Apparent Superluminal Velocities Research Articles

Lateral spreading effects on VLBI radio images of neutron star merger jets

ABSTRACT Very long baseline interferometry radio images recently proved to be essential in breaking the degeneracy in the ejecta model for the neutron star merger GW170817. We discuss the properties of synthetic radio images of merger jet afterglows by using semi-analytical models of laterally spreading or non-spreading jets. The image centroid initially moves away from the explosion point in the sky with apparent superluminal velocity. After reaching a maximum displacement, its motion is reversed. This behaviour is in line with that found in full hydrodynamic simulations. We show that the evolution of the centroid shift and the image size are significantly different when lateral spreading is considered. For Gaussian jet models with plausible model parameters, the morphology of the laterally spreading jet images is much closer to circular. The maximum displacement of the centroid shift and its occurrence time are smaller/earlier by a factor of a few for spreading jets. Our results indicate that it is crucial to include lateral spreading effects when analysing radio images of neutron star merger jets. We also obtain the viewing angle θobs by using the centroid shift of radio images provided the ratio of the jet core size θc and θobs is determined by afterglow light curves. We show that a simple method based on a point-source approximation provides reasonable angular estimates ($10{-}20{{\ \rm per\ cent}}$ errors at most). By taking a sample of laterally spreading structured Gaussian jets, we obtain θobs ∼ 0.32 for GW170817, consistent with previous studies.

Apparent superluminal core expansion and limb brightening in the candidate neutrino blazar TXS 0506+056

Context.IceCube has reported a very-high-energy neutrino (IceCube-170922A) in a region containing the blazarTXS 0506+056. Correlated gamma-ray activity has led to the first high-probability association of a high-energy neutrino with an extragalactic source. This blazar has been found to be in a radio outburst during the neutrino event.Aims.Our goal is to probe the sub-milliarcsecond properties of the radio jet right after the neutrino detection and during the further evolution of the radio outburst.Methods.We performed target of opportunity observations at 43 GHz frequency using very long baseline interferometry imaging, corresponding to 7 mm in wavelength, with the Very Long Baseline Array two and eight months after the neutrino event.Results.We produced two images of the radio jet of TXS 0506+056 at 43 GHz with angular resolutions of (0.2 × 1.1) mas and (0.2 × 0.5) mas, respectively. The source shows a compact, high brightness temperature core, albeit not approaching the equipartition limit and a bright and originally very collimated inner jet. Beyond approximately 0.5 mas from the millimeter-VLBI core, the jet loses this tight collimation and expands rapidly. During the months after the neutrino event associated with this source, the overall flux density is rising. This flux density increase happens solely within the core. Notably, the core expands in size with apparent superluminal velocity during these six months so that the brightness temperature drops by a factor of three despite the strong flux density increase.Conclusions.The radio jet of TXS 0506+056 shows strong signs of deceleration and/or a spine-sheath structure within the inner 1 mas, corresponding to about 70–140 pc in deprojected distance, from the millimeter-VLBI core. This structure is consistent with theoretical models that attribute the neutrino and gamma-ray production in TXS 0506+056 to interactions of electrons and protons in the highly relativistic jet spine with external photons originating from a slower moving jet region. Proton loading due to jet-star interactions in the inner host galaxy is suggested as the possible cause of deceleration.

Is there a non-stationary γ-ray emission zone 42 pc from the 3C 279 core?

We investigate the relationship between the variable gamma-ray emission and jet properties in the blazar 3C 279 by combining the Fermi-LAT data spanning a period of eight years and concurrent radio measurements made at multiple epochs with VLBA at 15 and 43 GHz within the MOJAVE and VLBA-BU monitoring programmes. The aim of this paper is to compare the flux variability of the different components found in the VLBA observations, to the variability in the gamma-rays. This analysis helps us to investigate whether any of the jet components can be associated with the gamma-ray variability. Through Spearman rank correlation we found that the gamma-ray variability is correlated with a particular region (feature B in the MOJAVE images) downstream from the observed base (core) of the jet. This jet component is therefore a likely location where an important fraction of the variable gamma-ray emission is produced. We also calculated the average proper motion of the component with respect to the VLBA core and found that it moves at an apparent superluminal velocity of (3.70 ± 0.35)c, implying that one of the gamma-ray emission zones is not stationary. This jet component is also found between 6.86 mas and 8.68 mas, which translates to a distance from the radio core of at least 42 pc.

