Parsec-scale Jet Research Articles

Polarization of active galactic nuclei with significant VLBI-Gaia displacements

Context. Numerous studies have reported significant displacements in the coordinates of active galactic nuclei (AGNs) between measurements using the very-long-baseline-interferometry (VLBI) technique and those obtained by the Gaia space observatory. There is consensus that these discrepancies do indeed manifest astrometrically resolved sub-components of AGNs rather than random measurement noise. Among other evidence, it has been reported that AGNs with VLBI-to-Gaia displacements (VGDs) pointing downstream of their parsec-scale radio jets exhibit higher optical polarization compared to sources with the opposite (upstream) VGD orientation. Aims. We aim to verify the previously reported connection between optical polarization and a VGD-jet angle using a larger dataset of polarimetric measurements and updated Gaia DR3 positions. We also seek further evidence supporting the disk-jet dichotomy as an explanation of such a connection by using millimeter-wave polarization and multiband optical polarization measurements. Methods. We performed optical polarimetric observations of 152 AGNs using three telescopes. These data are complemented by other publicly available polarimetric measurements of AGNs. We cross-matched public astrometric data from VLBI and Gaia experiments, obtained corresponding positional displacements, and combined this catalog with the polarimetric and jet direction data. Results. Active galactic nuclei with downstream VGDs are confirmed to have significantly higher optical fractional polarization than the upstream sample. At the same time, the millimeter-wavelength polarization of the two samples shows very similar distributions. Conclusions. Our results support the hypothesis that the VGDs pointing down the radio jet are likely caused by a component in the jet emitting highly polarized synchrotron radiation and dominating in the overall optical emission. The upstream-oriented VGDs are likely to be produced by the low-polarization emission of the central engine’s subcomponents, which dominate in the optical.

Discovery of Limb Brightening in the Parsec-scale Jet of NGC 315 through Global Very Long Baseline Interferometry Observations and Its Implications for Jet Models

We report the first observation of the nearby giant radio galaxy NGC 315 using a global very long baseline interferometry (VLBI) array consisting of 22 radio antennas located across five continents, including high-sensitivity stations, at 22 GHz. Utilizing the extensive u v-coverage provided by the array, coupled with the application of a recently developed superresolution imaging technique based on the regularized maximum-likelihood method, we were able to transversely resolve the NGC 315 jet at parsec scales for the first time. Previously known for its central ridge-brightened morphology at similar scales in former VLBI studies, the jet now clearly exhibits a limb-brightened structure. This finding suggests an inherent limb brightening that was not observable before due to limited angular resolution. Considering that the jet is viewed at an angle of ∼50°, the observed limb brightening is challenging to reconcile with the magnetohydrodynamic models and simulations, which predict that the Doppler-boosted jet edges should dominate over the nonboosted central layer. The conventional jet model that proposes a fast spine and a slow sheath with uniform transverse emissivity may pertain to our observations. However, in this model, the relativistic spine would need to travel at speeds of Γ ≳ 6.0–12.9 along the deprojected jet distance of (2.3–10.8) × 103 gravitational radii from the black hole. We propose an alternative scenario that suggests higher emissivity at the jet boundary layer, resulting from more efficient particle acceleration or mass loading onto the jet edges, and consider prospects for future observations with even higher angular resolution.

