arXiv subjects

F. Eppel

Publications and source records attributed to F. Eppel.

15 recordsLinked to original sources

arXiv2026

SKA-VLBI Probes of High-energy Emission Processes in Relativistic Jets

Relativistic jets in the nuclei of active galaxies are ubiquitous sources of high-energy emission. In particular, blazars represent the most luminous persistent X-ray and gamma-ray sources, whose defining characteristics are small jet inclination angles to the line of sight. Blazars can be detected in many cases up to TeV energies and the largest class of TeV emitting extragalactic AGN is represented by high-synchrotron peaked (HSP) BL Lac objects, which are generally comparably faint radio sources. Moreover, evidence has also been accumulated that high-energy cosmic neutrinos detected by IceCube can be associated with blazars. There is an increasing number of suggested blazar-neutrino associations, along with many cases of coincident flaring radio emission, but in a majority of cases, faint blazars on the level of millijanskies or below have to be considered. These high-energy photon and neutrino emission processes hold many unanswered questions including the unknown source of seed-photons for photo-pion production and the infamous Doppler crisis of TeV-emitting BL Lac objects. SKA-VLBI offers the opportunity to achieve superior sensitivity at milliarcsecond resolutions, provided by the combination of the phased SKA-Mid and global VLBI arrays. This opens the possibility to perform high-sensitivity and high-angular resolution imaging and polarimetric probes of faint blazars. The resulting high-fidelity spatially resolved parameterizations of structured jets in bright sources will yield key insights to constrain physical models of high-energy photon and particle emission in AGN jets.

astro-ph.HE↗

arXiv2026

A hidden ultra-relativistic spine in the jet of a neutrino-associated blazar

Supermassive black holes launch powerful jets of plasma that can accelerate particles to extreme energies, but the physical conditions required to produce high-energy neutrinos remain unknown. In the blazar TXS 0506+056, the first source individually linked to a high-energy neutrino, radio images had seemed to reveal a jet too slow to sustain the extreme conditions required for neutrino production. Here we resolve this tension using long-term radio monitoring, particularly Very Long Baseline Interferometry (VLBI) imaging. We uncover a disturbance in emission propagating with an apparent speed of 21+-1 times the speed of light, that is masked by slower, radio-bright features that dominated earlier analyses. We interpret this as the signature of a stratified jet: an ultra-relativistic spine with Lorentz factor Gamma>20 embedded within a slower outer sheath. As the disturbance travels along the spine, it progressively illuminates the sheath, producing the delayed radio flare and naturally accounting for the years-long offset between neutrino and radio emission. The same pattern recurs in a second neutrino-associated event, pointing to a repeatable multi-messenger engine. These findings challenge the standard interpretation of VLBI jet speeds and establish a concrete, testable framework connecting structured jets to the sources of the Universe's highest-energy neutrinos.

astro-ph.HE↗

arXiv2026

Parsec-scale polarimetry and kinematics of a spine-sheath jet in the neutrino-blazar TXS 0506+056

In 2017, the blazar TXS 0506+056 showed a remarkable gamma-ray outburst simultaneously with a high-energy neutrino detection by IceCube from the same sky region. The significance of this association was found to be on the order of 3 sigma thus providing a strong link between the neutrino emission and the blazar flare. The high-energy flare in TXS 0506+056 was followed by a delayed radio flare, peaking in $\sim2020$ in the aftermath of the neutrino event. We investigate the parsec-scale jet structure and dynamics of TXS 0506+056 using 15 GHz full-polarization VLBI observations obtained between 2009 and 2025 with the Very Long Baseline Array. Our kinematic analysis reveals moderate superluminal jet speeds of $\sim(1-2)$c and two quasi-stationary components. A new jet component with comparable speed was ejected contemporaneously with the 2017 IceCube neutrino event. The stacked polarization map is consistent with a stratified spine-sheath jet structure: an inner spine with EVPA aligned with the jet, surrounded by a sheath layer with perpendicular EVPA. Variability in total intensity and polarization further indicates interaction between these layers, particularly evident in characteristic linear polarization flares, associated with EVPA rotations of the quasi-stationary components. The multi-layered jet configuration is consistent with previous studies that were able to explain the neutrino emission in this TXS 0506+056 through a spine-sheath jet structure. We suggest that TXS 0506+056 represents an archetypal case and that similar polarization signatures and geometric light curve flares may be present in other neutrino-emitting sources, potentially offering a solution to the Doppler-crisis phenomenon observed in TeV-emitting blazars.

