arXiv subjects

D. C. Homan

Publications and source records attributed to D. C. Homan.

At least 19 recordsLinked to original sources

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.

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arXiv2026

Multi-messenger flare in the quasar PKS 0446+11

The physical mechanisms driving neutrino and electromagnetic flares in blazars remain poorly understood. We investigate a prominent multi-messenger flare in the quasar PKS 1424+240 to identify the processes responsible for its high-energy emission. We analyze the IceCube-240105A high-energy neutrino event together with contemporaneous observations in the gamma-ray, X-ray, optical, and radio bands. The on- and off-flare spectral energy distributions (SEDs) are modeled within a single-zone leptohadronic framework. Multi-epoch VLBA observations from the MOJAVE program provide parsec-scale polarization data that complement the multi-wavelength light curves. No significant time delay is detected between the neutrino arrival and the flares in different energy bands. This is consistent with an extremely small jet viewing angle below 1 deg, inferred from the parsec-scale polarization structure. The flare can be reproduced by the injection of a proton population and an increase of the Doppler factor from 18 to 24. We also detect an approximately 90 deg rotation of the EVPA in the parsec-scale core during the initial phase of the flare, indicating the emergence of a shock formed by the change in the bulk plasma speed. Our comprehensive multi-messenger analysis demonstrates that the extreme beaming and sub-degree viewing angle of this distant blazar can account for the observed neutrino and electromagnetic activity. These findings strengthen the case for blazars as efficient accelerators of hadrons and as significant contributors to the observed high-energy neutrino flux.

astro-ph.HE↗

arXiv2025

Studies of stationary features in jets: 3C 279 quasar I. On-sky scattering and dynamics

A recent study on the dynamics of the quasi-stationary component (QSC) in the jet of BL~ Lacertae highlighted its significance in evaluating the physical properties of relativistic transverse waves in the parsec-scale jet. Motivated by this finding, we selected a different type of blazar, the flat-spectrum radio quasar (FSRQ) 3C~279, which hosts a QSC at an angular median distance of 0.35~mas from the radio core, as revealed by 27 years of VLBA monitoring data at 15~GHz. We investigate the positional scatter and dynamics of a QSC in the 3C~279 jet, aiming to detect the presence of a relativistic transverse wave and estimate its characteristics. We employ an analytical statistical method to estimate the mean intrinsic speed of the QSC, while moving average and refinement methods are used to smooth its trajectory. Analysis of the QSC position scatter shows that the jet axis change its direction by about $21^{\circ}$ over 27 years and jet mean intrinsic full opening angle is $\approx 0.30^{\circ} \pm 0.03^{\circ}$. The apparent displacement vectors of the QSC exhibit strong asymmetry and anisotropy along the jet direction, indicating pronounced anisotropic displacements of the core along the jet axis. We estimated the mean intrinsic speed of the QSC to be superluminal, $\overline{β_{\rm s}} \approx 10$ in units of the speed of light, which, within the framework of the seagull-on-wave model, is interpreted as evidence for a relativistic transverse wave propagating through the QSC. Analysis of the reversing trajectory of the QSC enables the classification and characterisation of reversal patterns, which, in turn, allows the determination of key transverse wave parameters such as frequency, amplitude, inclination angle, and magnetic energy of the wave (abbrev.).

astro-ph.GA↗

arXiv2025

Looking into the Jet Cone of the Neutrino-Associated Very High Energy Blazar PKS 1424+240

The acceleration process of massive particles as well as the production of very high energy (VHE) photons and neutrinos remains a fundamental challenge in astrophysics. We investigate the parsec-scale jet structure and magnetic field of the blazar PKS 1424+240, that has been selected on the basis of strong VHE gamma-ray emission and identified with one of the highest peaks in the IceCube 9-year neutrino sky. We analyze 15 GHz VLBA observations of this BL Lac object, stacking 42 polarization-sensitive images collected in 2009-2025 to enhance the signal and reveal persistent parsec-scale structure. Our observations uncover a rare scenario. The object is viewed inside the jet cone, very close to the axis of its relativistic jet, with a viewing angle of <0.6 deg. This effectively maximizes Doppler boosting to values ~30 and enhances both electromagnetic and neutrino emission in the observer's direction. Based on polarimetric observations, we unambiguously detect a net toroidal component of the jet's magnetic field, indicating a current carrying jet flowing almost directly towards our line of sight. Blazars with very small jet viewing angles offer a solution to the longstanding mismatch between Doppler factors inferred from low VLBI apparent jet speed and those derived from VHE observations -- the so-called "Doppler factor crisis". We show that relativistic beaming plays the critical role in the gamma-ray and neutrino emission of blazars, with direct implications for models of their multi-messenger emission.

