arXiv subjects

S. Frey

Publications and source records attributed to S. Frey.

At least 19 recordsLinked to original sources

arXiv2026

Radio Properties of Narrow-Line and Broad-Line Seyfert 1 Galaxies

Narrow-line Seyfert 1 (NLS1) galaxies host active galactic nuclei (AGN) with narrow optical emission lines of the broad-line region. This is often explained with a relatively lower mass of the central supermassive black hole and super-Eddington accretion. We compared the radio properties of large samples of NLS1 and broad-line Seyfert 1 (BLS1) galaxies compiled from the Sloan Digital Sky Survey. We cross-matched the NLS1 and BLS1 samples with the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) sky survey at 1.4 GHz and the first and second epoch data of the Very Large Array Sky Survey (VLASS) at 3 GHz. We calculated the radio spectral indices, the 1.4-GHz radio power, and the radio loudness. We found lower 1.4-GHz radio detection rates for the NLS1 galaxies. The median radio loudness values, the fraction of radio-loud AGN, and the median 1.4-GHz radio power are also lower for the NLS1 sample. The median spectral indices imply a slightly steeper radio spectrum for the NLS1 sample than for the BLS1 sample. Comparison of the star formation rates estimated from the radio data and the infrared measurements of the Wide-field Infrared Survey Explorer satellite indicated that more than half of the FIRST- and VLASS-detected NLS1 and BLS1 galaxies contain radio-emitting AGN.

astro-ph.GA↗

arXiv2026

From compact jets to extended lobes: radio morphologies of distant quasars at z > 4

High-redshift quasars play an essential role in studying the growth and evolution of supermassive black holes and active galactic nuclei (AGN). Radio-loud quasars additionally enable us to investigate the interactions between the jets and their environment. We aimed to reveal the radio morphology of three radio quasars at redshifts z>4 that contain milliarcsecond (mas) scale compact radio features according to previous very long baseline interferometry (VLBI) observations, but show significant flux density at arcsecond scales, indicating the presence of extended radio structure that cannot be sampled by the highest-resolution observations. We analysed radio interferometric data obtained at various angular resolutions and multiple frequencies, including observations made by the international Low-Frequency Array and the enhanced Multi-Element Remotely-Linked Interferometer Network. We also re-imaged archival European VLBI Network observations of our targets. Two quasars (J0813+3508 and J1231+3816) exhibit complex radio structures with hotspots, extended to tens of kpc. They resemble Fanaroff--Riley II-type radio galaxies with extremely bent jet morphology. The third object (J1548+3335) shows a one-sided structure of $\lesssim 10$ kpc size. There was no sign of relativistic boosting at its previous mas-scale resolution radio observations. Its radio power and (inferred) linear size derived from the lower-resolution observations are similar to the known high-power compact steep-spectrum sources. We found that the previously detected mas-scale compact radio features are related to the centres of the AGN or in one case to one of the hotspots in an extended lobe.

astro-ph.GA↗

arXiv2026

SKA$-$VLBI view of AGN jets in the early Universe

Active Galactic Nuclei (AGN) are among the brightest sources in the Universe, and those that are also jetted are uniquely valuable at the earliest epochs, because their relativistic outflows can regulate the gas supply of their host galaxies, potentially affecting both early star formation and the rapid growth of supermassive black holes (SMBHs). Their compact, high-brightness-temperature radio cores provide the sharpest beacons for very long baseline interferometry (VLBI), enabling direct constraints on Doppler boosting, jet duty cycles, and jet$-$environment coupling at extreme redshifts. In this White Paper, we discuss how the SKA-VLBI will provide sub-$\mu$Jy sensitivity together with milliarcsecond (mas) angular resolution to image and characterise jetted AGN at $z>6$ across SKA-Mid and SKA-Low frequencies. These observations can directly test SMBHs ($>10^6$ M$_{\odot}$) formation/evolution models (including jet-assisted super-Eddington phases) and infer the geometry of the Universe, directly probing the cosmological framework at high precision. Synergies with current and next-generation multi-band facilities will also be crucial to fully understand their host galaxies and their environment, providing an unprecedented panchromatic knowledge of the first jetted AGN.

