arXiv subjects

Florian Rösch

Publications and source records attributed to Florian Rösch.

8 recordsLinked to original sources

arXiv2026

AGN Jets from Formation to Dissipation

Active Galactic Nuclei (AGN) are among the most energetic phenomena in the Universe, capable of launching powerful relativistic jets that extend from sub-parsec to megaparsec scales. These jets play a crucial role in regulating star formation, redistributing energy and matter, and shaping the evolution of galaxies and their environments. Despite decades of study, a comprehensive understanding of how AGN jets form, propagate, and dissipate remains elusive. The aim of this chapter is to highlight how the future capabilities of the the Square Kilometre Array (SKA), as a standalone array as well as in combination with Very Long Baseline Interferometry (VLBI) arrays and multi-wavelength facilities, will transform our capabilities to study the co-evolution of AGN jets and their host galaxies from jet formation to dissipation scales.

astro-ph.GA↗

arXiv2026

A Census of Variable and Transient Radio Sources Within High-Energy Neutrino Fields

The origin of high-energy cosmic neutrinos detected by the IceCube observatory is a hotly debated topic in astroparticle physics. Neutrinos can be produced via interactions of high-energy protons with photons. There are multiple candidate source classes which can accelerate cosmic particles to the energies required to emit high-energy cosmic neutrinos and which have in common that they lead to variable/transient radio emissions. However, so far only a few active galactic nuclei (AGNs) could be associated with high confidence with IceCube neutrinos. The bulk of the diffuse neutrino flux might be emitted from a rather faint and numerous source population. The sub-mJy low-frequency radio sky may harbor these neutrino emitters that have gone unnoticed in previous searches. SKA-Mid continuum observations of high-energy neutrino fields will yield the most complete census of coincident transient and variable radio sources that might be associated with the neutrino emission. In previous MeerKAT observations of the IceCube gold alert IC240929A at 815 MHz, we detect about 550 faint radio sources inside the 90% uncertainty region of this neutrino field with flux densities between $169\,\mathrm{μJy}$ and $140\,\mathrm{mJy}$. In its AA4 configuration, SKA-Mid will achieve about four times the sensitivity of MeerKAT, allowing for the detection of even fainter radio sources within such neutrino fields. By adding wide-field VLBI analysis of the fields under consideration, all neutrino-candidate radio sources can be tested for high brightness-temperature compact emission (indicative of an AGN classification) and milliarcsecond-scale resolved structures.

astro-ph.HE↗

arXiv2026

Tracking down the broadband polarimetric properties of PG 1553+113

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 multi-wavelength facilities. The source displayed pronounced variability across the electromagnetic spectrum, with X-ray flux changes by up to a factor of $\sim5$ and complex evolution of the optical polarization properties, including one of the largest (exceeding $150^{\circ}$) 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 $Π_{\rm X}\,=(\,18.4\,\pm\,5.8)\%$ and $Ψ_{\rm X}\,=\,74^{\circ} \pm 9^{\circ}$ 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 co-spatial regions within a dynamically evolving, magnetically structured jet.

astro-ph.HE↗

arXiv2026

The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

Multiwavelength polarimetry is a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. In this respect, particularly interesting are the sources whose synchrotron emission covers a broad range of frequencies, from radio to X-rays, such as the BL Lac objects of the HSP type. 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 found a strong frequency-dependency of the degree of polarization (with a ratio $Π_X/Π_O\simeq 5.2$). For RGB J0710+591, IXPE derived a 1$σ$ upper limit $Π_X<11.6\%$, comparable with the measured optical degree of polarization (average $Π_O\sim 12\%$). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that is able to reproduce the observed data of both sources.

astro-ph.HE↗

arXiv2024

PKS 1424-418: A persistent candidate source of the mm$-γ$-ray connection?