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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.

astro-ph.HE↗

arXiv2025

MOJAVE -- XXII. Brightness temperature distributions and geometric profiles along parsec-scale AGN jets

Radial gradients of the brightness temperatures along the parsec-scale jets of Active Galactic Nuclei (AGN) can be used to infer the energy balance and to estimate the parameter range of physical conditions in these regions. In this paper, we present a detailed study of the brightness temperature gradients and geometry profiles of relativistic jets of 447 AGN based on 15 GHz Very Long Baseline Array observations performed between 1994 and 2019. We used models of the jet structure using two-dimensional Gaussian components and analysed variations in their brightness temperatures and sizes along the jets. The size of the jet components, R, increases with projected distance from the jet base, r, as $R\propto r^{1.02\pm0.03}$, i.e., typically following a conically expanding streamline and therefore indicating that the size of jet components is a good tracer of jet geometry. The brightness temperature gradients along the jets typically follow a power-law $T_\mathrm{b} \propto r^{-2.82\pm0.07}$. Half of the sample sources show non-monotonic R(r) or Tb(r) profiles and their distributions were characterised by a double power-law model. We found at least six scenarios to explain the enhancement of the brightness temperature by a presence of inhomogeneities (shocks, jet recollimation) or curvature effects (helical structures, helical magnetic field, non-radial motion, bent jets). Our results are consistent with the scenario that the jet features can be simplified as optically thin moving blobs. In the sources demonstrating transition from a conical to parabolic jet shape, the gradient of the Tb(R) changes at the position of the break consistent with the model of magneto-hydrodynamic acceleration.

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arXiv2024

Studies of stationary features in jets: BL Lacertae II. Trajectory reversals and superluminal speeds on sub-parsec scales

High-resolution VLBI observations revealed a quasi-stationary component (QSC) in the relativistic jets of many blazars, which represents a standing recollimation shock. The VLBA monitoring of the BL Lacertae jet at 15~GHz shows the QSC at a projected distance of about 0.26~mas from the radio core.} We study the trajectory and kinematics of the QSC in BL Lacertae on sub-parsec scales using 15~GHz VLBA data of 164 observations over 20 years from the MOJAVE program and 2~cm VLBA Survey. To reconstruct the QSC's intrinsic trajectory, we use moving average and trajectory refinement procedures to smooth out the effects of core displacement and account for QSC positioning errors. We identified 22 QSC reversal patterns with a frequency of $\sim 1.5$ per year. Most reversals have an acute angle $<90\degr$ and few have a loop-shaped or arc-shaped trajectory. Where observed, combinations of reversals show reversible and quasi-oscillatory motion. We propose a model in which a relativistic transverse wave passes through the QSC, generating a short-lived reverse motion, similar to the transverse motion of a seagull on a wave. According to the model, relativistic waves are generated upstream and the reverse motion of the QSC is governed by the amplitude, velocity and tilt of the wave as it passes through. The apparent superluminal speeds of the QSC ($\sim 2\,c$) are then due to the relativistic speed of the jet's transverse wave ($<0.3\,c$ in the host galaxy rest frame) combined with the relativistic motion towards the observer. The measured superluminal speeds of QSC indirectly indicate the presence of relativistic transverse waves, and the size of the QSC scattering on the sky is proportional to the maximum amplitude of the wave. (abbreviated)

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arXiv2023

Residual Entropy as a Diagnostic and Stopping Metric for CLEAN

We propose the use of entropy, measured from the spatial and flux distribution of pixels in the residual image, as a potential diagnostic and stopping metric for the CLEAN algorithm. Despite its broad success as the standard deconvolution approach in radio interferometry, finding the optimum stopping point for the iterative CLEAN algorithm is still a challenge. We show that the entropy of the residual image, measured during the final stages of CLEAN, can be computed without prior knowledge of the source structure or expected noise levels, and that finding the point of maximum entropy as a measure of randomness in the residual image serves as a robust stopping criterion. We also find that, when compared to the expected thermal noise in the image, the maximum entropy of the residuals is a useful diagnostic that can reveal the presence of data editing, calibration, or deconvolution issues that may limit the fidelity of the final CLEAN map.