astro-ph.GA↗

arXiv2026

SDSSJ110546.07+145202.4: The first long-duration radio changing-look NLS1 galaxy

SDSSJ110546.07+145202.4 stands out as a unique radio changing-look Narrow-line Seyfert 1 (NLS1) galaxy that has brightened dramatically and shows an exceptionally long duration of its "on" phase. We present the first high-frequency radio observations, the first simultaneous radio spectral energy distributions (SEDs), the first optical--UV--X-ray SEDs, and the first X-ray monitoring and spectroscopy of this recently discovered event. Importantly for understanding the nature of the outburst, we show that the X-ray spectrum is soft with a photon index Gamma_X=2.5; line-of-sight absorption and extinction are low or absent; the radio SED is peaked at low frequencies ~2 GHz; and the radio outburst emission is very long-lived (t > 8 yr) and roughly constant. The softness of the X-ray spectrum, low supermassive black hole (SMBH) mass, and high Eddington ratio all corroborate the optical NLS1 classification. We discuss multiple outburst scenarios, including lensing, absorption, a binary SMBH merger, a long-duration giant-star tidal disruption, a newly ignited active galactic nucleus (AGN), and an accretion-rate change. While most of them can be either excluded or are deemed too rare and lack positive evidence so far, most or all types of these transients are expected to be detected in ongoing VLA and upcoming SKA surveys. SDSSJ110546.07+145202.4 itself is well explained by an accretion rate change that triggered the powerful radio jet emission. The low redshift and SMBH mass of this system offer a unique perspective of the physical processes of radio-jet ignition that are expected to operate in the early Universe around growing SMBHs.

astro-ph.HE↗

arXiv2025

High-resolution radio imaging of TGSSJ1530+1049, a radio galaxy in a dense environment at z=4

High-redshift radio galaxies can provide insights into the structure formation and galaxy evolution at earlier cosmological epochs. TGSSJ1530+1049 was selected as a candidate high-redshift radio galaxy. Subsequent observations with the James Webb Space Telescope (JWST) presented in a companion paper (Saxena et al., 2026) have shown that it is located at a redshift z=4.0. The JWST data furthermore showed that the radio source is part of one of the densest structures of galaxies and ionized gas known at these redshifts. The complex system qualitatively resembles a massive (cluster) galaxy forming early through a rapid succession of mergers. TGSSJ1530+1049 is an unresolved source down to ~0.6" scale in multiple radio surveys. To reveal its high-resolution radio structure and allow for a detailed comparison with JWST observations, we studied its morphology at various angular scales with different radio interferometric instruments. We observed TGSSJ1530+1049 at milliarcsecond (mas) scale angular resolution with the European VLBI Network (EVN), and at ~100-mas scale resolution with the enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN). We recovered a complex north--south oriented structure with steep-spectrum radio-emitting features, which are associated with lobes and hot spots of a jetted active galactic nucleus. The centre of the radio galaxy proved to be too faint at cm wavelengths to be unambiguously detected in our observations. Nevertheless, the linear size (~5.5 kpc) and the radio power place it among the so-called medium-sized symmetric objects, a smaller and/or confined version of larger radio galaxies. Comparison between the radio morphology and that of the ionized gas observed by JWST shows that the two are closely aligned. However, the optical emission line gas extends out to ~25 kpc, which is well beyond the detected radio structures. (Abridged)

astro-ph.GA↗

arXiv2025

Discovery of a giant radio outburst of the narrow-line Seyfert 1 galaxy SDSS J110546.07+145202.4

