arXiv subjects

J. A. Zensus

Publications and source records attributed to J. A. Zensus.

At least 19 recordsLinked to original sources

arXiv2026

Probing the $γ$-ray emission region and the connection to jet ejections in NRAO 150 with VLBI

Relativistic jets launched by active galactic nuclei are fundamental for understanding the physics of accreting supermassive black holes and their immediate environments, yet the origin of these jets remains an open question. NRAO 150 is a blazar with a complex relativistic jet morphology that evolves on short timescales due to strong projection effects, enabling detailed kinematic analysis. In this study, we utilise data by the Very Long Baseline Array and the European VLBI Network from 2010 until 2019 at 43 GHz, to understand the formation and launching processes of the jet in NRAO 150. We study the $γ$-ray and radio light-curves, together with total intensity and linear polarisation information to probe the connection between flaring events, $γ$-ray emission, and the ejection of new jet features. Furthermore, we investigate the magnetic field configuration in the innermost jet region, as captured in polarised light, to gain insights about its configuration before, during, and after a $γ$-ray flare. Our results indicate a close temporal link between the $γ$-ray flaring activity and the ejection of new VLBI jet components, suggesting that the high-energy emission is produced downstream of the VLBI core. The combined kinematic and polarimetric evidence further points to a toroidal magnetic field in the inner jet, highlighting the key role of magnetic fields in governing both jet dynamics and high-energy emission in NRAO 150.

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arXiv2026

Spectral index evolution of the limb-brightened jet in 3C 84

Relativistic jets launched by active galactic nuclei are fundamental for understanding the physics of accreting supermassive black holes and their immediate environment, yet the mechanisms driving jet launching remain uncertain. In this study, we investigate the sub-parsec jet of 3C 84 using multi-epoch, multi-frequency, very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array at 22 and 43 GHz. We analyse the evolution of the spectral index gradient in the core region to relate the observed structure to physical interpretations and to discriminate between competing jet launching models. Furthermore, we examine the impact of the ambient medium and magnetic field configuration on jet morphology and dynamics over time, and explore their connection to a coinciding $γ$-ray flare. Our spectral analysis reveals significant changes across three epochs, indicating dynamic activity between filamentary structures on sub-parsec scales, evolving magnetic fields, and a complex interaction with the surrounding medium, all of which shape the innermost jet and may influence its high-energy emission.

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arXiv2025

Compelling Evidence for a Harmonic in the Light Curve of the Supermassive Black Hole Binary Candidate PKS J1309+1154

We recently discovered a supermassive black hole binary (SMBHB) candidate, PKS J1309+1154, in the combined 46-yr University of Michigan Radio Astronomy Observatory (UMRAO) plus Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz. The light curve of PKS 1309+1154 exhibits a 17.9 year periodicity. We also reported a hint of a first harmonic with a 9 year periodicity in this object. Further analysis of the PKS J1309+1154 light curve provides compelling evidence that both the fundamental and the harmonic are real, confirming the existence of real periodicities in blazar light curves. This is the first case, to our knowledge, of watertight evidence for a fundamental and a harmonic periodicity in a blazar light curve. It makes PKS J1309+1154 a \textit{strong\/} supermassive black hole binary (SMBHB) candidate, and thus the third such candidate to be revealed through long-term radio monitoring, the other two being PKS J0805--0111 and PKS 2131--021, both discovered through the OVRO 40 m Telescope monitoring program. It is argued that hundreds of SMBHB candidates will be discovered by the Vera Rubin and Simons Observatories. Coherent searches for gravitational waves from a network of SMBHB candidates, starting immediately, are strongly motivated.

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arXiv2025

A Search for Supermassive Black Hole Binary Candidates in 46-Year Radio Light Curves of 83 Blazars

The combined University of Michigan Radio Astronomy Observatory (UMRAO) and Owens Valley Radio Observatory (OVRO) blazar monitoring programs at 14.5/15 GHz provide uninterrupted light curves of $\sim~46-50$ yr duration for 83 blazars, selected from amongst the brightest and most rapidly flaring blazars north of declination $-20^\circ$. In a search for supermassive black hole binary (SMBHB) candidates, we carried out tests for periodic variability using generalized Lomb-Scargle (GLS), weighted wavelet-Z (WWZ), and sine-wave fitting (SWF) analyses of this sample. We used simulations to test the effects of the power law spectrum of the power spectral density (PSD) on our findings, and show that the irregular sampling in the observed light curves has very little effect on the GLS spectra. Apparent periodicities and putative harmonics appear in all 83 of the GLS spectra of the blazars in our sample. We tested the reality of these apparent periodicities and harmonics with simulations, and found that in the overwhelming majority of cases they are due to the steep slope of the PSD, and should therefore be treated with great caution. We find one new SMBHB candidate: PKS 1309+1154, which exhibits a 17.9 year periodicity. The fraction of SMBHB candidates in our sample is $2.4_{-0.8}^{+3.2}\%$.