Multi-Frequency VLBA Polarimetry and the Twin-Jet Quasar 0850+581

We present the first multi-frequency VLBA study of the quasar 0850+581 which appears to have a two-sided relativistic jet. Apparent velocity in the approaching jet changes from 3.4c to 7c with the separation from the core. The jet-to-counter-jet ratio of about 5 and apparent superluminal velocities suggest that the observing angle of the inner jet is ≤ 17 ∘ . It is likely that this orientation significantly changes downstream due to an interaction of the jet with the surrounding medium; signs of this are seen in polarization. A dense inhomogeneous Faraday screen is detected in the innermost regions of this quasar. We suggest that there is a presence of ionized gas in its nucleus, which might be responsible for the free-free absorption of the synchrotron emission in the jet and counter-jet at frequencies below 8.4 GHz. The experiment makes use of slowly varying instrumental polarisation factors (polarization leakage or D-terms) in time. We report application of the “D-term connection” technique for the calibration of an absolute orientation of electric vector position angle (EVPA) observed by VLBA at 4.6, 5.0, 8.1, 8.4, 15.4, 22.3, and 43.3 GHz bands during the 2007–2011.

Studying the Superluminal Behavior of UWB Antennas and Its Effect on Near-Field Imaging

In this paper, the superluminal behavior of a radiated pulse in the near-field of an ideal dipole and an ultrawideband antenna is explained and demonstrated through simulations and measurements. The deformation of the radiated pulse in the near-field is illustrated using the ideal dipole antenna radiation as well as by a Vivaldi antenna through full-wave simulations and measurements. The apparent superluminal pulse velocity is due to the pulse reshaping of the radiated pulse in the near-field of the antenna. The pulse velocity is measured by tracking pulse peak, pulse envelope peak, and also calculating the pulse centrovelocity. The superluminal pulse peak velocity of the radiated pulse from a miniaturized Vivaldi antenna is confirmed through measurements in face-to-face and quasi-monostatic radar configurations. The effect of different values of pulse velocities on the quality of reconstructed images using synthetic aperture radar processing is demonstrated as well.

Multiwavelength observations of the γ-ray flaring quasar S4 1030+61 in 2009–2014

We present a study of the parsec-scale multi-frequency properties of the quasar S4 1030+61 during a prolonged radio and gamma-ray activity. Observations were performed within Fermi gamma-ray telescope, OVRO 40-m telescope and MOJAVE VLBA monitoring programs, covering five years from 2009. The data are supplemented by four-epoch VLBA observations at 5, 8, 15, 24, and 43 GHz, which were triggered by the bright gamma-ray flare, registered in the quasar in 2010. The S4 1030+61 jet exhibits an apparent superluminal velocity of (6.4+-0.4)c and does not show ejections of new components in the observed period, while decomposition of the radio light curve reveals nine prominent flares. The measured variability parameters of the source show values typical for Fermi-detected quasars. Combined analysis of radio and gamma-ray emission implies a spatial separation between emitting regions at these bands of about 12 pc and locates the gamma-ray emission within a parsec from the central engine. We detected changes in the value and direction of the linear polarization and the Faraday rotation measure. The value of the intrinsic brightness temperature of the core is above the equipartition state, while its value as a function of distance from the core is well approximated by the power-law. Altogether these results show that the radio flaring activity of the quasar is accompanied by injection of relativistic particles and energy losses at the jet base, while S4 1030+61 has a stable, straight jet well described by standard conical jet theories.