We have identified a high-amplitude radio outburst in the course of a large-sample study of the radio properties of narrow-line Seyfert 1 (NLS1) galaxies. We have analysed previous radio data and obtained new radio observations with the Effelsberg 100 m telescope, in order to measure the properties and understand the nature of the high-amplitude radio variability. We have also searched for signs of variability in the infrared and optical bands using archival data. We report the discovery of a rare high-amplitude radio outburst of a NLS1 galaxy, SDSS J110546.07+145202.4, with an amplitude of a factor of >20 at centimetre wavelengths within 18 yr, and remaining at high-state for at least 7.6 yr. Thus, the object transitioned to a radio-loud state with a radio-loudness parameter exceeding 150. The radio spectrum measured at gigahertz frequencies during the 2020s is flat. We did not find indications of a similar increase in brightness in optical surveys or in the infrared measurements of the Wide-field Infrared Survey Explorer. The variability characteristics are inconsistent with tidal disruption events, and hard to reconcile with blazar variability.

astro-ph.GA↗

arXiv2025

Characterization of Mega-Constellation Links for LEO Missions With Applications to EO and ISS Use Cases

Satellite missions demand ever greater connectivity, especially in the LEO regime. In this paper, we introduce the new mega-constellation services in space paradigm: we show that megaconstellations, deployed to offer innovative services to Earth's users, can provide excellent connectivity to LEO spacecraft as well. First, we characterise the communication link between space users and the actual OneWeb and Starlink constellations. A full set of results in terms of availability, access duration, Doppler, and path losses as a function of user orbital parameters, identifying optimal user orbits, is provided. The results achieved by a multi-system user able to communicate with both fleets are also presented. The potential improvements available if geostationary constellations are used to complement LEO megaconstellations in a multi-orbit system are discussed as well. Finally, we focus on two LEO use cases: the International Space Station and an Earth Observation Sun Synchronous satellite. All the results demonstrate the numerous advantages of the mega-constellation connectivity solution, which can transform LEO spacecraft into highly responsive nodes of a space-to-space network.

eess.SP↗

arXiv2025

Zooming into the neutrino-associated blazar candidate J1718+4239

The radio quasar NVSS~J171822+423948 (J1718+4239) was proposed as the counterpart of the IceCube neutrino event IC-201221A. To reveal the nature of the source, we conducted new very long baseline interferometry (VLBI) observations of this blazar candidate with the Very Long Baseline Array (VLBA). The observations were carried out in dual-band mode between $4$ and $7$~GHz. Archival radio data from the literature were also collected for comparison. Our analysis revealed highly variable, Doppler-boosted radio emission of the source, with compact structure at both kpc and pc scales, a slightly inverted spectrum, and a maximum jet inclination angle of $\theta\le5\degr$. These results confirm J1718+4239 as a blazar-type object.

astro-ph.HE↗

arXiv2025

Identifying compact symmetric objects with high-precision VLBI and Gaia astrometry

Compact symmetric objects (CSOs) trace the earliest phases of radio-galaxy growth; however, robust classification is difficult when radio cores are weak or invisible. We aim to develop and test a Gaia+VLBI approach that utilizes the high-precision optical astrometry of Gaia together with the high-resolution imaging of VLBI to reliably locate the central engine and classify CSOs. We analysed 40 literature CSO candidates by overlaying Gaia DR3 positions on VLBI maps and by examining spectral index distributions, whole-source variability, and hotspot kinematics over up to 25 years. A source is classified as a CSO when the Gaia centroid lies between two steep-spectrum lobes. Our method yields 20 confirmed CSOs. The confirmed CSOs show low integrated variability, slow hotspot advance speeds, and kinematic ages of 20-2000 yr. High-power CSOs tend to be larger and host faster hotspots, while many low-power systems remain sub-kiloparsec and environmentally confined. Gaia+VLBI registration is a powerful method for CSO classification, especially where radio cores are faint. The observed power-size-velocity-age relations support distinct multiple evolutionary tracks, with high-power CSOs plausibly growing into large radio galaxies, while low-power CSOs appear confined by their host galaxy environments. Taken together, our results indicate that CSO evolution is shaped not only by intrinsic jet power, but also by host-galaxy environment and the duty cycle of the central engine. High-sensitivity observations of low-power CSOs will be crucial to map the full diversity of formation channels and evolutionary pathways of radio galaxies.