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arXiv2025

Shared-optical-path VLBI frequency phase transfer from 86 to 258 GHz on an 8600 km baseline: Demonstrated with the APEX and IRAM 30 m telescopes

The receiver N3AR operating at a frequency range between 67 and 116 GHz has been commissioned at the APEX telescope in October 2024. This adds a new low-frequency band for APEX, with the capability of simultaneous dual-frequency observations using a dichroic beamsplitter. The 3 mm receiver also allows APEX to join the existing 3 mm global VLBI network. One of our commissioning goals was to perform simultaneous dual-band VLBI observations at 86 and 258 GHz using receivers with shared-optical-paths (SOP) to correct the atmospheric phase fluctuations using the frequency phase transfer (FPT) technique. This was possible together with the IRAM 30 m telescope, which has already developed such a capability. We aimed to verify the expected phase coherence and sensitivity improvement at the higher frequency achievable by applying FPT. With the dual-band, single baseline data, we applied the FPT method, which uses the lower frequency data to correct the simultaneously observed higher-frequency data. We evaluate the improvement compared to the conventional single-band observing mode by analyzing the coherence factor in the higher frequency data. Our results show that the phase fluctuations at the two bands are well correlated. After applying FPT, the interferometric phases at the higher frequency vary much slower and the coherence factor is significantly improved. Our analysis confirms the feasibility of applying FPT to frequencies above 250 GHz with SOP receivers. Future observations in this mode could dramatically improve the sensitivity and imaging fidelity of high-frequency VLBI.

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arXiv2025

PKS 2131-021 -- Discovery of Strong Coherent Sinusoidal Variations from Radio to Optical Frequencies: Compelling Evidence for a Blazar Supermassive Black Hole Binary

Haystack and Owens Valley Radio Observatory (OVRO) observations recently revealed strong, intermittent, sinusoidal total flux-density variations that maintained coherence between 1975 and 2021 in the blazar PKS 2131$-$021 ($z=1.283$). This was interpreted as possible evidence of a supermassive black hole binary (SMBHB). Extended observations through 2023 show coherence over 47.9 years, with an observed period $P_\textrm{15 GHz}=(1739.8 \pm 17.4)$ days}. We reject, with $p$-value = $2.09 \times 10^{-7}$, the hypothesis that the variations are due to random fluctuations in the red noise tail of the power spectral density. There is clearly a physical phenomenon in PKS 2131$-$021 producing coherent sinusoidal flux density variations. We find the coherent sinusoidal intensity variations extend from below 2.7 GHz to optical frequencies, from which we derive an observed period $P_\textrm{optical}=(1764 \pm 36)$ days. Across this broad frequency range there is a smoothly-varying monotonic phase shift in the sinusoidal variations with frequency. Hints of periodic variations are also observed at $γ$-ray energies. The importance of well-vetted SMBHB candidates to searches for gravitational waves is pointed out. We estimate the fraction of blazars that are SMBHB candidates to be > 1 in 100. Thus monitoring programs covering tens of thousands of blazars could discover hundreds of SMBHB candidates.

