Florian Eppel

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.

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.

Context. Among the blazars whose emission has been detected up to very high-energy (VHE; 100 GeV<Eγ rays, intermediate synchrotron-peaked BL Lacs (IBLs) are quite rare. The IBL B2 1811+31 (z = 0.117) exhibited intense flaring activity in 2020. Detailed characterization of the source emission from radio to γ-ray energies was achieved with quasi-simultaneous observations, which led to the first-time detection of VHE γ-ray emission from the source with the MAGIC telescopes. Aims. In this work, we present a comprehensive multiwavelength (MWL) view of B2 1811+31, with a specific focus on the 2020 VHE flare, employing data from MAGIC, Fermi-LAT, Swift-XRT, Swift-UVOT, and several optical and radio ground-based telescopes. Methods. Long-term MWL data were employed to contextualize the high-state episode within the source emissions over 18 years. We investigated the variability, cross-correlations, and classification of the source emissions during low and high states. We propose an interpretative leptonic model for the observed radiative high state. Results. During the 2020 flaring state, the synchrotron peak frequency shifted to higher values and reached the limit of the IBL classification. Variability in timescales of a few hours in the high-energy (HE; 100 MeV<Eγ-ray band poses an upper limit of 6×10¹⁴ δ_D cm on the size of the emission region responsible for the γ-ray flare, with δ_D being the relativistic Doppler factor of the region. During the 2020 high state, the average spectrum became harder in the HE γ-ray band compared to the low states. A similar behavior has been observed in X-rays. Conversely, during different activity periods, we find harder-when-brighter trends in X-rays and a hint of softer-when-brighter trends at HE γ rays. A long-term HE γ-ray and optical correlation indicates that the same emission regions dominate the radiative output in both ranges, whereas the evolution at 15 GHz shows no correlation with the fluxes at higher frequencies. We test one-zone and two-zone synchrotron-self-Compton models for describing the broadband spectral energy distribution during the 2020 flaring state and investigate the self-consistency of the proposed scenario.

Among the blazars whose emission has been detected up to very-high-energy (VHE; 100 GeV < E < 100 TeV) gamma rays, intermediate synchrotron-peaked BL Lacs (IBLs) are quite rare. The IBL B2 1811+31 (z = 0.117) exhibited intense flaring activity in 2020. Detailed characterization of the source emissions from radio to gamma-ray energies was achieved with quasi-simultaneous observations, which led to the first-time detection of VHE gamma-ray emission from the source with the MAGIC telescopes. In this work, we present a comprehensive multi-wavelength view of B2 1811+31 employing data from MAGIC, Fermi-LAT, Swift-XRT, Swift-UVOT and from several optical and radio ground-based telescopes. We investigate the variability, cross-correlations and classification of the source emissions during low and high states. During the 2020 flaring state, the synchrotron peak frequency shifted to higher values and reached the limit of the IBL classification. Variability in timescales of few hours in the high-energy (HE; 100 MeV < E < 100 GeV) gamma-ray band poses an upper limit of 6 x 10^{14} delta_D cm to the size of the emission region responsible for the gamma-ray flare, delta_D being the relativistic Doppler factor of the region. During the 2020 high state, the average spectrum became harder in the X-ray and HE gamma-ray bands compared to the low states. Conversely, during different activity periods, we find harder-when-brighter trends in X rays and a hint of softer-when-brighter trends at HE gamma rays. Gamma-optical correlation indicates the same emission regions dominate the radiative output in both ranges, whereas the levolution at 15 GHz shows no correlation with the flux at higher frequencies. We test one-zone and two-zone synchrotron-self-Compton models for describing the broad-band spectral energy distribution during the 2020 flare and investigate the self-consistency of the proposed scenario.

We report on multiwavelength observations of FRB 20240114A, a nearby (z = 0.13), hyperactive, repeating fast radio burst that was discovered in January 2024. We performed simultaneous observations of the source with the Effelsberg 100-m radio telescope, the Thai National Radio Telescope, the Astropeiler Stockert, and the X-ray satellite XMM-Newton in May 2024. On May 23, 2024, we detected 459 bursts from the source using the Ultra-Broad-Band (UBB) receiver of the Effelsberg telescope, covering a frequency range from 1.3 GHz to 6 GHz. All bursts have simultaneous X-ray coverage, which allows us to put stringent constraints on the X-ray-to-radio fluence ratio, η_x/r, of FRB 20240114A. In this work, we focus on the three brightest radio bursts detected during the campaign. The brightest burst exhibits a radio fluence of 1.4 × 10⁻¹⁷ erg cm⁻², while the 3σ upper limit of the 0.2−12 keV absorption-corrected X-ray burst fluence lies in the range of 3.4 × 10⁻¹¹ erg cm⁻² to 1.7 × 10⁻¹⁰ erg cm⁻², depending on the spectral model. Assuming a 10 keV black-body spectrum, the X-ray-to-radio fluence ratio can be constrained to η_x/r < 1.2 × 10⁷. A cutoff power law (Γ = 1.56, cutoff at 84 keV) is also considered, physically motivated by the Galactic magnetar SGR 1935+2154, which has previously shown X-ray bursts associated with FRB-like radio bursts at a measured X-ray-to-radio fluence ratio of η_x/r ∼ 2.5 × 10⁵ (1−250 keV). In this scenario, we find that η_x/r < 2.4 × 10⁶. Our results are consistent with FRB 20240114A being powered by a mechanism similar to that of SGR 1935+2154. We show that future multiwavelength campaigns will be able to improve this limit if sufficiently bright radio bursts are observed with simultaneous X-ray coverage.