astro-ph.GA↗

arXiv2025

Solid Identification of Extragalactic Gamma-Ray Source Using High-Resolution Radio Interferometric Observation

The dominant fraction of the extragalactic $\gamma$-ray sources are blazars, active galactic nuclei with jets inclined at a small angle to the line of sight. Apart from blazars, a few dozen narrow-line Seyfert 1 galaxies (NLS1) and a number of radio galaxies are associated with $\gamma$-ray sources. The identification of $\gamma$-ray sources requires multiwavelength follow-up observations since several candidates could reside within the relatively large $\gamma$-ray localisation area. The $\gamma$-ray source 4FGL 0959.6+4606 was originally associated with a radio galaxy. However, follow-up multiwavelength work suggested a nearby NLS1 as the more probable origin of the $\gamma$-ray emission. We performed high-resolution very long baseline interferometry (VLBI) observation at 5 GHz of both proposed counterparts of 4FGL 0959.6+4606. We clearly detected the NLS1 source SDSS J095909.51+460014.3 with relativistically boosted jet emission. On the other hand, we did not detect milliarcsecond-scale compact emission in the radio galaxy 2MASX J09591976+4603515. Our VLBI imaging results suggest that the NLS1 object is the origin of the $\gamma$-ray emission in 4FGL 0959.6+4606.

astro-ph.HE↗

arXiv2025

High-resolution radio observations of TeV candidate sources

Radio-loud active galactic nuclei (AGN) with their jets pointed close to our line of sight constitute the majority of extragalactic $\gamma$-ray sources and significantly contribute to the radiation observed in the even higher energy regime. The upcoming Cherenkov Telescope Array (CTA) is expected to detect fainter TeV objects, leading to an anticipated increase in the proportion of non-blazar extragalactic high-energy sources. Here we present the results of our dual-frequency (1.7 and 5~GHz) European VLBI Network (EVN) and enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) observations of two faint radio sources from the list of TeV candidate sources. They do not show signs of nuclear activity in their optical spectra, but they were hypothesized to contain faint AGN that is outshone by the host galaxy. We used the mas-scale resolution radio data to try to pinpoint the location of the compact radio emitting feature, determine its spectral index, radio power, brightness temperature and radio-X-ray luminosity ratio and thus identify the origin of the radio emission. Our results suggest that both optically passive-looking galaxies host faint compact radio-emitting AGN with steep spectra.

astro-ph.HE↗

arXiv2025

What is Inside the Double-Double Structure of the Radio Galaxy J0028+0035?

The radio source J0028+0035 is a recently discovered double--double radio galaxy at redshift z=0.398. Its relic outer lobes are separated by about 3' in the sky, corresponding to ~1 Mpc projected linear size. Inside this large-scale structure, the inner pair of collinear lobes span about 100 kpc. In the arcsec-resolution radio images of J0028+0035, there is a central radio feature that offers the intriguing possibility of being resolved into a pc-scale, third pair of innermost lobes. This would make this radio galaxy a rare triple-double source where traces of three distinct episodes of radio activity could be observed. To reveal the compact radio structure of the central component, we conducted observation with the European Very Long Baseline Interferometer Network and the enhanced Multi Element Remotely Linked Interferometer Network. Our 1.66 GHz image with high (~5 milliarcsec) resolution shows a compact central radio core with no indication of a third, innermost double feature. The observation performed in multi-phase-centre mode also revealed that the physically unrelated but in projection closely separated background source 5BZU J0028+0035 has a single weak, somewhat resolved radio feature, at odds with its blazar classification.

astro-ph.GA↗

arXiv2025

New look at old friends: EVN imaging of prominent radio-loud active galactic nuclei with extremely large radio-optical positional offsets

