Jet Precession Research Articles

2251+158 (3C 454.3): detection of an arc-like structure on parsec scales

Context. 2251+158 (3C 454.3) is a well-studied quasar with rather unusual properties. It is among the most variable and brightest gamma-ray emitting active galactic nucleus (AGN) in the sky observed by Fermi. The multi-wavelength flux density emission of this source is peculiar, and so is the very long baseline interferometry (VLBI) structure. While it is usually assumed that the jet and jet-components in a given AGN show more or less the same properties (e.g., apparent velocities and paths) with time, we demonstrate here that unusual morphologies (arc-like structure) can occur as temporal phenomena. We also show that the kinematic properties of jet components change from slow to fast apparent motion. Aims. We present the detection of an arc around the core region. To understand the physical nature of this and other peculiar kinematic properties of the parsec-scale jet of 2251+158 we performed detailed radio-interferometric studies of this prototypical AGN. Methods. We (re-)analyzed 32 Very Long Baseline Array (VLBA) observations covering 16 years in time (between 1995.57 and 2011.48), observed at 15 GHz from the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE/ 2c m) survey and derived the parameters of the observed VLBI structure. Three 43 GHz VLBA observations from 2001.22, 2005.38, and 2005.76 have been reprocessed. We fitted the components of the VLBI structure with Gaussian components. We studied the properties of light-curves obtained in the radio (4.8, 8.0, and 14.5 GHz) and optical regime (R-band), and studied the correlations between the flaring properties and the VLBI structure of the source. Results. We report the detection of an arc-like structure around the core of an AGN. This ring-structure becomes visible in VLBI maps around 1996. It expands with an apparent proper motion between 0.12 ± 0.01 mas yr −1 and 0.19 ± 0.01 mas yr −1 (corresponding to 5.4 ± 0.3c to 9.0 ± 0.4c) and dominates the parsec-scale structure for about 14 years. In addition, we report a significant change in the kinematic properties of jet components close to the core. A phase of slow apparent radial motion (directed away from the core) of two inner jet components (A and B) and no component ejection after a major radio-flare in 1995 is atypical for AGN. This phase is followed by a more typical behaviour in which the same components separate with higher apparent speeds from the core and two new components appear to be ejected from the central region. Conclusions. We observe significant changes in the morphological and kinematic properties of the parsec-scale jet. Taken together, it is tempting to see a causal connection between the different phenomena. We briefly suggest and discuss several possible physical scenarios to explain the observed phenomena. The kinematic changes as well as the changes in the flaring characteristics could be explained geometrically because of a change in the angle to the line of sight towards the observer assuming a helical structure of the jet. Another possible explanation for the observed phenomena and the correlations among them could be a precessing jet (precession period on timescale of 14.5 years) in combination with its interaction with an external inhomogeneous medium surrounding the core. These and other possible explanations will be investigated further and manuscripts presenting the results are in preparation.

FINE-SCALE STRUCTURE OF THE QUASAR 3C 279 MEASURED WITH 1.3 mm VERY LONG BASELINE INTERFEROMETRY

We report results from 5-day VLBI observations of the well-known quasar 3C 279 at 1.3 mm (230 GHz) in 2011. The measured nonzero closure phases on triangles including stations in Arizona, California and Hawaii indicate that the source structure is spatially resolved. We find an unusual inner jet direction at scales of $\sim$1 parsec extending along the northwest-southeast direction (PA = $127^{\circ}\pm3^{\circ}$), as opposed to other (previously) reported measurements on scales of a few parsecs showing inner jet direction extending to the southwest. The 1.3 mm structure corresponds closely with that observed in the central region of quasi-simultaneous super-resolution VLBA images at 7 mm. The closure phase changed significantly on the last day when compared with the rest of observations, indicating that the inner jet structure may be variable on daily timescales. The observed new direction of the inner jet shows inconsistency with the prediction of a class of jet precession models. Our observations indicate a brightness temperature of $\sim 8\times10^{10}$ K in the 1.3 mm core, much lower than that at centimeter wavelengths. Observations with better uv coverage and sensitivity in the coming years will allow the discrimination between different structure models and will provide direct images of the inner regions of the jet with 20--30 $\mu$as (5--7 light months) resolution.