X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei: NGC 1068

Context. NGC 1068 is the most observed radio-quiet active galactic nucleus (AGN) in polarimetry, yet its high-energy polarization has never been probed before due to a lack of dedicated polarimeters. Aims. Using the first X-ray polarimeter sensitive enough to measure the polarization of AGNs, we want to probe the orientation and geometric arrangement of (sub)parsec-scale matter around the X-ray source. Methods. We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2–8 keV polarization of NGC 1068. We pointed IXPE at the target for a net exposure time of 1.15 Ms, in addition to using two Chandra snapshots of ∼10 ks each in order to account for the potential impact of several ultraluminous X-ray sources (ULXs) within IXPE’s field of view. Results. We measured a 2–8 keV polarization degree of 12.4% ± 3.6% and an electric vector polarization angle of 101° ± 8° at a 68% confidence level. If we exclude the spectral region containing bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises to 21.3% ± 6.7% in the 3.5–6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec-scale radio jet. Using a combined Chandra and IXPE analysis plus multiwavelength constraints, we estimated that the circumnuclear “torus” may sustain a half-opening angle of 50–55° (from the vertical axis of the system). Conclusions. Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of the polarization angle orientation with respect to the radio jet and the torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN: the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic objects is now feasible thanks to X-ray polarimetry.

Evolution of the Termination Region of the Parsec-scale Jet of 3C 84 Over the Past 20 yr

We present the kinematics of the parsec-scale jet in 3C 84 from 2003 November to 2022 June observed with the Very Long Baseline Array at 43 GHz. We find that the C3 component, a bright feature at the termination region of the jet component ejected from the core in 2003, has maintained a nearly constant apparent velocity of 0.259 ± 0.003c over the period covered by observations. We observe the emergence of four new subcomponents from C3, each exhibiting apparent speeds higher than that of C3. Notably, the last two subcomponents exhibit apparent superluminal motion, with the fastest component showing an apparent speed of 1.22 ± 0.14c. Our analysis suggests that a change in viewing angle alone cannot account for the fast apparent speeds of the new subcomponents, indicating that they are intrinsically faster than C3. We identify jet precession (or reorientation), a jet–cloud collision, and magnetic reconnection as possible physical mechanisms responsible for the ejection of the new subcomponents.

Non-thermal emission from microquasar jets: The case of GRS 1915+105

Microquasars (μQs) represent a prominent population of Galactic sources, from which broadband non-thermal emission was detected. At present, the potential of μQs as sources of gamma-ray emission is poorly understood and constrained. In most cases, even the power of μQ jets remains largely uncertain, and this hinders the study of gamma-ray generation in these sources. Using observational data obtained in the radio band, we derive an estimate of the jet power in one of the most famous Galactic μQ, in GRS 1915+105. Compared to other studies of this subject, we additionally consider constraints related to the energy dissipated at internal shocks in the jet, the conditions at which are obtained using properties of the radio blobs detected in the jets. We show that even if one subtracts the rest-energy of matter in the jet, the jet power in this source should be very high, 1039ergs−1, or even higher. The properties of the detected radio emission favor jets with a Gauss-strength magnetic field at parsec distances from the black hole. Such a strong magnetic field should suppress inverse Compton (IC) emission from relativistic electrons, at least from parsec-scale jet. On the other hand, this strong magnetic field makes μQ jets perfect proton accelerators. If present, protons should easily be accelerated in the ultra-high-energy regime. However, due to a diluted target, protons should not lose a notable fraction of their energy in the jet, injected into the interstellar medium at the jet termination. This should significantly enhance the density of cosmic rays in the vicinity of GRS 1915+105. This may manifest itself as an extended gamma-ray source or as several compact sources if the target gas has a highly inhomogeneous distribution. This analysis has direct implication for studying Galactic μQ with such gamma-ray observatories as LHAASO, CTA, and SWGA in the next decades.