When comparing modern fundamental reference frames in the radio (International Celestial Reference Frame) and optical (Gaia), a couple of bright radio reference sources appear to have very large radio-optical offsets, from tens up to hundreds of milliarcseconds (mas). The amount of these positional misalignments exceeds the uncertainty of each individual technique by at least an order of magnitude. In most cases, complex and extended radio structure and its time variability, and thus the difficulty in pinpointing the true location of the central engine, is responsible for the large apparent offsets. Sometimes distant parts of the radio structure are not properly detected due to a lack of shorter interferometer baselines. For our 5-GHz very long baseline interferometry (VLBI) experiment using antennas of the European VLBI Network and the enhanced Multi Element Radio Linked Interferometer Network, we selected 10 bright radio-loud active galactic nuclei with extremely large radio-optical offsets. Sensitive imaging involving a wide range of projected baseline lengths, as well as phase-referencing to nearby sources shed light on the possible causes of positional inconsistencies. Here we show results for 3 selected sources from this project.

astro-ph.GA↗

arXiv2024

Deep silence: radio properties of little red dots

To investigate the radio properties of the recently found high-redshift population, we collected a sample of $919$ little red dots (LRDs) from the literature. By cross-matching their coordinates with the radio catalogues based on the first- and second-epoch observations of the Very Large Array Sky Survey (VLASS) and the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey, we found no radio counterparts coinciding with any of the LRDs. To uncover possible sub-mJy level weak radio emission, we performed mean and median image stacking analyses of empty-field 'Quick Look' VLASS and FIRST image cutouts centred on the LRD positions. We found no radio emission above $3\sigma$ noise levels ($\sim11$ and $\sim18~\mu$Jy~beam$^{-1}$ for the VLASS and FIRST maps, respectively) in either of the stacked images for the LRD sample, while the noise levels of the single-epoch images are comparable to those found earlier in the stacking of high-redshift radio-quiet active galactic nuclei (AGNs). The non-detection of radio emission in LRDs suggests these sources host weaker (or no) radio AGNs.

astro-ph.GA↗

arXiv2023

Milliarcsecond-scale radio structure of the most distant BL Lac object candidate at redshift 6.57

The existence of accreting supermassive black holes up to billions of solar masses at early cosmological epochs (in the context of this work, redshifts z>=6) requires very fast growth rates which is challenging to explain. The presence of a relativistic jet can be a direct indication of activity and accretion status in active galactic nuclei (AGN), constraining the radiative properties of these extreme objects. However, known jetted AGN beyond z~6 are still very rare. The radio-emitting AGN J2331+1129 has recently been claimed as a candidate BL Lac object at redshift z=6.57, based on its synchrotron-dominated emission spectrum and the lack of ultraviolet/optical emission lines. It is a promising candidate for the highest-redshift blazar known to date. The aim of the observations described here was to support or refute the blazar classification of this peculiar source. We performed high-resolution radio interferometric imaging observations of J2331+1129 using the Very Long Baseline Array at 1.6 and 4.9 GHz in 2022 Feb. The images revealed a compact but slightly resolved, flat-spectrum core feature at both frequencies, indicating that the total radio emission is produced by a compact jet and originates from within a central 10-pc scale region. While these are consistent with the radio properties of a BL Lac object, the inferred brightness temperatures are at least an order of magnitude lower than expected from a Doppler-boosted radio jet, leaving the high-redshift BL Lac identification still an open question.

astro-ph.GA↗

arXiv2023

Signatures of a spinning supermassive black hole binary on the mas-scale jet of the quasar S5 1928+738 based on 25 years of VLBI data