astro-ph.HE↗

arXiv2026

Rapid jet ejection from PKS 0215+015 coincident with a high-energy neutrino event

Aims. We present a new neutrino-blazar multiwavelength flare coincidence observed in the blazar PKS 0215+015, which showed a strong multiwavelength outburst in 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. Methods. We performed target-of-opportunity observations with the Very Long Baseline Array (VLBA) at 15, 23, and 43 GHz in full polarization for six epochs with monthly cadence following the neutrino event. We combine the VLBA observations with monitoring data from the Effelsberg 100-m telescope, the Australia Telescope Compact Array, and Fermi/LAT. Results. Based on our VLBI kinematic analysis, we identified a new rapid jet component with an apparent speed of ~60-80c, which was ejected around the arrival 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 VLBI results with radio variability data, we estimated a bulk Lorentz factor of $\Gamma = 105 \pm 56$ and a jet viewing angle of $\vartheta = (1.47 \pm 0.31)^\circ$. Conclusions. We note that the properties of the rapid component exceed previously reported maximum apparent jet speeds and Lorentz factors from continuous VLBI monitoring programs. This is likely only possible because we are observing an exceptional flaring event at high redshift (z=1.72) with higher observing cadence than in typical monitoring programs. We suggest that neutrino production in PKS 0215+015 can occur through p{\gamma}-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 is consistent with the presence of quasi-stationary features revealed in our analysis.

astro-ph.HE↗

arXiv2025

A broadband study of FRB20240114A with the Effelsberg 100-m radio telescope

We present Effelsberg 100-m telescope observations of the hyperactive repeating fast radio burst source FRB 20240110A, discovered by CHIME/FRB in January 2024. Using the Ultra BroadBand (UBB) receiver, spanning 1.3-6.0 GHz, we detected over 700 unique bursts across four observing epochs. A comprehensive analysis of their temporal and spectral properties reveals four distinct spectro-temporal morphologies, including simple, complex and frequency-drifting structures. No bursts were detected across the full UBB band, confirming the band-limited emission typical of repeating FRBs. We find modest frequency evolution in burst widths but constant fractional bandwidths, and strong variability in burst rates that may be influenced by scintillation. The waiting-time distributions indicate predominantly independent burst events, with occasional clustering suggesting a characteristic emission timescale of $\sim$10 ms. Additionally, this study presents a multi-frequency analysis of waiting-time distributions, offering new insights into the complex frequency drifts commonly observed in repeating FRBs. These broadband observations provide a detailed view of the frequency-dependent burst behavior of FRB 20240110A and offer insights into the variability and temporal structure of repeating FRB emission.

astro-ph.HE↗

arXiv2025

Constraints on the X-ray-to-radio fluence ratio of FRB 20240114A

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, $\eta_{x/r}$, 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 $1.4\times 10^{-17}$ erg cm$^{-2}$, while the $3\sigma$ upper limit of the 0.2$-$12 keV absorption-corrected X-ray burst fluence lies in the range of $3.4\times 10^{-11}$ erg cm$^{-2}$ to $1.7\times 10^{-10}$ 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 $\eta_{x/r}<1.2\times10^{7}$. A cutoff power law ($\Gamma=1.56$, 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 $\eta_{x/r}\sim2.5\times 10^{5}$ (1$-$250 keV). In this scenario, we find that $\eta_{x/r}<2.4\times 10^6$. 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.

astro-ph.HE↗

arXiv2024

Very-long-baseline interferometry study of the flaring blazar TXS 1508+572 in the early Universe

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 ($0.1<\mathrm{E}<100$ 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 $\gamma$-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 \textit{Fermi} Large Area Telescope. Our goal is to monitor the morphological changes, spectral index and opacity variations that could be associated with the preceding $\gamma$-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. 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.

astro-ph.HE↗

arXiv2024

TELAMON: Effelsberg monitoring of AGN jets with very-high-energy astroparticle emission -- I. Program description and sample characterization