astro-ph.IM↗

arXiv2023

MOJAVE XXI. Decade-long linear polarization variability in AGN jets at parsec scales

Using stacking of images obtained at different epochs, we studied the variability properties of linear polarization of active galactic nucleus (AGN) jets on parsec-scales. Our sample is drawn from the MOJAVE programme, and consists of 436 AGNs manifesting core-jet morphology and having at least five VLBA observing epochs at 15 GHz from January 1996 through August 2019, with some additional archival VLBA data reduced by us. We employed a stacking procedure and constructed maps of (i) standard deviation of fractional polarization and electric vector position angle (EVPA) over epochs as the measure of variability and (ii) median polarization degree to quantify typical values in time. The distributions of these values along and across the jet were analysed for the whole sample for the first time. We found that core EVPA variability is typically higher than that of the jet, presumably due to component blending and outflow bends in the core. The BL Lacertae object cores have lower EVPA variability, compared to that of quasars, possibly due to lower Faraday rotation measure, suggesting a stronger ordered magnetic field component. The EVPA becomes more stable down the jet. Most of the sources showing this trend have a time coverage of more than 12 years and at least 15 epochs. The possible cause could be the increase of stability in the magnetic field direction, reflecting an increase in the fraction of the magnetic field that is ordered. There are no significant optical-class-dependent or spectral-class-dependent relations in the EVPA variability properties in AGN jets.

astro-ph.HE↗

arXiv2023

MOJAVE. XX. Persistent Linear Polarization Structure in Parsec-scale AGN Jets

We analysed the parsec-scale linear polarization properties of 436 active galactic nuclei (AGN) based on 15 GHz polarimetric Very Long Baseline Array (VLBA) observations. We present polarization and total intensity images averaged over at least five epochs since 1996 January 19 through 2019 August 4. Stacking improves the image sensitivity down to $\sim$30 $μ$Jy/beam and effectively fills out the jet cross-section both in total intensity and linear polarization. It delineates the long-term persistent magnetic field configuration and its regularity by restoring spatial distributions of the electric vector position angle (EVPA) and fractional polarization, respectively. On average, about ten years of stacking period is needed to reveal the stable and most-complete polarization distribution of a source. We find that the degree of polarization significantly increases down and across the jet towards its edges, typically manifesting U or W-shaped transverse profiles, suggesting a presence of a large-scale helical magnetic field associated with the outflow. In some AGN jets, mainly BL Lacs, we detect quasi-constant fractional polarization profiles across the jet, accompanied by EVPAs that closely follow the outflow. BL Lacs show higher fractional polarization values in their cores and jets than those in quasars up to hectoparsec de-projected scales, while on larger scales, they become comparable. High-synchrotron-peaked BL Lac jets are found to be less polarized than intermediate and low-synchrotron-peaked BL Lacs. The spatial distribution of the EVPAs in BL Lacs tend to align with the local jet direction, while quasars show an excess of orthogonal polarization orientation.

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arXiv2021

MOJAVE XIX: Brightness Temperatures and Intrinsic Properties of Blazar Jets

We present multi-epoch, parsec-scale core brightness temperature observations of 447 AGN jets from the MOJAVE and 2cm Survey programs at 15 GHz from 1994 to 2019. The brightness temperature of each jet over time is characterized by its median value and variability. We find that the range of median brightness temperatures for AGN jets in our sample is much larger than the variations within individual jets, consistent with Doppler boosting being the primary difference between the brightness temperatures of jets in their median state. We combine the observed median brightness temperatures with apparent jet speed measurements to find the typical intrinsic Gaussian brightness temperature of (4.1 +- 0.6)*10^10 K, suggesting that jet cores are at or below equipartition between particle and magnetic field energy in their median state. We use this value to derive estimates for the Doppler factor for every source in our sample. For the 309 jets with both apparent speed and brightness temperature data, we estimate their Lorentz factors and viewing angles to the line of sight. Within the BL Lac optical class, we find that high-synchrotron-peaked (HSP) BL Lacs have smaller Doppler factors, lower Lorentz factors, and larger angles to the line of sight than intermediate and low-synchrotron-peaked (LSP) BL Lacs. We confirm that AGN jets with larger Doppler factors measured in their parsec-scale radio cores are more likely to be detected in gamma rays, and we find a strong correlation between gamma-ray luminosity and Doppler factor for the detected sources.