In a previous work, we have identified the spin of the dominant black hole of a binary from its jet properties. Analysing Very Long Baseline Array (VLBA) observations of the quasar S5~1928+738, taken at $15$-GHz during $43$ epochs between $1995.96$ and $2013.06$, we showed that the inclination angle variation of the inner ($<2$~mas) jet symmetry axis naturally decomposes into a periodic and a monotonic contribution. The former emerges due to the Keplerian orbital evolution, while the latter is interpreted as the signature of the spin-orbit precession of the jet emitting black hole. In this paper, we revisit the analysis of the quasar S5~1928+738 by including new $15$-GHz VLBA observations extending over $29$ additional epochs, between $2013.34$ and $2020.89$. The extended data set confirms our previous findings which are further supported by the flux density variation of the jet. By applying an enhanced jet precession model that can handle arbitrary spin orientations $\kappa$ with respect to the orbital angular momentum of a binary supermassive black hole system, we estimate the binary mass ratio as $\nu=0.21\pm0.04$ for $\kappa=0$ (i.e. when the spin direction is perpendicular to the orbital plane) and as $\nu=0.32\pm0.07$ for $\kappa=\pi/2$ (i.e. when the spin lies in the orbital plane). We estimate more precisely the spin precession velocity, halving its uncertainty from $(-0.05\pm0.02)^\circ\,\mathrm{yr}^{-1}$ to $(-0.04\pm0.01)^\circ\,\mathrm{yr}^{-1}$.

astro-ph.HE↗

arXiv2023

From binary to singular: the AGN PSO J334.2028+1.4075 under the high-resolution scope

PSO J334.2028+1.4075 (PSO J334) is a luminous quasar located at redshift z=2.06. The source gained attention when periodic flux density variations were discovered in its optical light curve. These variations were initially interpreted as the variability due to the orbital motion of a supermassive black hole binary (SMBHB) residing in a single circumbinary accretion disk. However, subsequent multiwavelength observations provided evidence against the binary hypothesis as no optical periodicity was found on extended time baselines. On the other hand, detailed radio analysis with the Karl G. Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA) revealed a lobe-dominated quasar at kpc scales, and possibly a precessing jet, which could retain PSO J334 as a binary SMBH candidate. We aim to study both the large- and small-scale radio structures in PSO J334 to provide additional evidence for or against the binary scenario. We observed the source at 1.7 GHz with the European Very Long Baseline Interferometry Network (EVN), and at 1.5 and 6.2 GHz with the VLA, at frequencies that complement the previous radio interferometric study. Our images reveal a single component at parsec scales slightly resolved in the southeast-northwest direction and a lobe-dominated quasar at kiloparsec scales with a complex structure. The source morphology and polarization in our VLA maps suggest that the jet is interacting with dense clumps of the ambient medium. While we also observe a misalignment between the inner jet and the outer lobes, we suggest that this is due to the restarted nature of the radio jet activity and the possible presence of a warped accretion disk rather than due to the perturbing effects of a companion SMBH. Our analysis suggests that PSO J334 is most likely a jetted AGN with a single SMBH, and there is no clear evidence of a binary SMBH system in its central engine.

astro-ph.GA↗

arXiv2023

The Detection of a Compact Radio Feature in a Seyfert Galaxy After an Accretion Rate Change

X-ray binaries are known to show state transitions related to accretion rate changes which are often accompanied with dramatic changes in the jet emission. However, it is not clear whether this characteristics of stellar-mass black hole systems can be scaled up to the accretion disk of active galactic nuclei. The Seyfert 1 galaxy, KUG 1141+371 has been showing a steadily increasing X-ray flux since 2007, and exhibited variability behaviour similar to the state transitions observed in X-ray binaries. It was hypothesised to undergo a rapid boost of mass accretion. If the X-ray binary analogy holds then the appearance of jet emission can also be expected in KUG 1141+371. While the source was not detected in the Faint Images of the Radio Sky at Twenty-centimeters in 1994, it appears in the VLA Sky Survey in 2019 and at 22 GHz in a VLA observation in 2018 at mJy flux density level. Our VLBI observations revealed a compact, flat-spectrum radio feature. Its high brightness temperature indicates the radio emission originates from an AGN.

astro-ph.GA↗