Pulsations in short gamma ray bursts from black hole-neutron star mergers

The precise of short gamma ray bursts (SGRBs) remains an important open question in relativistic astrophysics. Increasingly, observational evidence suggests the merger of a binary compact object system as the source for most SGRBs, but it is currently unclear how to distinguish observationally between a binary neutron star progenitor and a black hole-neutron star progenitor. We suggest the quasiperiodic signal of jet precession as an observational signature of SGRBs originating in mixed binary systems, and quantify both the fraction of mixed binaries capable of producing SGRBs and the distributions of precession amplitudes and periods. The difficulty inherent in disrupting a neutron star outside the horizon of a stellar mass black hole biases the jet precession signal towards low amplitude and high frequency. Precession periods of $\ensuremath{\sim}0.01--0.1\text{ }\text{ }\mathrm{s}$ and disk-black hole spin misalignments $\ensuremath{\sim}10\ifmmode^\circ\else\textdegree\fi{}$ are generally expected, although sufficiently high viscosity may prevent the accumulation of multiple precession periods during the SGRB. The precessing jet will naturally cover a larger solid angle in the sky than would standard SGRB jets, enhancing observability for both prompt emission and optical afterglows.

DO JETS PRECESS… OR EVEN MOVE AT ALL?

Observations of accreting black holes often provoke suggestions that their jets precess. The precession is usually supposed to result from a combination of the Lense-Thirring effect and accretion disc viscosity. We show that this is unlikely for any type of black hole system, as the disc generally has too little angular momentum compared with a spinning hole to cause any significant movement of the jet direction across the sky on short timescales. Uncorrelated accretion events, as in the chaotic accretion picture of active galactic nuclei, change AGN jet directions only on timescales \gtrsim 10^7 yr. In this picture AGN jet directions are stable on shorter timescales, but uncorrelated with any structure of the host galaxy, as observed. We argue that observations of black-hole jets precessing on timescales short compared to the accretion time would be a strong indication that the accretion disc, and not the standard Blandford-Znajek mechanism, is responsible for driving the jet. This would be particularly convincing in a tidal disruption event. We suggest that additional disc physics is needed to explain any jet precession on timescales short compared with the accretion time. Possibilities include the radiation warping instability, or disc tearing.

THE STRUCTURE AND LINEAR POLARIZATION OF THE KILOPARSEC-SCALE JET OF THE QUASAR 3C 345

Deep Very Large Array imaging of the quasar 3C\,345 at 4.86 and 8.44 GHz has been used to study the structure and linear polarization of its radio jet on scales ranging from 2 to 30 kpc. There is a 7--8 Jy unresolved core with spectral index $\alpha \simeq -0.24$ ($I_\nu \propto \nu^{\alpha}$). The jet (typical intensity 15 mJy/beam) consists of a $2.5\arcsec$ straight section containing two knots, and two additional non-co-linear knots at the end. The jet's total projected length is about 27 kpc. The spectral index of the jet varies over $-1.1 \lesssim \alpha \lesssim -0.5$. The jet diverges with a semi-opening angle of about $9^\circ$, and is nearly constant in integrated brightness over its length. A faint feature north-east of the core does not appear to be a true counter-jet, but rather an extended lobe of this FR-II radio source seen in projection. The absence of a counter-jet is sufficient to place modest constraints on the speed of the jet on these scales, requiring $\beta \ga 0.5$. Despite the indication of jet precession in the total intensity structure, the polarization images suggest instead a jet re-directed at least twice by collisions with the external medium. Surprisingly, the electric vector position angles in the main body of the jet are neither longitudinal nor transverse, but make an angle of about $55^\circ$ with the jet axis in the middle while along the edges the vectors are transverse, suggesting a helical magnetic field. There is no significant Faraday rotation in the source, so that is not the cause of the twist. The fractional polarization in the jet averages 25% and is higher at the edges. In a companion paper it is shown that differential Doppler boosting in a diverging relativisitic velocity field can explain the electric vector pattern in the jet.