We present a long-term strong correlation between millimeter (mm) radio and $γ$-ray emission in the flat-spectrum radio quasar (FSRQ) PKS 1424-418. The mm$-γ$-ray connection in blazars is generally thought to originate from the relativistic jet close to the central engine. We confirm a unique long-lasting mm$-γ$-ray correlation of PKS 1424-418 by using detailed correlation analyses and statistical tests, and we find its physical meaning in the source. We employed ~8.5 yr of (sub)mm and $γ$-ray light curves observed by ALMA and Fermi-LAT, respectively. From linear and cross-correlation analyses between the light curves, we found a significant, strong mm$-γ$-ray correlation over the whole period. We did not find any notable time delay within the uncertainties for the mm$-γ$-ray correlation, which means zero lag. The mm wave spectral index values (S$_ν$ $\propto$ $ν_α$) between the band 3 and 7 flux densities indicate a time-variable opacity of the source at (sub)mm wavelengths. Interestingly, the mm wave spectral index becomes temporarily flatter (i.e., $α$ > $-$0.5) when the source flares in the $γ$-rays. We relate our results with the jet of PKS 1424-418, and we discuss the origin of the $γ$-rays and opacity of the inner (sub)parsec-scale jet regions.

astro-ph.HE↗

arXiv2024

A gamma-ray flare from TXS 1508+572: characterizing the jet of a $z=4.31$ blazar in the early Universe

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 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 February to August 2022, reaching a peak luminosity comparable to the most luminous flares ever detected with Fermi -LAT. We conducted a multiwavelength observing campaign involving XMM-Newton, Swift, 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 NEOWISE at optical and infrared wavelengths, respectively. We find that the source is particularly variable in the infrared band on daily time scales. 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.

astro-ph.HE↗

arXiv2023

TANAMI: Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry. III. First-epoch S band images

With the emergence of very high energy astronomy (VHE; E>100 GeV), new open questions were presented to astronomers studying the multi-wavelength emission from blazars. Answers to these open questions, such as the Doppler crisis, and finding the location of the high-energy activity have eluded us thus far. Recently, quasi-simultaneous multi-wavelength monitoring programs have shown considerable success in investigating blazar activity. After the launch of the Fermi Gamma-ray Space Telescope in 2008, such quasi-simultaneous observations across the electromagnetic spectrum became possible. In addition, with very long baseline interferometry (VLBI) observations we can resolve the central parsec region of active galactic nuclei (AGN) and compare morphological changes to the gamma-ray activity to study high-energy emitting blazars. To achieve our goals, we need sensitive, long-term VLBI monitoring of a complete sample of VHE detected AGN. We performed VLBI observations of TeV-detected AGN and high likelihood neutrino associations as of December of 2021 with the Long Baseline Array (LBA) and other southern hemisphere radio telescopes at 2.3 GHz. In this paper we present first light TANAMI S-band images, focusing on the TeV-detected sub-sample of the full TANAMI sample. Apart from these very high energy-detected sources, we also show images of the two flux density calibrators and two additional sources included in the observations. We study the redshift, 0.1-100 GeV photon flux and S-band core brightness temperature distributions of the TeV-detected objects, and find that flat spectrum radio quasars and low synchrotron peaked sources on average show higher brightness temperatures than high-synchrotron-peaked BL Lacs. Sources with bright GeV gamma-ray emission also show higher brightness temperature values than gamma-low sources.

astro-ph.HE↗

arXiv2016

VLBI studies of TANAMI radio galaxies

Radio galaxies are relatively faint at $γ$-ray energies, where they make up only 1-2% of all AGN detected by Fermi-LAT. However, they offer a unique perspective to study the intrinsic properties of AGN jets. For this reason, the combination of $γ$-ray and multi-wavelength data with high-resolution VLBI monitoring is a powerful tool to tackle the basic unanswered questions about AGN jets. Here we present preliminary results from a sample study of radio galaxies in the Southern hemisphere observed by the TANAMI VLBI monitoring program. We obtain high-resolution maps at 8.4 and 22.3 GHz, and study the jet kinematics using multi-epoch data. We present a preliminary kinematic analysis for the peculiar $γ$-ray AGN PKS 0521$-$36.

astro-ph.HE↗