Aims. We introduce the TELAMON program which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, specifically TeV blazars and candidate neutrino-associated AGN. Here, we present and characterize our main sample of TeV-detected blazars. Methods. We analyze the data sample from the first ~2.5 years of observations between August 2020 and February 2023 in the range from 14 GHz to 45 GHz. During this pilot phase, we have observed all 59 TeV-detected blazars in the Northern Hemisphere (i.e., Dec. >0{\deg}) known at the time of observation. We discuss the basic data reduction and calibration procedures used for all TELAMON data and introduce a sub-band averaging method used to calculate average light curves for the sources in our sample. Results. The TeV-selected sources in our sample exhibit a median flux density of 0.12 Jy at 20 mm, 0.20 Jy at 14 mm and 0.60 Jy at 7 mm. The spectrum for most of the sources is consistent with a flat radio spectrum and we find a median spectral index ($S(\nu)\propto\nu^\alpha$) of $\alpha=-0.11$. Our results on flux density and spectral index are consistent with previous studies of TeV-selected blazars. Compared to the GeV-selected F-GAMMA sample, TELAMON sources are significantly fainter in the radio band. This is consistent with the double-humped spectrum of blazars being shifted towards higher frequencies for TeV-emitters (in particular for high-synchrotron peaked BL Lac type objects), which results in a lower radio flux density. The spectral index distribution of our TeV-selected blazar sample is not significantly different from the GeV-selected F-GAMMA sample. Moreover, we present a strategy to track the light curve evolution of sources in our sample for future variability and correlation analysis.

astro-ph.HE↗

arXiv2023

A Collection of German Science Interests in the Next Generation Very Large Array

The Next Generation Very Large Array (ngVLA) is a planned radio interferometer providing unprecedented sensitivity at wavelengths between 21 cm and 3 mm. Its 263 antenna element array will be spatially distributed across North America to enable both superb low surface brightness recovery and sub-milliarcsecond angular resolution imaging. The project was developed by the international astronomy community under the lead of the National Radio Astronomy Observatory (NRAO), and is anticipated to be built between 2027 and 2037. Two workshops have been held in 2022 and 2023 with the goal to discuss and consolidate the scientific interests in the ngVLA within the German astronomical community. This community paper constitutes a collection of 48 science ideas which the German community aims to pursue with the ngVLA in the 2030s. This is not a complete list and the ideas are not developed at the level of a "Science Book", such that the present document is mainly meant provide a basis for further discussion within the community. As such, additional contributions are welcome, and will be considered for inclusion in future revisions.

astro-ph.IM↗

arXiv2023

The Impact of Southern-Hemisphere Radio Blazar Observations on Neutrino Astronomy

The origin of high-energy cosmic neutrinos detected by the IceCube observatory is a hotly debated topic in astroparticle physics. There is growing evidence that some of these neutrinos can be associated with active galactic nuclei (AGN) and especially with blazars. Several recent studies have revealed a statistical correlation between radio-bright AGN samples and IceCube neutrino event catalogs. In addition, a growing number of individual high-energy neutrinos have been found to coincide with individual radio-flaring blazars. These observational results strongly call for high-quality, high angular-resolution radio observations of such neutrino-associated blazars to study their parsec-scale jet structures. TANAMI is the only large and long-term VLBI monitoring program focused on the Southern sky. Within TANAMI, we put an emphasis on Southern IceCube neutrino candidate blazars at 2.3 GHz and 8.4 GHz. Here we present first results of the first high-quality, high angular-resolution VLBI observations of nine Southern-Hemisphere blazars that were associated to IceCube neutrino hotspots in the Southern sky. In the near future, the rapidly growing KM3NeT will complement IceCube by being sensitive to high-energy neutrinos mainly from the Southern Hemisphere. This will increase the importance of Southern-Hemisphere radio monitoring programs of neutrino-associated blazars, like TANAMI.

astro-ph.HE↗

arXiv2023

TELAMON: Effelsberg Monitoring of AGN Jets with Very-High-Energy Astroparticle Emissions -- Polarization properties

We present recent results of the TELAMON program, which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, namely TeV blazars and neutrino-associated AGN. Our sample includes all known Northern TeV-emitting blazars as well as blazars positionally coincident with IceCube neutrino alerts. Polarization can give additional insight into the source properties, as the polarized emission is often found to vary on different timescales and amplitudes than the total intensity emission. Here, we present an overview of the polarization properties of the TeV-emitting TELAMON sources at four frequencies in the 20 mm and 7 mm bands. While at 7 mm roughly $82\,\%$ of all observed sources are found to be significantly polarized, for 20 mm the percentage is $\sim58\,\%$. We find that most of the sources exhibit mean fractional polarizations of $<5\%$, matching the expectations of rather low polarization levels in these sources from previous studies at lower radio frequencies. Nevertheless, we demonstrate examples of how the polarized emission can provide additional information over the total intensity.