Pressure Balance and Energy Budget of the Nuclear Superbubble of NGC 3079

Superbubbles in the nuclear region of galaxies could be produced by active galactic nuclei (AGN) or nuclear starburst via different driving forces. We report analysis of the multiwavelength data of the kiloparsec-scale nuclear superbubble in NGC 3079 in order to probe the mechanisms driving the expansion of the superbubble. Based on the Chandra X-ray observations, we derive the hot-gas thermal pressure inside the bubble, which is about 1 order of magnitude higher than that of the warm ionized gas traced by optical lines. We derive a [C ii]-based star formation rate (SFR) of SFR ∼ 1.3 M ⊙ yr−1 from the nuclear region using the SOFIA/FIFI-LS observation. This SFR infers a radiation pressure toward the bubble shells much lower than the thermal pressure of the gases. The Very Large Array radio image infers that magnetic pressure at the northeast cap above the superbubble is less than the thermal pressure of the hot gas enclosed in the bubble but has a clearly larger extension. The magnetic field may thus still help to reconcile the expansion of the bubble. The observed thermal energy of the hot gas enclosed in the bubble requires an energy injection rate of ≳1042 erg s−1 within the bubble’s dynamical age, which is probably larger than the power provided by the current nuclear starburst and the parsec-scale jet. If this is true, stronger past AGN activity may provide an alternative energy source to drive the observed bubble expansion.

Observational evidence to support a dense ambient medium shaping the jet in 3C 84

Highly collimated relativistic jets are a defining feature of certain active galactic nuclei (AGN), yet their formation mechanism remains elusive. Previous observations and theoretical models have proposed that the ambient medium surrounding the jets could exert pressure, playing a crucial role in shaping the jets. However, a direct observational confirmation of such a medium has been lacking. In this study, we present very long baseline interferometric (VLBI) observations of 3C 84 (NGC 1275), located at the center of the Perseus Cluster. Through monitoring observations with the Very Long Baseline Array (VLBA) at 43 GHz, a jet knot was detected to have been ejected from the sub-parsec scale core in the late 2010s. Intriguingly, this knot propagated in a direction significantly offset from the parsec-scale jet direction. To delve deeper into the matter, we employed follow-up VLBA 43 GHz observations, tracing the knot’s trajectory until the end of 2022. We discovered that the knot abruptly changed its trajectory in the early 2020s, realigning itself with the parsec-scale jet direction. Additionally, we present results from an observation of 3C 84 with the Global VLBI Alliance (GVA) at 22 GHz, conducted near the monitoring period. By jointly analyzing the GVA 22 GHz image with a VLBA 43 GHz image observed about one week apart, we generated a spectral index map, revealing an inverted spectrum region near the edge of the jet where the knot experienced deflection. These findings suggest the presence of a dense, cold ambient medium characterized by an electron density exceeding ∼105 cm−3, which guides the jet’s propagation on parsec scales and significantly contributes to the overall shaping of the jet.

Unveiling the Small-scale Jets in the Rapidly Growing Supermassive Black Hole IZw1

Accretion of black holes at near-Eddington or super-Eddington rates represents the most powerful episode driving black hole growth, potentially occurring across various types of objects. However, the physics governing accretion and jet–disk coupling in such states remains unclear, primarily due to the difficulty in detecting associated jets, which may emit extremely weakly or exhibit episodic behavior. Only a few near/super-Eddington systems have demonstrated radio activity, and it remains uncertain whether jets exist and what their properties are in super-Eddington active galactic nuclei (AGNs) and ultraluminous X-ray sources. This uncertainty stems mainly from the complex radio emission mix, which includes contributions from jets, star formation activity, photoionized gas, accretion disk wind, and coronal activity. In this work, we conducted high-resolution, very long baseline interferometry observations to investigate jets in the highly accreting narrow-line Seyfert I system I Zw 1. Our observations successfully revealed small-scale jets (with a linear size of ∼45 pc) at both 1.5 and 5 GHz, based on the high radio brightness temperature, radio morphology, and spectral index distribution. Additionally, the parsec-scale jet observed in I Zw 1 displays a knotted morphology reminiscent of other sources accreting at similar rates. In summary, the high accretion rates and jet properties observed in the AGN I Zw 1 may support the AGN/X-ray binary analogy in this extreme state.