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arXiv2025

SMILE: Discriminating milli-lens systems in a VLBI pilot project

Dark Matter (DM) remains poorly probed on critical, sub-galactic scales, where predictions from different models diverge in terms of abundance and density profiles of halos. Gravitational lens systems on milli-arcsecond scales (milli-lenses) are expected for a population of dense DM halos (free-floating or sub-halos) and free-floating supermassive black holes in the mass range of $10^6$ to $10^9\,M_\odot$. In this paper, we aim to look for milli-lens systems via a systematic search in a large sample of radio-loud AGN observed with very-long-baseline interferometry (VLBI). We present the observational strategy to discriminate milli-lenses from contaminant objects mimicking a milli-lens morphology. In a pilot project, we have investigated VLBI images from 13,828 sources from the Astrogeo VLBI image database and reduced the number of candidates to 40 in a first step. We present here the images and analysis of new sensitive follow-up observations with the EVN at 5 and 22 GHz and streamline our analysis to reject milli-lens candidates. By using constraints such as the surface brightness ratio, conservation of spectral shape, stability of flux ratios over time, and changes in morphology, we can confidently discriminate between milli-lenses and contaminant objects that mimick them. Using the above constraints, we rule out 31 out of our initial 40 candidates of milli-lens systems, demonstrating the power of our approach. Also, we found many new candidate compact symmetric objects, which are thought to be primarily short-lived jetted radio sources. This serves as a pathfinder for the final sample used for the Search for MIlli-LEnses (SMILE) project, which will allow us to constrain DM models by comparing the results to theoretical predictions. This SMILE sample will consist of $\sim$5,000 sources based on the VLA CLASS survey, including many observations obtained for this project specifically.

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arXiv2025

Investigating launching of black hole jets with the combined power of the EVN and the EHT

AGN-launched jets are a crucial element in the study of supermassive black holes (SMBH) and their closest surroundings. The formation of such jets, whether they are launched by magnetic field lines anchored to the accretion disc or directly connected to the black hole's (BH) ergosphere, is the subject of ongoing, extensive research. 3C84, the compact radio source in the central galaxy NGC1275 of the Perseus super-cluster, is a prime laboratory for testing such jet launching scenarios, as well as studying the innermost, sub-parsec AGN structure and jet origin. Very long baseline interferometry (VLBI) offers a unique view into the physical processes in action, in the immediate vicinity of BHs, unparalleled by other observational techniques. With VLBI at short wavelengths particular high angular resolutions are obtained. Utilising such cm and mm-VLBI observations of 3C84 with the European VLBI Network and the Event Horizon Telescope, we study the magnetic field strength and associated accretion flow around its central SMBH. This is possible, as higher frequency VLBI measurements are capable of peering through the accretion flow surrounding the central engine of 3C84, which is known to block the line of sight to the sub-parsec counter-jet via free-free absorption. Furthermore, we study the magnetic field's signature in the core region, as manifested in polarised light. As part of this analysis we compare our observations to relativistic magneto-hydrodynamic simulations. Finally, we investigate the effect of instabilities on the shape of the jet's parsec-scale funnel and try to connect them to its historical evolution.

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arXiv2024

Jet formation studies in AGN: a search for new targets

In recent years, the jet formation region in active galaxies has been imaged through mm-VLBI in few ideal targets, first and foremost M87. An important leap forward for understanding jet launching could be made by identifying a larger number of suitable objects, characterized by different accretion modes and jet powers. In this article, we present 1 cm and 7 mm VLBI data of a sample of 16 poorly explored radio galaxies, comprising both High-Excitation (HEG) and Low-Excitation Galaxies (LEG) and spanning a large range in radio power. The combination of the sources vicinity (z<0.1) with a large black hole mass ($\log{M_{\rm BH}}$>8.5) results in a high spatial resolution in units of Schwarzschild radii ($<10^3-10^4$ $R_{\rm S}$), necessary for probing the region where the jet is initially accelerated and collimated. We aim at identifying the best candidates for follow-up observations with current and future VLBI facilities. The observations were performed with the High Sensitivity Array, including Effelsberg and the phased-VLA, which has allowed us to characterize the sub-parsec properties of these faint jets and to estimate their core brightness temperature and orientation. The number of sources imaged on scales $\lesssim 10^3$ $R_{\rm S}$ is more than doubled by our study. All targets were detected at both frequencies, and several present two-sided jet structures. Several LEG jets show hints of limb-brightening. The core brightness temperatures are generally below the equipartition value, indicating that equipartition has not yet been reached and/or that the emission is de-boosted. Among LEG, we identify 3C31, 3C66B, and 3C465 as the most promising, combining a relatively high flux density (>50 mJy) with superb spatial resolution (<500 $R_{\rm S}$) at 7 mm. The powerful HEG 3C452 is interesting as well due to its highly symmetric, two-sided jet base.