astro-ph.HE↗

arXiv2021

MOJAVE: XVIII. Kinematics and Inner Jet Evolution of Bright Radio-Loud Active Galaxies

We have analyzed the parsec-scale jet kinematics of 447 bright radio-loud AGN, based on 15 GHz VLBA data obtained between 1994 August 31 and 2019 August 4. We present new total intensity and linear polarization maps obtained between 2017 January 1 to 2019 August 4 for 143 of these AGN. We tracked 1923 bright features for five or more epochs in 419 jets. A majority (60%) of the well-sampled jet features show either accelerated or non-radial motion. In 47 jets there is at least one non-accelerating feature with an unusually slow apparent speed. Most of the jets show variations of 10 to 50 deg in their inner jet position angle (PA) over time, although the overall distribution has a continuous tail out to 200 deg. AGN with SEDs peaked at lower frequencies tend to have more variable PAs, with BL Lacs being less variable than quasars. The Fermi LAT gamma-ray associated AGN also tend to have more variable PAs than the non-LAT AGN in our sample. We attribute these trends to smaller viewing angles for the lower spectral peaked and LAT-associated jets. We identified 13 AGN where multiple features emerge over decade-long periods at systematically increasing or decreasing PAs. Since the ejected features do not fill the entire jet cross-section, this behavior is indicative of a precessing flow instability near the jet base. Although some jets show indications of oscillatory PA evolution, we claim no bona fide cases of periodicity since the fitted periods are comparable to the total VLBA time coverage.

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arXiv2020

TXS 0128+554: A Young Gamma-Ray Emitting AGN With Episodic Jet Activity

We have carried out a Chandra X-ray and multi-frequency radio VLBA study of the AGN TXS 0128+554, which is associated with the Fermi gamma-ray source 4FGL J0131.2+5547. The AGN is unresolved in a target 19.3 ks Chandra image, and its spectrum is well fit by a simple absorbed power law model, with no distinguishable spectral features. Its relatively soft X-ray spectrum compared to other CSOs may be indicative of a thermal emission component, for which we were able to obtain an upper temperature limit of kT = 0.08 keV. The compact radio morphology and measured advance speed of 0.32c +- 0.07c indicate a kinematic age of only 82 y +- 17 y, placing TXS 0128+554 among the youngest members of the compact symmetric object (CSO) class. The lack of compact, inverted spectrum hotspots and an emission gap between the bright inner jet and outer radio lobe structure indicate that the jets have undergone episodic activity, and were re-launched a decade ago. The predicted gamma-ray emission from the lobes, based on an inverse Compton-emitting cocoon model, is three orders of magnitude below the observed Fermi LAT flux. A comparison to other Fermi-detected and non-Fermi detected CSOs with redshift z<0.1 indicates that the gamma-ray emission likely originates in the inner jet/core region, and that nearby, recently launched AGN jets are primary candidates for detection by the Fermi LAT instrument.

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arXiv2019

MOJAVE. XVII. Jet Kinematics and Parent Population Properties of Relativistically Beamed Radio-Loud Blazars

We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual bright features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the >3sigma level. Although most features within a jet typically have speeds within ~40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc de-projected) of the core feature. Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution, and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope -1.4 +- 0.2. The parent jets of the brightest radio quasars have a space density of 261 +- 19 Gpc$^{-3}$ and unbeamed 15 GHz luminosities above ~$10^{24.5}$ W/Hz, consistent with FR II class radio galaxies.

astro-ph.GA↗

arXiv2017

MOJAVE: XV. VLBA 15 GHz Total Intensity and Polarization Maps of 437 Parsec-Scale AGN Jets From 1996-2017

