Galactic Jets Research Articles

UHECRs and multiple shock acceleration in Active Galactic Nuclei Jets

A prominent source for the production of ultra high energy cosmic rays (UHECRs) is the relativistic shocks in the jets of Active Galactic Nuclei (AGN). As multiple or conical-like shock features have been theorized or observed along the jets of several AGN, we propose here an acceleration model which allows a single injection of particles accelerated consecutively by more than one relativistic shocks (subluminal or/and superluminal) along an AGN jet, by conducting numerical simulations. We find that the first shock of a relativistic shock sequence, establishes a power-law spectrum with ∼ E−2.7, while the following consecutive shocks push the spectrum up to the UHECR region of the spectrum, rendering natter distributions, with characteristic depleted spectra at lower energies. The consequences of this numerical study, shed light into understanding the multiple-shock role in AGN jets, as well as the puzzling source power fed into UHECRs.

AGN jets under the microscope: A divide?

AbstractA new paradigm for active galactic jet kinematics has emerged through detailed investigations of BL Lac objects using very long baseline radio interferometry. In this new scheme, most, if not all, jet components appear to remain stationary with respect to the core but show significant non‐radial motions. This paper presents results from our kinematic investigation of the jets of a statistically complete sample of radio‐loud flat‐spectrum active galaxies, focusing on the comparison between the jet kinematic properties of BL Lacs and flat‐spectrum radio‐quasars. It is shown that there is a statistically significant difference between the kinematics of the two AGN classes, with BL Lacs showing more bent jets, that are wider and show slower movement along the jet axis, compared to flat‐spectrum radio‐quasars. This is interpreted as evidence for helically structured jets (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interpretation of neutrino flux limits from neutrino telescopes on the Hillas plot

We discuss the interplay between spectral shape and detector response beyond a simple ${E}^{\ensuremath{-}2}$ neutrino flux at neutrino telescopes, using the example of time-integrated point source searches using IceCube-40 data. We use a self-consistent model for the neutrino production, in which protons interact with synchrotron photons from coaccelerated electrons, and we fully take into account the relevant pion and kaon production modes, the flavor composition at the source, flavor mixing, and magnetic field effects on the secondaries (pions, muon, and kaons). Since some of the model parameters can be related to the Hillas parameters $R$ (size of the acceleration region) and $B$ (magnetic field), we relate the detector response to the Hillas plane. In order to compare the response to different spectral shapes, we use the energy flux density as a measure for the pion production efficiency times luminosity of the source. We demonstrate that IceCube has a very good reach in this quantity for active galactic nuclei and jets for all source declinations, while the spectra of sources with strong magnetic fields are found outside the optimal reach. We also demonstrate where neutrinos from kaon decays and muon tracks from $\ensuremath{\tau}$ decays can be relevant for the detector response. Finally, we point out the complementarity between IceCube and other experiments sensitive to high-energy neutrinos, using the example of 2004--2008 Earth-skimming neutrino data from Auger. We illustrate that Auger, in principle, is more sensitive to the parameter region in the Hillas plane from which the highest-energetic cosmic rays may be expected in this model.

THE STRUCTURE OF THE M87 JET: A TRANSITION FROM PARABOLIC TO CONICAL STREAMLINES

The structure of the M87 jet, from milli-arcsec to arcsecond scales, is extensively investigated, utilizing the images taken with the EVN, MERLIN and VLBA. We discover that the jet maintains a parabolic streamline over a range in sizescale equal to $10^{5}$ times the Schwarzschild radius. The jet then transitions into a conical shape further downstream. This suggests that the magnetohydrodynamic jet is initially subjected to the confinement by the external gas which is dominated by the gravitational influence of the supermassive black hole. Afterwards the jet is then freely expanding with a conical shape. This geometrical transition indicates that the origin of the HST-1 complex may be a consequence of the over-collimation of the jet. Our result suggests that when even higher angular resolution is provided by a future submm VLBI experiment, we will be able to explore the origin of active galactic nuclei jets.

