arXiv subjects

J. Heßdörfer

Publications and source records attributed to J. Heßdörfer.

7 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

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 $Γ= 105 \pm 56$ 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 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 is consistent with the presence of quasi-stationary features revealed in our analysis.

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 $γ$-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 \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 $γ$-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. 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°) 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 ($S(ν)\proptoν^α$) of $α=-0.11$. 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

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

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↗

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↗