We present 5321 milliarcsecond-resolution total intensity and linear polarization maps of 437 active galactic nuclei (AGNs) obtained with the VLBA at 15 GHz as part of the MOJAVE survey, and also from the NRAO data archive. The former is a long-term program to study the structure and evolution of powerful parsec-scale outflows associated with AGNs. The targeted AGNs are drawn from several flux-limited radio and gamma-ray samples, and all have correlated VLBA flux densities greater than about 50 mJy at 15 GHz. Approximately 80% of these AGNs are associated with gamma-ray sources detected by the Fermi LAT instrument. The vast majority were observed with the VLBA on 5 to 15 occasions between 1996 January 19 and 2016 December 26, at intervals ranging from a month to several years, with the most typical sampling interval being six months. A detailed analysis of the linear and circular polarization evolution of these AGN jets are presented in other papers in this series.

astro-ph.GA↗

arXiv2016

MOJAVE XIII. Parsec-Scale AGN Jet Kinematics Analysis Based on 19 years of VLBA Observations at 15 GHz

We present 1625 new 15 GHz (2 cm) VLBA images of 295 jets associated with active galactic nuclei (AGNs) from the MOJAVE and 2 cm VLBA surveys, spanning observations between 1994 Aug 31 and 2013 Aug 20. For 274 AGNs with at least 5 VLBA epochs, we have analyzed the kinematics of 961 individual bright features in their parsec-scale jets. A total of 122 of these jets have not been previously analyzed by the MOJAVE program. In the case of 451 jet features that had at least 10 epochs, we also examined their kinematics for possible accelerations. At least half of the well-sampled features have non-radial and/or accelerating trajectories, indicating that non-ballistic motion is common in AGN jets. Since it is impossible to extrapolate any accelerations that occurred before our monitoring period, we could only determine reliable ejection dates for about 24% of those features that had significant proper motions. The distribution of maximum apparent jet speeds in all 295 AGNs measured by our program to date is peaked below 5c, with very few jets with apparent speeds above 30c. The fastest speed in our survey is about 50c, measured in the jet of the quasar PKS 0805-07, and is indicative of a maximum jet Lorentz factor of about 50 in the parent population. The Fermi LAT-detected gamma-ray AGNs in our sample have, on average, higher jet speeds than non LAT-detected AGNs, indicating a strong correlation between pc-scale jet speed and gamma-ray Doppler boosting factor. We have identified 11 moderate-redshift (z<0.35) AGNs with fast apparent speeds (>10c) that are strong candidates for future TeV gamma-ray detection. Of the five gamma-ray loud narrow-lined Seyfert I AGNs in our sample, three show highly superluminal jet motions, while the others have sub-luminal speeds. (abridged)

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arXiv2015

Studies of the Jet in BL Lacertae. II. Superluminal Alfvén Waves

We study the kinematics of ridge lines on the pc-scale jet of the active galactic nucleus BL Lac. We show that the ridge lines display transverse patterns that move superluminally downstream, and that the moving patterns are analogous to waves on a whip. Their apparent speeds $β_\mathrm{app}$ (units of $c$) range from 3.9 to 13.5, corresponding to $β_\mathrm{wave}^\mathrm{gal}= 0.981 - 0.998$ in the galaxy frame. We show that the magnetic field in the jet is well-ordered with a strong transverse component, and assume that it is helical and that the transverse patterns are Alfvén waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic ($β_\mathrm{tr}^\mathrm{gal} \lesssim 0.09$). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behaviour. The Alfvén waves appear to be excited by changes in the position angle of the recollimation shock, in analogy to exciting a wave on a whip by shaking the handle. A simple model of the system with plasma sound speed $β_\mathrm{s}=0.3$ and apparent speed of a slow MHD wave $β_\mathrm{app,S}=4$ yields Lorentz factor of the beam $Γ_\mathrm{beam} \sim 4.5$, pitch angle of the helix (in the beam frame) $α\sim 67^\circ$, Alfvén speed $β_\mathrm{A}\sim 0.64$, and magnetosonic Mach number $M_\mathrm{ms}\sim 4.7$. This describes a plasma in which the magnetic field is dominant and in a rather tight helix, and Alfvén waves are responsible for the moving transverse patterns.

astro-ph.HE↗