Doppler Boosting May Have Played No Significant Role in the Finding Surveys of Radio-Loud Quasars

There appears to be a fundamental problem facing Active Galactic Nuclei jet models that require highly relativistic ejection speeds and small jet viewing angles to explain the large apparent superluminal motions seen in so many of the radio-loud quasars with high redshift. When the data are looked at closely it is found that only a small percentage of the observed radio frequency flux density from these sources can be Doppler boosted. Without a highly directed, Doppler boosted component that dominates the observed flux, radio sources found in low-frequency finding surveys cannot be preferentially selected with small jet viewing angles. The distribution of jet orientations will then follow the sin$i$ curve associated with a random distribution, where only a very few sources (~1%) will have the small viewing angles (<8 deg) required to explain apparent superluminal motions v{app > 10c, and this makes it difficult to explain how around 33% of the radio-loud AGNs with high redshift can exhibit such highly superluminal motions. When the boosted component is the dominant one it can be argued that in a flux limited sample only those members with small viewing angles would be picked up while those with larger viewing angles (the un-boosted ones) would be missed. However, this is not the case when the boosted component is small and a new model to explain the high apparent superluminal motions may be needed if the redshifts of high-redshift quasars are to remain entirely cosmological.

NON-THERMAL EMISSION FROM GALACTIC JETS

Jets are ubiquitous in the Universe. They are collimated outflows whose origin is associated to an accretion disc and a central object, and can be very powerful non-thermal emitters. Jets form in active galactic nuclei, gamma-ray bursts, microquasars, and young stellar objects. Galactic jets emitting non-thermal emission are typically associated to microquasars, although the jets of massive young stellar objects are also non-thermal sources. The production of non-thermal radiation, in particular radio synchrotron emission, is a clear indication that particle acceleration is taking place in the source, which hints to the generation of photons even at high energies. In this work, we will discuss the emitting sites in, or related to, microquasar jets, and briefly comment on the possibility of high-energy emission in jets from young stellar objects.

INTRACLUSTER MEDIUM REHEATING BY RELATIVISTIC JETS

Galactic jets are powerful energy sources reheating the intra-cluster medium in galaxy clusters. Their crucial role in the cosmic puzzle, motivated by observations, has been established by a great number of numerical simulations missing the relativistic nature of these jets. We present the first relativistic simulations of the very long term evolution of realistic galactic jets. Unexpectedly, our results show no buoyant bubbles, but large cocoon regions compatible with the observed X-ray cavities. The reheating is more efficient and faster than in previous scenarios, and it is produced by the shock wave driven by the jet, that survives for several hundreds of Myrs. Therefore, the X-ray cavities in clusters produced by powerful relativistic jets would remain confined by weak shocks for extremely long periods, whose detection could be an observational challenge.

On the interaction of jet plasma with dark matter subhaloes in active galaxies

The interaction of fragmented plasma of active galactic nuclei jets with galactic haloes via gravitational scattering and lensing by dark matter subhaloes is studied using analytical calculations and numerical Monte-Carlo method. The lensing of jet radiation by halo masses is found to be negligible and unobservable. Moving through a galactic halo jet plasma fragments are sequentially deflected on hyperbolic orbits by gravitational field of subhaloes and deviates at some angles when leaving halo, causing widening of the jet. Based on this model jet opening angles are calculated numerically for various values of jet and halo characteristics. Though these angles are very small, gravitational scattering by halo masses results in specific radial profile of jet radiation intensity, that does not depend on halo mass distribution and jet properties. The intensity of jet radiation, obeying the derived profile, decreases by reasonable observable factors giving possibility to probe the presence of dark matter subhaloes.

Is there really a dichotomy in active galactic nucleus jet power?

To gain new insights into the radio-loud/radio-quiet dichotomy reported for active galactic nuclei, we examine radio loudness as a function of Eddington ratio for a previously published sample of 199 AGN from five different populations. After initially considering radio loudnesses derived using total radio luminosities, we repeat the investigation using core radio luminosities only, applying a previously established mass correction for these core luminosities. In both cases, for Eddington ratios < 1 per cent, Fanaroff-Riley type I and broad-line radio galaxies are on average more radio-loud than Seyfert and low-ionization nuclear emission-line region galaxies. However, the distribution of radio loudnesses for the mass-corrected, core-only sample is much narrower than that of the clearly bimodal total radio loudness distribution. The advantages and disadvantages of using core- or lobe-dominated radio luminosity as a measure of instantaneous jet power are discussed. We furthermore compare the core and total radio luminosities for the entire sample, as well as illustrating the importance of the mass term by comparing the AGN with a sample of black hole X-ray binaries. We conclude that if the mass-corrected core radio luminosity is a good measure of jet power, then black hole spin may have considerably less impact on jet power than previously reported, or that our sample does not include the extremes of spin. If the spread in jet power is small then we suggest that characteristics of the ambient environment and/or the radio source age could be equally as important in producing a radio-loud/radio-quiet dichotomy seen in total radio luminosity.