Identification of the optical emission detected by Gaia with radio structures in parsec-scale active galactic nucleus jets

Aims. We investigated the location of the optical emission with respect to the parsec-scale radio structure and attempted to identify whether the optical centroid is coincident with the radio core or a radio component of the jet. Methods. We used optical Gaia DR3 and 8 GHz ICRF3 positions and a model fitting of 15 GHz imaging by the MOJAVE VLBA survey for 422 sources, mostly blazars. We searched for possible associations between optical centroids and radio components. We also compared the astrometric and photometric properties of each category, looked for trends between the various source parameters, and discussed possible biases. Results. Sources can be separated into four categories based on whether their optical centroid (i) falls onto the radio core (category C), (ii) lies close to the base of the jet (category B), (iii) coincides with a radio component downstream in the jet (category J), or (iv) is not found to coincide with a detected radio component (category O). Due to a number of random and systematic errors, the number of sources falling into each category remains approximate but close to 32%, 36%, 22%, and 10%, respectively. The family of quasars, mostly flat spectrum radio quasars (FSRQs), clearly dominates the C and B categories, with their percentage decreasing from categories C to O. Conversely, the family of BL Lacs is spread over the four classes and dominates the category O. Radio galaxies are mainly in classes O and J, and the radio-loud narrow-line Seyfert 1 all belong to class C. An individual analysis of O sources shows that, despite the absence of a direct association with a specific radio component, the optical Gaia centroid is globally related to the radio VLBI jet structure. Conclusions. Our study shows that the spatial distribution of the optical counterparts seen by Gaia below the 0.2-arcsec scale around the radio cores appears in the vast majority of cases to be related to the AGN core or to its jet, with most of them located downstream in the jet. Those associated to the core also exhibit a bluer color index, suggesting a possible contribution of the accretion disk to the optical emission, while those associated to a radio knot in the jet appear redder and tend on average to be found in more polarized radio sources. Most BL Lacs have their optical emission coincident with the jet base or a knot in the jet, while sources with an optical emission on or close to the radio core are mostly FSRQs. Radio components associated with the optical centroid at the jet base or along the jet are mainly stationary or quasi-stationary features, with low apparent MOJAVE velocities. There are indications that the apparent proper motion of the Gaia centroids may be higher than the speed of the associated radio components, but the significance of this trend requires further investigation.

A Large Jet Narrow-line Seyfert 1 Galaxy: Observations from Parsec to 100 kpc Scales

We present new 1.5–8.5 GHz Very Long Baseline Array (VLBA) observations and 0.32–1.26 GHz Giant Meterwave Radio Telescope (GMRT) observations of J0354−1340, which is the only known radio-quiet (RQ) or radio-intermediate (RI) narrow-line Seyfert 1 galaxy with a 100 kpc, two-sided radio jet. A parsec-scale, one-sided jet in the southeastern direction from the core emission is found in the VLBA observations, while the kiloparsec-scale jet observed with the Karl G. Jansky Very Large Array (VLA) and GMRT is in the south–north direction. Core spectra on parsec and kiloparsec scales are presented in combination with archival VLA Sky Survey observations at 3.0 GHz and VLA C-configuration observations at 5.5 GHz. The parsec-scale emission dominates the kiloparsec-scale emission above ∼5 GHz, and the spectrum is inverted due to synchrotron self-absorption. This indicates a compact synchrotron source with a size of ∼0.04 pc, which is associated with either the jet base or the corona. A subkiloparsec-scale jet, which is unresolved on scales of ∼3″, probably dominates the emission below ∼5 GHz. Future radio observations can explore the jet structure between the parsec and 100 kpc scales, the origin of their direction mismatch, and the parsec-scale jet proper motion. It remains to be explored how common such large-scale jets are in RQ or RI active galactic nuclei.

