high-energy emission.

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.

The location in which γ -ray are created and emitted within extra-galactic jets is a matter of active debate. One particularly well-suited source for determining the location is the nearby bright radio galaxy 3C 84, harbouring a powerful jet. We investigated the origin of γ -rays that were measured during a recent γ -ray flare by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is almost unpolarised overall, we find that close in time to the γ -ray flare peak regions at parsec-scale distances from the central engine, the linear polarisation increases fractionally. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the γ -ray flare, and combined with insights from concurrent X-ray polarisation measurements, a physically motivated scenario is that the γ -rays are created in this region, in a process consistent with the synchrotron self-Compton mechanism.

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.

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.

The source of extragalactic neutrinos in the TeV–PeV range is a matter of very active research, with blazar jets having been postulated to be the origin of at least some of the detections. The blazar PKS 0735+178 is a prominent example; during its multiband flare in late 2021, a neutrino event was reported by four observatories, with its origin consistent with the direction of that source. While no new jet component was observed to be ejected during that narrow time frame, our analysis shows that a propagating shock front originating from the core region was the likely source of the multiband flare, using very-long-baseline interferometry images of PKS 0735+178 in polarized light. Taken together, our findings are suggestive of a coherent scenario in which the shock may contribute to the acceleration of protons, with the target photons potentially originating either from the ambient medium surrounding the jet or from proton synchrotron radiation. The necessary conditions for neutrino emission via proton–photon interactions are, hence, present in this jet.

Context. Active galactic nuclei (AGN) are some of the most luminous and extreme environments in the Universe. The central engines of AGN are believed to be super-massive black-holes (SMBHs) are fed by accretion discs threaded by magnetic fields within a dense magneto-ionic medium. Aims. We report our findings from polarimetric very-long-baseline interferometry (VLBI) observations of quasar NRAO 150 taken in October 2022 using a combined network of the Very Long Baseline Array (VLBA) and Effelsberg 100-m Radio Telescope. These observations comprise the first co-temporal multi-frequency polarimetric VLBI observations of NRAO 150 at frequencies above 15 GHz. Methods. We used the new VLBI polarization calibration procedure, GPCAL , with polarization observations of frequencies of 12 GHz, 15 GHz, 24 GHz, and 43 GHz of NRAO 150. From these observations, we were able to measure the Faraday rotation and use it to derive the intrinsic electric vector position angle (EVPA ₀ ) for the source. As a complementary measurement, we determined the behavior of polarization as a function of observed frequency. Results. The polarization from NRAO 150 only comes from the core region, with a peak polarization intensity occurring at 24 GHz. Across the core region of NRAO 150, we see clear gradients in Faraday rotation and EVPA ₀ values that are aligned with the direction of the jet curving around the core region. We find that for the majority of the polarized region the polarization fraction is greater at higher frequencies, with intrinsic polarization fractions in the core ≈3%. Conclusions. The Faraday rotation gradients and circular patterns in EVPA ₀ offer strong evidence to support the presence of a helical+toroidal magnetic field. Furthermore, the presence of low intrinsic polarization fractions indicate that the polarized emission and, hence, the helical+toroidal magnetic field, is present within the innermost jet.

The spatial scales of relativistic radio jets, probed by relativistic magneto-hydrodynamic (RMHD) jet launching simulations and by most very long baseline interferometry (VLBI) observations differ by an order of magnitude. Bridging the gap between these RMHD simulations and VLBI observations requires selecting nearby active galactic nuclei (AGN), the parsec-scale region of which can be resolved. The radio source 3C 84 is a nearby bright AGN fulfilling the necessary requirements: it is launching a powerful, relativistic jet powered by a central supermassive black hole, while also being very bright. Using 22 GHz globe-spanning VLBI measurements of 3C 84 we studied its sub-parsec region in both total intensity and linear polarisation to explore the properties of this jet, with a linear resolution of ∼0.1 parsec. We tested different simulation set-ups by altering the bulk Lorentz factor Γ of the jet, as well as the magnetic field configuration (toroidal, poloidal, helical). We confirm the persistence of a limb brightened structure, which reaches deep into the sub-parsec region. The corresponding electric vector position angles (EVPAs) follow the bulk jet flow inside but tend to be orthogonal to it near the edges. Our state-of-the-art RMHD simulations show that this geometry is consistent with a spine-sheath model, associated with a mildly relativistic flow and a toroidal magnetic field configuration.