Three-dimensional vortex analysis and aeroacoustic source characterization of jet core breakdown

The three-dimensional behavior of jet core breakdown is investigated with experiments conducted on a free water jet at Re = 5000 by time-resolved tomographic particle image velocimetry (TR-TOMO PIV). The investigated domain encompasses the range between 0 and 10 jet diameters. The characteristic pulsatile motion of vortex ring shedding and pairing culminates with the growth of four primary in-plane and out-of-plane azimuthal waves and leads to the formation of streamwise vortices. Vortex ring humps are tilted and ejected along the axial direction as they are subjected to higher axial velocities. By the end of the potential core, this process causes the breakdown of the vortex ring regime and the onset of streamwise filaments oriented at 30°-45° to the jet axis and “C” shaped peripheral structures. The latter re-organize further downstream in filaments oriented along the azimuthal direction at the jet periphery. Instead, in the vicinity of the jet axis the filaments do not exhibit any preferential direction resembling the isotropic turbulent regime. Following Powell's aeroacoustic analogy, the instantaneous spatial distribution of the acoustic source term is mapped by the second time derivative of the Lamb vector, revealing the highest activity during vortex ring breakdown. A three-dimensional modal analysis of velocity, vorticity, Lamb vector, and Lamb vector second time derivative fields is conducted by proper orthogonal decomposition (POD) within the first 10 modes. The decomposed velocity fluctuations describe a helical organization in the region of the jet core-breakdown and, further downstream, jet axis flapping and precession motions. By the end of the potential core, vorticity modes show that vortex rings are dominated by travelling waves of radial and axial vorticity with a characteristic 40°-45° inclination to the jet axis. The Lamb vector and the Lamb vector second time derivative modes exhibit similar patterns for the azimuthal component, whereas the vortex ring coherence is described by the radial and the axial components. While velocity, vorticity, and Lamb vector modes are typically associated with Strouhal numbers (St) smaller than 0.9, the modes of the Lamb vector second time derivative are also related to higher frequencies (1.05 ≤ St ≤ 1.9) ascribed to the three-dimensional travelling waves. Far-field acoustic predictions are obtained on the basis of direct evaluation of Powell's analogy with TR-TOMO PIV data. The spectral analysis returns peaks at pairing (St = 0.36) and shedding (St = 0.72) frequency. A broader distribution with a hump between St = 1 and 2.25 is observed, which corresponds to the breakdown of ring vortices.

The jet of the Low Luminosity AGN of M81

In this contribution, we summarize our main results of a big campaign of global VLBI observations of the AGN in M81 (M81*) phase-referenced to the radio supernova SN 1993J. Thanks to the precise multi-epoch and multi-frequency astrometry, we have determined the normalized core-shift of the relativistic jet of M81* and estimated both the magnetic field and the particle density at the jet base. We have also found evidence of jet precession in M81* coming from the systematic time evolution of the jet orientation correlated with changes in the overall flux density.

GRAVITATIONAL RADIATIONS FROM THE PRECESSION CENTRAL ENGINE IN GAMMA-RAY BURSTS

The ultra-relativistic precessing jet in gamma-ray bursts (GRBs) may be responsible for the complex structure in GRBs' light curves. In this work, we study the gravitational radiations of jet precession induced by neutrino-dominated accretion disks around black holes. In our model, the jet and the inner part of the disk may precess along with the black hole, which is driven by the outer part of the disk. Gravitational radiations are therefore expected to be significant from this precession system. Based our numerical results, we find that it is possible for DECIGO and BBO to detect such gravitational radiations regardless of GRBs' black hole masses, particularly for GRBs in the Local Group.

Parsec-scale jet precession in BL Lacertae (2200+420)

BL Lacertae is the prototype of the BL Lac class of active galactic nuclei, exhibiting intensive activity on parsec (pc) scales, such as intense core variability and multiple ejections of jet components. In particular, in previous works the existence of precession motions in the pc-scale jet of BL Lacertae has been suggested. In this work we revisit this issue, investigating temporal changes of the observed right ascension and declination offsets of the jet knots in terms of our relativistic jet-precession model. The seven free parameters of our precession model were optimized via a heuristic cross-entropy method, comparing the projected precession helix with the positions of the jet components on the plane of the sky and imposing constraints on their maximum and minimum superluminal velocities. Our optimized best model is compatible with a jet having a bulk velocity of 0.9824c, which is precessing with a period of about 12.1 yr in the observer's reference frame and changing its orientation in relation to the line of sight between 4 and 5 degrees, approximately. Assuming that the jet precession has its origin in a supermassive binary black hole system, we show that the 2.3-yr periodic variation in the structural position angle of the very-long-baseline interferometry (VLBI) core of BL Lacertae reported by Stirling et al. is compatible with a nutation phenomenon if the secondary black hole has a mass higher than about six times that of the primary black hole.