Lost in the curve: Investigating the disappearing knots in blazar 3C 454.3

One of the most well-known extragalactic sources in the sky, quasar 3C 454.3, shows a curved parsec-scale jet that has been exhaustively monitored with very-long-baseline interferometry (VLBI) over the recent years. In this work, we present a comprehensive analysis of four years of high-frequency VLBI observations at 43 GHz and 86 GHz, between 2013–2017, in total intensity and linear polarization. The images obtained from these observations enabled us to study the jet structure and the magnetic field topology of the source on spatial scales down to 4.6 parsec in projected distance. The kinematic analysis reveals the abrupt vanishing of at least four new superluminal jet features in a characteristic jet region (i.e., region C), which is located at an approximate distance of 0.6 milliarcsec from the VLBI core. Our results support a model in which the jet bends, directing the relativistic plasma flow almost perfectly toward our line of sight, co-spatially with the region where components appear to stop.

Acceleration and transport of relativistic electrons in the jets of the microquasar SS 433.

SS 433 is a microquasar, a stellar binary system that launches collimated relativistic jets. We observed SS 433 in gamma rays using the High Energy Stereoscopic System (H.E.S.S.) and found an energy-dependent shift in the apparent position of the gamma-ray emission from the parsec-scale jets. These observations trace the energetic electron population and indicate that inverse Compton scattering is the emission mechanism of the gamma rays. Our modeling of the energy-dependent gamma-ray morphology constrains the location of particle acceleration and requires an abrupt deceleration of the jet flow. We infer the presence of shocks on either side of the binary system, at distances of 25 to 30 parsecs, and that self-collimation of the precessing jets forms the shocks, which then efficiently accelerate electrons.

Magnetic field properties inside the jet of Mrk 421

Aims. We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes. Methods. We conducted a polarimetry campaign from radio to X-ray wavelengths of the high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry Explorer (IXPE) measurements from 2022 December 6–8. During the IXPE observation, we also monitored Mrk 421 using Swift-XRT and obtained a single observation with XMM-Newton to improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months. Results. We detected X-ray polarization of Mrk 421 with a degree of ΠX = 14 ± 1% and an electric-vector position angle ψX = 107 ± 3° in the 2–8 keV band. From the time variability analysis, we find a significant episodic variation in ψX. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May, ψX varied in the range 0° to 180°, while ΠX remained relatively constant within ∼10–15%. Furthermore, a swing in ψX in 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that ΠX was generally ∼2–3 times greater than Π at longer wavelengths, while ψ fluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation of ψ occurred in the opposite direction with respect to the rotation of ψX and over longer timescales at similar epochs. Conclusions. The polarization behavior observed across multiple wavelengths is consistent with previous IXPE findings for HSP blazars. This result favors the energy-stratified shock model developed to explain variable emission in relativistic jets. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation of ψX. The accompanying spectral variation during the ψX rotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction of ψ between the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode of ψX rotation, although it is unclear whether there is any connection between the two events.

Cold gas in the heart of Perseus A

We present new Karl G. Jansky Very Large Array (JVLA) and Very Long Baseline Array (VLBA) observations that traced the H I in the central region of 3C 84 (Perseus A). This radio source is hosted by the bright cluster galaxy NGC 1275 in the centre of the iconic Perseus cluster. With the JVLA, we detected very broad (FWHM ∼500 km s−1) H I absorption at arcsecond resolution (∼300 pc) that is centred at the systemic velocity of NGC 1275 against the bright radio continuum, suggesting that the detected gas is very close to the supermassive black hole (SMBH). However, we did not detect any absorption in the higher-resolution VLBA data against the parsec-scale radio core and jet. Based on a comparison of the properties of the H I absorption with those of the molecular circum-nuclear disc (CND) known to be present in NGC 1275, we argue that the H I seen in absorption arises from H I in this fast-rotating CND, and that neutral atomic hydrogen is present as close as ∼20 pc from the SMBH. The radio continuum providing the background for absorption arises from non-thermal synchrotron emission from the star formation activity in the CND, whose presence has been reported by earlier VLBA studies. We did not detect any signature that the H I gas is affected by an interaction with the radio jet. Thus, at this stage of the evolution of the source, the impact of the radio jet on the gas in NGC 1275 mainly creates cavities on much larger galaxy scales. This prevents the circum-galactic gas from cooling, and it does not produce gas outflows. Overall, the properties of the CND in Perseus A present a very similar case to that of Mrk 231, suggesting that, unlike often assumed, H I absorption can arise against the radio emission from star formation in a CND and is perhaps common in radio AGN. With the JVLA, we serendipitously detected a new, faint absorbing system that is redshifted by ∼2660 km s−1, in addition to the already known high-velocity absorption system that is redshifted 2850 km s−1 with respect to NGC 1275. We identify this new system as gas that is stripped from a foreground galaxy falling into the Perseus cluster. This new absorption remains undetected with the VLBA.