The lower limit of the Doppler factor for a Fermi blazar sample

We selected 457 blazars (193 flat spectrum radio quasars, 61 low-synchrotron peaked blazars, 69 intermediate-synchrotron peaked blazars and 134 high-synchrotron peaked blazars) from the second Fermi-LAT catalog (2FGL) of γ-ray sources, which have X-ray observations. We calculated the lower limits for their Doppler factors, δγ, and compared the lower limits with the available Doppler factors and the apparent superluminal velocities in the literature.

Analogous of Hartman Effect for Relativistic Particles Through “Transparent” Barrier

The phase time in a nonrelativistic monodimensional tunneling is, for short barrier length, greater than the time it takes for the light in vacuum to travel the same path. Nevertheless, if the length is increased to the limit of “opaque” barrier (κL ≫ 1), where κ is the decay constant of the evanescent wave, the phase time becomes independent from the length (Hartman effect), so that apparent superluminal velocities (independently from unresolved questions about their physical meaning) are possible increasing length in this limit. Here is analytically demonstrated that apparent superluminal behavior can be found also in another situation: for highly relativistic particles, in the limit of “transparent barrier” (κL ≪ 1), is found that the phase time is always shorter than the correspondent time for light in vacuum, and that apparent superluminal velocities (following the Hartman definition) are always possible independently from the barrier length.

Quantum temporal probabilities in tunneling systems

We study the temporal aspects of quantum tunneling as manifested in time-of-arrival experiments in which the detected particle tunnels through a potential barrier. In particular, we present a general method for constructing temporal probabilities in tunneling systems that (i) defines ‘classical’ time observables for quantum systems and (ii) applies to relativistic particles interacting through quantum fields. We show that the relevant probabilities are defined in terms of specific correlation functions of the quantum field associated with tunneling particles. We construct a probability distribution with respect to the time of particle detection that contains all information about the temporal aspects of the tunneling process. In specific cases, this probability distribution leads to the definition of a delay time that, for parity-symmetric potentials, reduces to the phase time of Bohm and Wigner. We apply our results to piecewise constant potentials, by deriving the appropriate junction conditions on the points of discontinuity. For the double square potential, in particular, we demonstrate the existence of (at least) two physically relevant time parameters, the delay time and a decay rate that describes the escape of particles trapped in the inter-barrier region. Finally, we propose a resolution to the paradox of apparent superluminal velocities for tunneling particles. We demonstrate that the idea of faster-than-light speeds in tunneling follows from an inadmissible use of classical reasoning in the description of quantum systems.

SBS 0846+513: a new γ-ray-emitting narrow-line Seyfert 1 galaxy

We report Fermi-LAT observations of the radio-loud AGN SBS 0846+513 (z=0.5835), optically classified as a Narrow-Line Seyfert 1 galaxy, together with new and archival radio-to-X-ray data. The source was not active at gamma-ray energies during the first two years of Fermi operation. A significant increase in activity was observed during 2010 October-2011 August. In particular a strong gamma-ray flare was observed in 2011 June reaching an isotropic gamma-ray luminosity (0.1-300 GeV) of 1.0x10^48 erg/s, comparable to that of the brightest flat spectrum radio quasars, and showing spectral evolution in gamma rays. An apparent superluminal velocity of (8.2+/-1.5)c in the jet was inferred from 2011-2012 VLBA images, suggesting the presence of a highly relativistic jet. Both the power released by this object during the flaring activity and the apparent superluminal velocity are strong indications of the presence of a relativistic jet as powerful as those of blazars. In addition, variability and spectral properties in radio and gamma-ray bands indicate blazar-like behaviour, suggesting that, except for some distinct optical characteristics, SBS 0846+513 could be considered as a young blazar at the low end of the blazar's black hole mass distribution.