A COMPREHENSIVE ANALYSIS OFFERMIGAMMA-RAY BURST DATA. II.EpEVOLUTION PATTERNS AND IMPLICATIONS FOR THE OBSERVED SPECTRUM-LUMINOSITY RELATIONS

We present a time-resolved spectral analysis of 51 long and 11 short bright GRBs observed with the {\em Femri}/GBM, paying special attention to $E_{\rm p}$ evolution within a same burst. Among 8 single-pulse long GRBs, 5 show hard-to-soft evolution, while 3 show intensity-tracking. The multi-pulse long GRBs have more complicated patterns. Among the GRBs whose time-resolved spectrum is available for the first pulse, almost half (15/32 GRBs) show clear hard-to-soft evolution, and the other half (17/32 GRBs) show clear intensity-tracking. Later pulses typically show the tracking behavior, although a hard-to-soft evolution pattern was identified in the 2nd pulse of 2 GRBs whose pulses are well separated. Statistically, the hard-to-soft evolution pulses tend to be more asymmetric than the intensity-tracking ones, with a steeper rising wing than the falling wing. Short GRBs have $E_{\rm p}$ tracking intensity exclusively with the 16ms time resolution analysis. We performed a simulation analysis, and suggest that at least for some bursts, the late intensity-tracking pulses could be a consequence of overlapping hard-to-soft pulses. However, the fact that the intensity-tracking pattern exists in the first pulse of multi-pulse long GRBs and some single-pulse GRBs suggest that intensity tracking is an independent component, which may operate in some late pulses as well. For the GRBs with measured redshifts, we present a time-resolved $E_{\rm p}-L_{\gamma, \rm iso}$ correlation analysis and show that the scatter of the correlation is comparable to that of the global Amati/Yonetoku relation. We discuss the predictions of various radiation models regarding $E_{\rm p}$ evolution, as well as the possibility of a precession jet in GRBs. It seems that the data pose great challenge to all these models, and hold the key to unveil the physics of GRB prompt emission.

Correlations between the peak flux density and the position angle of inner-jet in three blazars

We aim to investigate the relation between the long-term flux density and the position angle (PA) evolution of inner-jet in blazars. We have carried out the elliptic Gaussian model-fit to the `core' of 50 blazars from 15 GHz VLBA data, and analyzed the variability properties of three blazars from the model-fit results. Diverse correlations between the long-term peak flux density and the PA evolution of the major axis of the `core' have been found in $\sim$ 20% of the 50 sources. Of them, three typical blazars have been analyzed, which also show quasi-periodic flux variations of a few years (T). The correlation between the peak flux density and the PA of inner-jet is positive for S5~0716+714, and negative for S4~1807+698. The two sources cannot be explained with the ballistic jet models, the non-ballistic models have been analyzed to explain the two sub-luminal blazars. A correlation between the peak flux density and the PA (with a T/4 time lag) of inner-jet is found in [HB89]~1823+568, this correlation can be explained with a ballistic precession jet model. All the explanations are based mainly on the geometric beaming effect; physical flux density variations from the jet base would be considered for more complicated situations in future, which could account for the no or less significance of the correlation between the peak flux density and the PA of inner-jet in the majority blazars of our sample.

Jet precession in neutrino-cooled disks for gamma-ray bursts: The effects of the mass and spin of a black hole

We present a model of jet precession driven by a neutrino-cooled disk around a spinning black hole to explain the quasi-periodic features observed in some gamma-ray burst light curves. The different orientations of the rotational axes between the outer part of a neutrino-cooled disk and a black hole result in precessions of the central black hole and the inner part of the disk. Hence, the jet arising from the neutrino annihilation above the inner disk is driven to precession. We find that the period of precession is positively correlated with the mass as well as the spin of a black hole.