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 3C 84 (NGC 1275) at the centre of the Perseus cluster. The observations were carried out during a perigee passage of the Spektr-R spacecraft 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 μas at 5 GHz and 27 μas 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.

Probing Neutrino Production in Blazars by Millimeter VLBI

The advancement of neutrino observatories has sparked a surge in multi-messenger astronomy. Multiple neutrino associations among blazars are reported while neutrino production sites are located within their central (sub)parsecs. Yet, many questions remain on the nature of those processes. The next generation Event Horizon Telescope (ngEHT) is uniquely positioned for these studies, as its high frequency and resolution can probe both the accretion disk region and the parsec-scale jet. This opens up new opportunities for connecting the two regions and unraveling the proton acceleration and neutrino production in blazars. We outline observational strategies for ngEHT and highlight what it can contribute to the multi-messenger study of blazars.

Precession-induced Variability in AGN Jets and OJ 287

The combined study of the flaring of active galactic nuclei (AGNs) at radio wavelengths and parsec-scale jet kinematics with Very Long Baseline Interferometry has led to the view that (i) the observed flares are associated with ejections of synchrotron blobs from the core, and (ii) most of the flaring follows a one-to-one correlation with the ejection of the component. Recent results have added to the mounting evidence showing that the quasi-regular component injections into the relativistic jet may not be the only cause of the flux variability. We propose that AGN flux variability and changes in jet morphology can both be of deterministic nature, i.e., having a geometric/kinetic origin linked to the time-variable Doppler beaming of the jet emission as its direction changes due to precession (and nutation). The physics of the underlying jet leads to shocks, instabilities, or ejections of plasmoids. The appearance (morphology, flux, etc.) of the jet can, however, be strongly affected and modulated by precession. We demonstrate this modulating power of precession for OJ 287. For the first time, we show that the spectral state of the spectral energy distribution (SED) can be directly related to the jet’s precession phase. We model the SED evolution and reproduce the precession parameters. Further, we apply our precession model to 11 prominent AGNs. We show that for OJ 287 precession seems to dominate the long-term variability (≳1 yr) of the AGN flux, SED spectral state, and jet morphology, while stochastic processes affect the variability on short timescales (≲0.2 yr).

HOPS 361-C’s Jet Decelerating and Precessing through NGC 2071 IR

We present a two-epoch Hubble Space Telescope study of NGC 2071 IR highlighting HOPS 361-C, a protostar producing an arced 0.2 parsec-scale jet. The proper motions for the brightest knots decrease from 350 to 100 km s−1 with increasing distance from the source. The [Fe ii] and Paβ emission line intensity ratio gives a velocity jump through each knot of 40–50 km s−1. A new [O i] 63 μm spectrum, taken with the German REciever for Astronomy at Terahertz frequencies instrument aboard Stratospheric Observatory for Infrared Astronomy, shows a low line-of-sight velocity indicative of high jet inclination. Proper motions and jump velocities then estimate 3D flow speed for knots. Subsequently, we model knot positions and speeds with a precessing jet that decelerates. The measurements are matched with a precession period of 1000–3000 yr and half opening angle of 15°. The [Fe ii] 1.26-to-1.64 μm line intensity ratio determines visual extinction to each knot from 5 to 30 mag. Relative to ∼14 mag of extinction through the cloud from C18O emission maps, the jet is embedded at a 1/5–4/5 fractional cloud depth. Our model suggests the jet is dissipated over a 0.2 pc arc. This short distance may result from the jet sweeping through a wide angle, allowing the cloud time to fill cavities opened by the jet. Precessing jets contrast with nearly unidirectional protostellar jets that puncture host clouds and can propagate significantly farther.