Apparent superluminal advancement of a single photon far beyond its coherence length

We present experimental results relative to superluminal propagation based on a single photon traversing an optical system, called a 4f-system, which acts singularly on the photon's spectral component phases. A single photon is created by a continuous wave (CW) laser light down-conversion process. The introduction of a linear spectral phase function will lead to the shift of the photon peak far beyond the coherence length of the photon itself (an apparent superluminal propagation of the photon). Superluminal group velocity detection is done by interferometric measurement of the temporally shifted photon with its correlated untouched reference. The observed superluminal photon propagation complies with causality. The operation of the optical system allows the origin of the apparent superluminal photon velocity to be elucidated. The experiment foresees a superluminal effect with single-photon wavepackets.

Characterization of apparent superluminal effects in the focus of an axicon lens using terahertz time-domain spectroscopy

We describe time-resolved measurements of the propagating interference pattern at the focus of an axicon lens. The technique of terahertz time-domain spectroscopy permits a direct observation of the electric field within the line focus formed by a converging conical wave front. We extract the apparent superluminal velocity of this pulse from the peak of the time-domain waveform, and also in the frequency domain by Fourier transform. From these measurements, we conclude that pulse reshaping, arising from strong frequency-dependent propagation effects, does not play a substantial role. As a result, tracking the peak of the pulse in the time domain is a valid method for determining the average group velocity of the disturbance. We also elucidate a new mechanism which influences the value of the superluminal velocity, involving the curvature of the incident wave front.

Multiepoch Very Long Baseline Array Observations of EGRET‐detected Quasars and BL Lacertae Objects: Superluminal Motion of Gamma‐Ray Bright Blazars

We present the results of a program to monitor the structure of the radio emission in 42 ?-ray bright blazars (31 quasars and 11 BL Lac objects) with the Very Long Baseline Array (VLBA) at 43, 22, and occasionally 15 and 8.4 GHz, over the period from 1993 November to 1997 July. We determine proper motions in 33 sources and find that the apparent superluminal motions in ?-ray sources are much faster than for the general population of bright compact radio sources. This follows the strong dependence of the ?-ray flux on the level of relativistic beaming for both external radiation Compton and synchrotron self-Compton emission. There is a positive correlation (correlation coefficient r = 0.45) between the flux density of the VLBI core and the ?-ray flux and a moderate correlation (partial correlation coefficient r = 0.31) between ?-ray apparent luminosity and superluminal velocities of jet components, as expected if the ?-ray emission originates in a very compact region of the relativistic jet and is highly beamed. In 43% of the sources the jet bends by more than 20? on parsec scales, which is consistent with amplification by projection effects of modest actual changes in position angle. In 27 of the sources in the sample there is at least one non-core component that appears to be stationary during our observations. Different characteristics of stationary features close to and farther from the core lead us to suggest two different classes of stationary components: those within about 2 mas of the core, probably associated with standing hydrodynamical compressions, and those farther down the jet, which tend to be associated with bends in the jet.

Prospects for realism in quantum mechanics

Abstract Quantum mechanics has seemed to defy all attempts to construe it realistically, but antirealism, like the many‐worlds hypothesis, is even more difficult to accept. In order to give a realist construal of quantum mechanics, we need first to distinguish the objective and rational aspect of reality from the paradigmatic thing‐like aspects of having determinate physical properties: quantum‐mechanical entities may be real in the former sense though not in the latter. Anti‐realist arguments are based on the difficulty of giving an account of quantum‐mechanical collapse and the apparent superluminal velocities involved. Objections to superluminal velocities on the score of the special theory of relativity are found not to be conclusive, and the price—there being some preferred frame of reference—to be acceptable. A sketch of a probabilistic account of quantum‐mechanical collapse is offered, which makes the difference between the macro‐ and the micro‐world a matter of degree rather than kind. If that, or...