astro-ph.HE↗

arXiv2023

VLBI Probes of Jet Physics in Neutrino-Candidate Blazars

In recent years, evidence has accumulated that some high-energy cosmic neutrinos can be associated with blazars. The strongest evidence for an individual association was found in the case of the blazar TXS 0506+056 in 2017. In July 2019, another track-like neutrino event (IC190730A) was found spatially coincident with the well-known bright flat-spectrum radio quasar PKS 1502+106. PKS 1502+106 was not found to be in a particularly elevated gamma-ray state, but exhibited a remarkably bright radio outburst at the time of the neutrino detection, similar to TXS 0506+056. We have performed a multi-frequency VLBI study from 15 GHz up to 86 GHz on TXS 0506+056, PKS 1502+106 and one additional neutrino-candidate blazar (PKS 0215+015) to study the radio structure of neutrino candidate blazars in response to their neutrino association. We have obtained target of opportunity observations with the VLBA for all three sources within $\sim$1 month from their associated neutrino events and are performing multi-epoch studies of the jet kinematics at 15 GHz as part of the MOJAVE program. Here, we present first results on TXS 0506+056 at 86 GHz and one additional 43 GHz image obtained 27 days after IC170922A, closer in time to the neutrino event than previously published images. We also give an overview about our recent work on PKS 1502+106 and PKS 0215+015.

astro-ph.HE↗

arXiv2023

VLBI Scrutiny of a New Neutrino-Blazar Multiwavelength-Flare Coincidence

In the past years, evidence has started piling up that some high-energy cosmic neutrinos can be associated with blazars in flaring states. On February 26, 2022, a new blazar-neutrino coincidence has been reported: the track-like neutrino event IC220225A detected by IceCube is spatially coincident with the flat-spectrum radio quasar PKS 0215+015. Like previous associations, this source was found to be in a high optical and ${\gamma}$-ray state. Moreover, the source showed a bright radio outburst, which substantially increases the probability of a true physical association. We have performed six observations with the VLBA shortly after the neutrino event with a monthly cadence and are monitoring the source with the Effelsberg 100m-Telescope, and with the Australia Compact Telescope Array. Here, we present first results on the contemporary parsec-scale jet structure of PKS 0215+015 in total intensity and polarization to constrain possible physical processes leading to neutrino emission in blazars.

astro-ph.HE↗

arXiv2021

Multi-wavelength study of Mrk 421 during a TeV flare

The blazar Mrk 421 shows frequent, short flares in the TeV energy regime. Due to the fast nature of such episodes, we often fail to obtain sufficient simultaneous information about flux variations in several energy bands. To overcome this lack of multi-wavelength (MWL) coverage, especially for the pre- and post-flare periods, we have set up a monitoring program with the FACT telescope (TeV energies) and the Neil Gehrels Swift Observatory (X-rays). On 2019 June 9, Mrk 421 showed a TeV outburst reaching a flux level of more than two times the flux of the Crab Nebula at TeV energies. We acquired simultaneous data in the X-rays with additional observations by XMM-Newton and INTEGRAL. For the first time, we can study a TeV blazar in outburst taking advantage of highly sensitive X-ray data from XMM-Newton and INTEGRAL combined. Our dataset is complemented by pointed radio observations by Effelsberg at GHz frequencies. We present our first results, including the {\gamma}-ray and X-ray light curves, a timing analysis of the X-ray data obtained with XMM-Newton , as well as the radio spectra before, during and after the flare.

astro-ph.HE↗

arXiv2021

TELAMON: Effelsberg Monitoring of AGN Jets with Very-High-Energy Astroparticle Emissions

We introduce the TELAMON program, which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, namely TeV blazars and candidate neutrino-associated AGN. Thanks to its large dish aperture and sensitive instrumentation, the Effelsberg telescope can yield radio data superior over other programs in the low flux-density regime down to several 10 mJy. This is a particular strength in the case of TeV-emitting blazars, which are often comparatively faint radio sources of the high-synchrotron peaked type. We perform high-cadence high-frequency observations every 2-4 weeks at multiple frequencies up to 44 GHz. This setup is well suited to trace dynamical processes in the compact parsec-scale jets of blazars related to high-energy flares or neutrino detections. Our sample currently covers about 40 sources and puts its focus on AGN with very-high-energy astroparticle emission, i.e., TeV blazars and neutrino-associated AGN. Here, we introduce the TELAMON program characteristics and present first results obtained since fall 2020.

astro-ph.HE↗