Spectral analysis of a parsec-scale jet in M 87: Observational constraint on the magnetic field strengths in the jet

Context. Because of its proximity and the large size of its black hole, M 87 is one of the best targets for studying the launching mechanism of active galactic nucleus jets. Currently, magnetic fields are considered to be an essential factor in the launching and accelerating of the jet. However, current observational estimates of the magnetic field strength of the M 87 jet are limited to the innermost part of the jet (≲100 rs) or to HST-1 (∼105 rs). No attempt has yet been made to measure the magnetic field strength in between. Aims. We aim to infer the magnetic field strength of the M 87 jet out to a distance of several thousand rs by tracking the distance-dependent changes in the synchrotron spectrum of the jet from high-resolution very long baseline interferometry observations. Methods. In order to obtain high-quality spectral index maps, quasi-simultaneous observations at 22 and 43 GHz were conducted using the KVN and VERA Array (KaVA) and the Very Long Baseline Array (VLBA). We compared the spectral index distributions obtained from the observations with a model and placed limits on the magnetic field strengths as a function of distance. Results. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from α22 − 43 GHz ∼ −0.7 at ∼2 mas to α22 − 43 GHz ∼ −2.5 at ∼6 mas. In the KaVA observations, the spectral index remains unchanged until ∼10 mas, but this trend is unclear in the VLBA observations. A spectral index model in which nonthermal electron injections inside the jet decrease with distance can adequately reproduce the observed trend. This suggests the magnetic field strength of the jet at a distance of 2−10 mas (∼900 rs − ∼4500 rs in the deprojected distance) has a range of B = (0.3−1.0 G)(z/2mas)−0.73. Extrapolating to the Event Horizon Telescope scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to ∼4500 rs without significant dissipation.

Jets in FR0 radio galaxies

Context. The local radio-loud AGN population is dominated by compact sources named FR0s. These sources show features, for example the host type, the mass of the supermassive black hole (SMBH), and the multi-band nuclear characteristics, that are similar to those of FRI radio galaxies. However, in the radio band, while FR0 and FRI share the same nuclear properties, the kiloparsec-scale diffuse component dominant in FRI is missing in FR0s. Previous very-long-baseline interferometry (VLBI) observations of a small sample of FR0s show a complex structure, mostly symmetric (two-sided jets) with respect to the central core. Aims. With this project we would like to study the parsec-scale structure in FR0s in comparison with that of FRI sources. Jets in FRI are relativistic on the parsec scale and decrease their velocity becoming subrelativistic on the kiloparsec scale. We would like to test whether this result also applies to the jets in FR0s or, alternatively, whether they are subrelativistic on the parsec scale. This might be the reason why they are unable to grow, because of instabilities, related to a low jet bulk velocity. Methods. To this end we observed 18 FR0 galaxies with the VLBA at 1.5 and 5 GHz and/or with the EVN at 1.7 GHz and produced detailed images at milliarcsec resolution of their nuclear emission to study the jet and core structure. Results. All sources have been detected but one. Four sources are unresolved, even in these high-resolution images; jets have been detected in all other sources. We derived the distribution of the jet-to-counter-jet ratio of FR0s and found that it is significantly different from that of FRIs, suggesting different jet bulk speed velocities. Conclusions. Combining the present data with published data of FR0 with VLBI observations, we derive that the radio structure of FR0 galaxies shows strong evidence that parsec-scale jets in FR0 sources are mildly relativistic with a bulk velocity on the order of 0.5c or less. A jet structure with a thin inner relativistic spine surrounded by a low-velocity sheath could be in agreement with the SMBH and jet launch region properties.