GRAVITATIONAL WAVES OF JET PRECESSION IN GAMMA-RAY BURSTS

The physical nature of gamma-ray bursts (GRBs) are believed to involve an ultra-relativistic jet. The observed complex structure of light curves motivate the idea of jet precession. In this work, we study the gravitational waves of jet precession based on neutrino-dominated accretion disks around black holes, which may account for the central engine of GRBs. In our model, the jet and the inner part of the disk may precess along with the black hole, which is driven by the outer part of the disk. Gravitational waves are therefore expected to be significant from this black hole-inner disk precession system. By comparing our numerical results with the sensitivity of some detectors, we find that it is possible for DECIGO and BBO to detect such gravitational waves, particularly for GRBs in the Local Group.

Effects of Swirl on Intermittency Characteristics in Non-Premixed Flames

Swirl effects on velocity, mixture fraction, and temperature intermittency have been analyzed for turbulent methane flames using large eddy simulation (LES). The LES solves the filtered governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modeling based on the localized dynamic Smagorinsky and the steady laminar flamelet models, respectively. Probability density function (PDF) distributions demonstrate a Gaussian shape closer to the centerline region of the flame and a delta function at the far radial position. However, non-Gaussian PDFs are observed for velocity and mixture fraction on the centerline in a region where center jet precession occurs. Non-Gaussian behavior is also observed for the temperature PDFs close to the centerline region of the flame. Due to the occurrence of recirculation zones, the variation from turbulent to nonturbulent flow is more rapid for the velocity than the mixture fraction and therefore indicates how rapidly turbulence affects the molecular transport in these regions of the flame.

Observing Lense-Thirring Precession in Tidal Disruption Flares

When a star is tidally disrupted by a supermassive black hole (SMBH), the streams of liberated gas form an accretion disk after their return to pericenter. We demonstrate that Lense-Thirring precession in the spacetime around a rotating SMBH can produce significant time evolution of the disk angular momentum vector, due to both the periodic precession of the disk and the nonperiodic, differential precession of the bound debris streams. Jet precession and periodic modulation of disk luminosity are possible consequences. The persistence of the jetted x-ray emission in the Swift J164449.3+573451 flare suggests that the jet axis was aligned with the spin axis of the SMBH during this event.

Swirl effects on external intermittency in turbulent jets

In this paper we present the modelled results of velocity and mixture fraction intermittency for selected isothermal swirling jet flows using Large Eddy Simulation (LES). The modelled problems considered three different swirl numbers S=0.3, 0.54 and 0.8 respectively. The probability density functions (pdfs) and intermittency of velocity and mixture fraction are presented. The simulations capture well complex flow features such as centre jet precession, a bluff body stabilised recirculation zone, and swirl induced vortex breakdown and also describe the effects of far field flow as well as scalar distributions. The calculated probability density distributions exhibit changes from a Gaussian to a delta function with increased radial distance from the jet centreline, and also a non-Gaussian shape on the jet centreline near the precession region. The radial variation of the intermittency shows differences between results with increased swirl number, especially inside the downstream swirl-induced recirculation region.

Non-ballistic superluminal motion in the plane of the sky - II.

The model of non-ballistic jet motion proposed in 2008 provides a simple explanation to the inward jet motion and bent jet. Recently, evidences of such a non-radial motion increase rapidly, and more complicated morphologies appear. On the other hand, the ballistic plus precession model likely holds in majority samples of jet motion. This paper discusses the relationship between the ballistic and non-ballistic model of jet motion, which suggests that the interaction of ejectors with ambient matter can produce knots at different stages of evolution and hence different separations to the core. And as a jet precesses, knots produced between the core and the deceleration radius result in spiral pattern expected by the model of ballistic plus precession; and knots generated at the deceleration radius display non-radial motion such as bent jet or oscillation of ridge-line. This paper develops the first non-ballistic model in four aspects. Firstly, it provides a numerical simulation to the production of multi-knot for a precessing jet. Secondly, it fits the precession behavior of multi-knot and interprets the oscillation of ridge lines like S5 1803+784. Thirdly, it gives an unified interpretation to the bent jet applicable to both multi-knot and single knot. And fourthly, the problem of very large numbers of observed outward motions as opposed to the inward ones is addressed in a new scope.