Blazars can be detected from very large distances due to their high luminosity. However, the detection of γ-ray emission of blazars beyond z = 3 has only been confirmed for a small number of sources. Such observations probe the growth of supermassive black holes close to the peak of star formation in the history of galaxy evolution. As a result from a continuous monitoring of a sample of 80 z > 3 blazars with the Fermi Large Area Telescope (Fermi-LAT), we present the first detection of a γ-ray flare from the z = 4.31 blazar TXS 1508+572. This source showed high γ-ray activity from 2022 February to August, reaching a peak luminosity comparable to the most luminous flares ever detected with Fermi-LAT. We conducted a multiwavelength observing campaign involving XMM-Newton, the Neil Gehrels Swift Observatory, the Effelsberg 100 m radio telescope, and the Very Long Baseline Array. In addition, we make use of the monitoring programs by the Zwicky Transient Facility and the Near-Earth Object Wide-field Infrared Survey Explorer at optical and infrared wavelengths, respectively. We find that the source is particularly variable in the infrared band on daily timescales. The spectral energy distribution collected during our campaign is well described by a one-zone leptonic model, with the γ-ray flare originating from an increase of external Compton emission as a result of a fresh injection of accelerated electrons.

Context. High-redshift blazars provide valuable input to studies of the evolution of active galactic nuclei (AGN) jets and provide constraints on cosmological models. Detections at high energies (0.1 < E < 100 GeV) of these distant sources are rare, but when they exhibit bright gamma-ray flares, we are able to study them. However, contemporaneous multi-wavelength observations of high-redshift objects (z > 4) during their different periods of activity have not been carried out so far. An excellent opportunity for such a study arose when the blazar TXS 1508+572 (z = 4.31) exhibited a γ-ray flare in 2022 February in the 0.1 − 300 GeV range with a flux 25 times brighter than the one reported in the in the fourth catalog of the Fermi Large Area Telescope. Aims. Our goal is to monitor the morphological changes, spectral index and opacity variations that could be associated with the preceding γ-ray flare in TXS 1508+572 to find the origin of the high-energy emission in this source. We also plan to compare the source characteristics in the radio band to the blazars in the local Universe (z < 0.1). In addition, we aim to collect quasi-simultaneous data to our multi-wavelength observations of the object, making TXS 1508+572 the first blazar in the early Universe (z > 4) with contemporaneous multi-frequency data available in its high state. Methods. In order to study the parsec-scale structure of the source, we performed three epochs of very-long-baseline interferometry (VLBI) follow-up observations with the Very Long Baseline Array (VLBA) supplemented with the Effelsberg 100-m Telescope at 15, 22, and 43 GHz, which corresponds to 80, 117, and 228 GHz in the rest frame of TXS 1508+572. In addition, one 86 GHz (456 GHz) measurement was performed by the VLBA and the Green Bank Telescope during the first epoch. Results. We present total intensity images from our multi-wavelength VLBI monitoring that reveal significant morphological changes in the parsec-scale structure of TXS 1508+572. The jet proper motion values range from 0.12 mas yr⁻¹ to 0.27 mas yr⁻¹, which corresponds to apparent superluminal motion β_app ≈ 14.3 − 32.2 c. This is consistent with the high Lorentz factors inferred from the spectral energy distribution (SED) modeling for this source. The core shift measurement reveals no significant impact by the high-energy flare on the distance of the 43-GHz radio core with respect to the central engine, that means this region is probably not affected by e.g., injection of new plasma as seen in other well-studied sources like CTA 102. We determine the average distance from the 43-GHz radio core to the central supermassive black hole to be 46.1 ± 2.3 μas, that corresponds to a projected distance of 0.32 ± 0.02 pc. We estimate the equipartition magnetic field strength 1 pc from the central engine to be on the order of 1.8 G, and the non-equipartition magnetic field strength at the same distance to be about 257 G, the former of which values agrees well with the magnetic field strength measured in low to intermediate redshift AGN. Conclusions. Based on our VLBI analysis, we propose that the γ-ray activity observed in February 2022 is caused by a shock-shock interaction between the jet of TXS 1508+572 and new plasma flowing through this component. Similar phenomena have been observed, for example, in CTA 102 in a shock-shock interaction between a stationary and newly emerging component. In this case, however, the core region was also affected by the flare as the core shift stays consistent throughout the observations.