An Improved Measure of Quasar Orientation

Radio core dominance, usually measured by R, the rest frame ratio of core to lobe flux density, has been used as an indicator of Doppler boosting of a quasar's radio jets and hence the inclination of the central engine's spin axis to the line of sight. The use of lobe flux density as a means of normalizing the boosted core flux density to the available intrinsic power of the central engine introduces scatter. This is because the emission from the radio lobes depends strongly on the interaction of the jet with the environment at distances beyond several kiloparsecs from the nucleus. Total kinetic power in the extended radio emission is approximately proportional to emission-line luminosity, and emission-line luminosity is proportional to the luminosity of the featureless continua of active galactic nuclei?both over four orders of magnitude. Thus, a quasar's optical luminosity may be an excellent measure of available intrinsic jet power. Therefore, we define a new core dominance parameter, RV, the ratio of radio core to optical (rest frame V band) continuum luminosity, that is not directly dependent on jet interactions with the super-kiloparsec-scale environment. We show that the use of RV, rather than R, results in significantly improved inverse correlations with the beaming angle as deduced from apparent superluminal velocities and inverse-Compton-scattered X-ray emission and with the FWHM of a quasar's broad H? emission line. We discuss some implications and applications of the new parameter.

The velocity of the emitting plasma of the superluminal galactic source GRS 1915+105

We calculate the flux and size ratio of radio emitting features moving in antiparallel directions at relativistic speeds, taking into account that the pattern we observe may move at a velocity different from the one of the emitting plasma. Both velocities can be determined in sources in which both the approaching and the receding features are observed. This is the case of the galactic superluminal source GRS 1915+105, for which the pattern speed (responsible for the apparent superluminal velocity) was found to be 0.92$c$. For this source we find that the velocity of the plasma is 0.73$c$. The found velocity helps decreasing the estimate of the associated kinetic power. Since intrinsically identical knots will be observed to have different sizes, high resolution radio observations of GRS 1915+105 will be a test for the proposed difference of the pattern and plasma velocities.

Relativistic Jets with Stellar Wind Entrainment

Observations suggest that the flow speeds in extragalactic radio jets are relativistic on parsec scales. This evidence includes rapid variability, brightness temperatures in excess of the inverse Compton limit and even the motion of individual features with apparent superluminal velocities. In contrast, FR–1 flow speeds appear to be non–relativistic on kiloparsec scales (the case is not clear for FR–2 sources); consequently, the possibility of jet deceleration has been explored.

Non-constant Superluminal Velocities in AGN

AbstractLarge apparent superluminal velocities are observed in nuclear jets in Active Galaxies, indicating the presence of relativistic velocities almost along the line of sight. If the flow is well collimated, as suggested by the large scale radio structure, the inferred alignment leads to difficulties with source statistics. Here a modification of the usual relativistic beam model is proposed, in which the jet is assumed to contain azimuthal (swirling) flow. Perturbation analysis is used to show that the jet is unstable to a Kelvin-Helmholtz helical standing wave, the wavelength of which increases without bound in the limit of vanishing swirl. This instability may cause a cylindrical jet to follow a helical path in space, thereby reducing the implied alignment of a superluminal source, and providing a natural interpretation of non-constant superluminal velocities.

Evolution of the Milliarcsecond Polarization Structure of the BL Lacertae Object OJ287

The λ6 cm total intensity and polarization structures of OJ287 have been determined at two epochs a year apart. At each epoch the source can be modeled as a linear structure consisting of a core and two knots at position angle PA ≃ −115°. This structure expanded slightly over a year, each knot increasing its separation from the core by ≃ 0.3 mas, suggesting an apparent superluminal velocity of ≃ 3.3h−1c for each.

Apparent superluminal velocities due to the curvature of space

Instead of the difficult concept of a three-dimensional curved space one can use two partial models of two-dimensional curved subspaces. If a two-dimensional subspace is curved the arbitrary angle of view must have a distorted shape which has in consequence an optical illusion. The distorted shape of ϕ must be corrected to the real angle1φ, valid in uncurved space according to the relation φ=1φf(ρ) where ρ is the central angle in theoretical sphere. If a two-dimensional subspace is just a spheric surface, the correction factor isfs(ρ)≅ρ/sin ρ, appropriate to the angle distance ρ from observer to the observed object.