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arXiv2024

PKS~J0805$-$0111: A Second Owens Valley Radio Observatory Blazar Showing Highly Significant Sinusoidal Radio Variability -- The Tip of the Iceberg

Owens Valley Radio Observatory (OVRO) observations of supermassive black hole binary (SMBHB) candidate PKS~2131$-$021 revealed, for the first time, six likely characteristics of the phenomenology exhibited by SMBHB in blazars, of which the most unexpected and critical is sinusoidal flux density variations. We have now identified a second blazar, PKS~J0805$-$0111, showing significant sinusoidal variations, with an observed period that translates to $1.422 \pm 0.005$ yr in the rest frame of the $z = 1.388$ object. We generate $10^6$ simulated light curves to reproduce the radio variability characteristics of PKS~J0805$-$0111, and show that the global probability, considering the \textit{look-elsewhere effect}, indicates that the observed periodicity can be attributed to the red noise tail of the power spectral density, with a $p_0$ value of $7.8 \times 10^{-5}$ (i.e. 3.78$σ$). PKS J0805$-$0111 displays all six characteristics observed in PKS 2131$-$021. Taking into account the well-defined OVRO sample size, the false positive probability $\sim 0.22$, but the rare behavior makes this a strong SMBHB candidate. The discovery of a second SMBHB candidate exhibiting these rare characteristics reveals that PKS~2131$-$021 is not a unique, isolated case. With these two strong cases we are clearly seeing only the tip of the iceberg. We estimate that the number of SMBHB candidates amongst blazars $\sim$ 1 in 100.

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arXiv2024

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.

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arXiv2024

Evidence for a toroidal magnetic field in the core of 3C 84

The spatial scales of relativistic radio jets, probed by relativistic magneto-hydrodynamic jet launching simulations (RMHDs) and by most very-long-baseline interferometry (VLBI) observations differ by an order of magnitude. Bridging the gap between these RMHD simulations and VLBI observations requires selecting nearby active galactic nuclei (AGN), the parsec-scale region of which can be resolved. 3C 84 is a nearby bright AGN fulfilling the necessary requirements: it is launching a powerful, relativistic jet powered by a central supermassive black hole, while also being very bright. Using 22 GHz global VLBI measurements of 3C 84 we aim to study its sub-parsec region in both total intensity and linear polarisation, to explore the properties of this jet, with a linear resolution of $\sim0.1$ parsec. We test different simulation setups by altering the bulk Lorentz factor $Γ$ of the jet, as well as the magnetic field configuration (toroidal, poloidal, helical). We confirm the persistence of a limb brightened structure, which reaches deep into the sub-parsec region. The corresponding electric vector position angles (EVPAs) follow the bulk jet flow inside but tend to be orthogonal to it near the edges. Our state-of-the-art RMHD simulations show that this geometry is consistent with a spine-sheath model, associated with a mildly relativistic flow and a toroidal magnetic field configuration.

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arXiv2024

Ordered magnetic fields around the 3C 84 central black hole

3C84 is a nearby radio source with a complex total intensity structure, showing linear polarisation and spectral patterns. A detailed investigation of the central engine region necessitates the use of VLBI above the hitherto available maximum frequency of 86GHz. Using ultrahigh resolution VLBI observations at the highest available frequency of 228GHz, we aim to directly detect compact structures and understand the physical conditions in the compact region of 3C84. We used EHT 228GHz observations and, given the limited (u,v)-coverage, applied geometric model fitting to the data. We also employed quasi-simultaneously observed, multi-frequency VLBI data for the source in order to carry out a comprehensive analysis of the core structure. We report the detection of a highly ordered, strong magnetic field around the central, SMBH of 3C84. The brightness temperature analysis suggests that the system is in equipartition. We determined a turnover frequency of $ν_m=(113\pm4)$GHz, a corresponding synchrotron self-absorbed magnetic field of $B_{SSA}=(2.9\pm1.6)$G, and an equipartition magnetic field of $B_{eq}=(5.2\pm0.6)$G. Three components are resolved with the highest fractional polarisation detected for this object ($m_\textrm{net}=(17.0\pm3.9)$%). The positions of the components are compatible with those seen in low-frequency VLBI observations since 2017-2018. We report a steeply negative slope of the spectrum at 228GHz. We used these findings to test models of jet formation, propagation, and Faraday rotation in 3C84. The findings of our investigation into different flow geometries and black hole spins support an advection-dominated accretion flow in a magnetically arrested state around a rapidly rotating supermassive black hole as a model of the jet-launching system in the core of 3C84. However, systematic uncertainties due to the limited (u,v)-coverage, however, cannot be ignored.