Detection of jet precession in the active nucleus of M 81

We report on very-long-baseline-interferometry (VLBI) monitoring observations of the low-luminosity active galactic nucleus (LLAGN) in the galaxy M 81 at the frequencies of 1.7, 2.3, 5.0, and 8.4 GHz. The observations reported here are phase-referenced to the supernova SN 1993J (located in the same galaxy) and cover from late 1993 to late 2005. The large amount of available observations allows us to study the stability of the AGN position in the frame of its host galaxy at different frequencies and chromatic effects in the jet morphology, together with their time evolution. The source consists at all frequencies of a slightly resolved core and a small jet extension towards the northeast direction (position angle of ~65 degrees) in agreement with previous publications. We find that the position of the intensity peak in the images at 8.4 GHz is very stable in the galactic frame of M 81 (proper motion upper limit about 10 μas per year). We confirm previous reports that the peaks at all frequencies are systematically shifted among them, possibly due to opacity effects in the jet as predicted by the standard relativistic jet model. We use this model, under plausible assumptions, to estimate the magnetic field in the jet close to the jet base and the mass of the central black hole. We obtain a black-hole mass of ~2 × 107 M⊙, comparable to estimates previously reported using different approaches, but the magnetic fields obtained are 103−104 times lower than previous estimates. We find that the positions of the cores at 1.7, 2.3, and 5.0 GHz are less stable than that at 8.4 GHz and evolve systematically, shifting southward at a rate of several tens of μas per year. The evolution in the jet orientation seems to be related to changes in the inclination of the cores at all frequencies. These results can be interpreted as due to a precessing jet. The evolving jet orientation also seems to be related to a flare in the peak flux densities at 5.0 and 8.4 GHz, which lasts ~4 years (from mid 1997 to mid 2001). An increase in the accretion rate of the black hole, and its correlation with the jet luminosity via the disk-jet connection model, seems insufficient to explain this long flare and the simultaneous evolution in the jet orientation. A continued monitoring of the flux density and the jet structure evolution in this LLAGN will be necessary to further confirm our jet precession model.

Multi-epoch parsec-scale observations of the blazar PKS 1510−089

(Abridged) We investigate the flux density variability and changes in the parsec-scale radio structure of the flat spectrum radio quasar PKS 1510-089. This source was target of multi-epoch VLBI and Space-VLBI observations at 4.8, 8.4 and 22 GHz carried out between 1999 and 2001. The comparison of the parsec-scale structure observed at different epochs shows the presence of a non-stationary jet feature moving with a superluminal apparent velocity of 16.2c+-0.7c. Over three epochs at 8.4 GHz during this period the core flux density varies of about 50%, while the scatter in the jet flux density is within 10%. The polarization percentage of both core and jet components significantly change from 2 to 9 per cent, while the polarization angle of the core shows an abrupt change of about 90 degrees becoming roughly perpendicular to the jet direction, consistent with a change in the opacity. To complete the picture of the physical processes at work, we complemented our observations with multi-epoch VLBA data at 15 GHz from the MOJAVE programme spanning a time baseline from 1995 to 2010. Since 1995 jet components are ejected roughly once per year with the same position angle and an apparent speed between 15c and 20c, indicating that no jet precession is taking place on a timescale longer than a decade in our frame. The variability of the total intensity flux density together with variations in the polarization properties may be explained assuming either a change between the optically-thick and -thin regimes produced by a shock that varies the opacity, or a highly ordered magnetic field produced by the compression of the relativistic plasma by a shock propagating along the jet. Taking into account the high gamma-ray emission observed from this source by the AGILE and Fermi satellites we investigated the connection between the radio and gamma-ray activity during 2007-2010.

Gravitational Waves Produced by Ejection of Jet Superluminal Components, Precession and Gravito-Magnetic Distortion of Accretion Disks in Active Galactic Nuclei, Micro-Quasars, and T-Tauri Stars Dynamically Driven by Bardeen-Petterson Effect

Gravitational Waves Produced by Ejection of Jet Superluminal Components, Precession and Gravito-Magnetic Distortion of Accretion Disks in Active Galactic Nuclei, Micro-Quasars, and T-Tauri Stars Dynamically Driven by Bardeen-Petterson Effect