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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.

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arXiv2023

An extensive analysis of the sub-parsec region of 3C84

The study of jet launching in AGN is an important research method to better understand supermassive black holes (SMBHs) and their immediate surroundings. The main theoretical jet launching scenarios invoke either magnetic field lines anchored to the black hole's (BH) accretion disc (Blandford & Payne 1982) or a magnetic field, which is directly connected to its rotating ergosphere (Blandford & Znajek 1977). The nearby and bright radio galaxy 3C84 (NGC1275) is a very suitable target for testing different jet launching mechanisms, as well as for the study of the innermost, sub-parsec scale AGN structure and the jet origin. Very long baseline interferometry (VLBI) - specifically at millimetre wavelengths - offers an unparalleled view into the physical processes in action, in the close vicinity of SMBHs. Utilising such mm-VLBI observations of 3C84, we study the jet kinematics of the VLBI core region of 3C84 by employing all available, high sensitivity 3 mm-VLBI data sets of this source. As part of this analysis we associate the component ejection events with the variability light-curves at different radio frequencies and in the $γ$-rays. Furthermore, by cross-correlating these light-curves, we determine their time-lags and draw conclusions regarding the location of the high energy emission close to the jet base.

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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.

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arXiv2023

The RoboPol sample of optical polarimetric standards

Optical polarimeters are typically calibrated using measurements of stars with known and stable polarization parameters. However, there is a lack of such stars available across the sky. Many of the currently available standards are not suitable for medium and large telescopes due to their high brightness. Moreover, as we find, some of the used polarimetric standards are in fact variable or have polarization parameters that differ from their cataloged values. Our goal is to establish a sample of stable standards suitable for calibrating linear optical polarimeters with an accuracy down to $10^{-3}$ in fractional polarization. For five years, we have been running a monitoring campaign of a sample of standard candidates comprised of 107 stars distributed across the northern sky. We analyzed the variability of the linear polarization of these stars, taking into account the non-Gaussian nature of fractional polarization measurements. For a subsample of nine stars, we also performed multiband polarization measurements. We created a new catalog of 65 stars (see Table 2) that are stable, have small uncertainties of measured polarimetric parameters, and can be used as calibrators of polarimeters at medium- and large-size telescopes.

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arXiv2023

RadioAstron discovery of a mini-cocoon around the restarted parsec-scale jet in 3C 84

We present RadioAstron space-based very long baseline interferometry (VLBI) observations of the nearby radio galaxy 3C84 (NGC1275) at the centre of the Perseus cluster. The observations were carried out on September 21-22, 2013 and involved a global array of 24 ground radio telescopes observing at 5 GHz and 22 GHz, together with the Space Radio Telescope (SRT). Furthermore, the Very Long Baseline Array (VLBA) and the phased Very Large Array (VLA) observed the source quasi-simultaneously at 15 GHz and 43 GHz. Fringes between the ground array and the SRT were detected on baseline lengths up to 8.1 times the Earth's diameter, providing unprecedented resolution for 3C 84 at these wavelengths. We note that the corresponding fringe spacing is 125 microarcsec at 5 GHz and 27 microarcsec at 22 GHz. Our space-VLBI images reveal a previously unseen sub-structure inside the compact 1 pc long jet that was ejected about ten years earlier. In the 5 GHz image, we detected, for the first time, low-intensity emission from a cocoon-like structure around the restarted jet. Our results suggest that the increased power of the young jet is inflating a bubble of hot plasma as it carves its way through the ambient medium of the central region of the galaxy. Here, we estimate the minimum energy stored in the mini-cocoon, along with its pressure, volume, expansion speed, and the ratio of heavy particles to relativistic electrons, as well as the density of the ambient medium. About half of the energy delivered by the jet is dumped into the mini-cocoon and the quasi-spherical shape of the bubble suggests that this energy may be transferred to a significantly larger volume of the interstellar medium than what would be accomplished by the well-collimated jet on its own. The pressure of the hot mini-cocoon also provides a natural explanation for the almost cylindrical jet profile seen in the 22 GHz RadioAstron image.

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