A new method for estimating frequency-dependent core shifts in active galactic nucleus jets

We discuss the opacity in the core regions of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI), and describe a new method for deriving the frequency-dependent shifts of the VLBI core from the frequency-dependent time lags of flares observed with single-dish observations. Application of the method to the core shifts of the quasar 3C 345 shows a very good agreement between the core shifts directly measured from VLBI observations and derived from flares in the total flux density using the proposed method. The frequency-dependent time lags of flares can be used to derive physical parameters of the jets, such as distance from the VLBI core to the base of the jet and the magnetic fields in the core region. Our estimates for 3C 345 indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.

Probing dark matter with active galactic nuclei jets

We study the possibility of detecting a signature of particle dark matter in the spectrum of gamma-ray photons from active galactic nuclei (AGNs) resulting from the scattering of high-energy particles in the AGN jet off of dark matter particles. We consider particle dark matter models in the context of both supersymmetry and universal extra dimensions , and we present the complete lowest-order calculation for processes where a photon is emitted in dark matter-electron and/or dark matter-proton scattering, where electrons and protons belong to the AGN jet. We find that the process is dominated by a resonance whose energy is dictated by the particle spectrum in the dark matter sector (neutralino and selectron for the case of supersymmetry, Kaluza-Klein photon and electron for universal extra dimensions ). The resulting gamma-ray spectrum exhibits a very characteristic spectral feature, consisting of a sharp break to a hard power-law behavior. Although the normalization of the gamma-ray flux depends strongly on assumptions on both the AGN jet geometry, composition and particle spectrum as well as on the particle dark matter model and density distribution, we show that for realistic parameters choices, and for two prominent nearby AGNs (Centaurus A and M87), the detection of this effect is in principle possible. Finally, we compare our predictions and results with recent gamma-ray observations from the Fermi, H.E.S.S., and VERITAS telescopes.

Scalings of the synchrotron cut-off and turbulent correlation of active galactic nucleus jets

We propose a new analytic scaling of the cut-off frequency of synchrotron radiation from active galactic nucleus (AGN) jets that are non-uniformly filled with many filaments. The theoretical upper limit is provided independent of magnetic intensity, spectral index, coherence, correlation length of filamentary turbulence, etc., such that νc≃ 6 × 1020δ[(r− 1)/r]4/3 (b/10−4) Hz, where δ, r and b are the Doppler beaming factor, shock-compression ratio and energy-density ratio of the perturbed/local mean magnetic field of the filaments, respectively. Combining our results with observational data for 18 extragalactic sources, a constraint on the filament correlation length is found, in order to give the number scaling of filaments. The results suggest that, in particular, the jets of compact BL Lacs possess a large number of filaments with transverse size scale smaller than the emission-region size. The novel concept of the quantization of flowing plasma is suggested.

Magnetic field strength and spectral distribution of six parsec-scale active galactic nuclei jets

We use observations of six "blazars" with the Very Long Baseline Array (VLBA), at eight frequencies (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22.2, 43.1 GHz), to investigate the frequency-dependent position of their VLBI cores ("core-shift") and their overall jet spectral distribution. By cross-correlating the optically thin jet emission, we are able to accurately align the multi-frequency images of three of the jets (1418+546, 2007+777, 2200+420), whose core-shifts and spectra we find consistent with the equipartition regime of the Blandford & Konigl conical jet model, where the position of the radio core from the base of the jet follows $r_{core}\propto\nu^{-1}$. For the jet of 0954+658, we align the higher frequency images using our lower frequency measurements assuming equipartition in the radio core from 4.6-43 GHz. The jet emission of the other two sources in our sample (1156+295, 1749+096) is too sparse for our alignment technique to work. Using our measured core-shifts, we calculate equipartition magnetic field strengths of the order of 10's to 100's of mG in the radio cores of these four AGN from 4.6-43 GHz. Extrapolating our results back to the accretion disk and black hole jet-launching distances, we find magnetic field strengths consistent with those expected from theoretical models of magnetically powered jets.

HYDROSTATIC GAS CONSTRAINTS ON SUPERMASSIVE BLACK HOLE MASSES: IMPLICATIONS FOR HYDROSTATIC EQUILIBRIUM AND DYNAMICAL MODELING IN A SAMPLE OF EARLY-TYPE GALAXIES

We present new mass measurements for the supermassive black holes (SMBHs) in the centers of three early-type galaxies. The gas pressure in the surrounding, hot interstellar medium (ISM) is measured through spatially resolved spectroscopy with the Chandra X-ray Observatory, allowing the SMBH mass (M BH) to be inferred directly under the hydrostatic approximation. This technique does not require calibration against other SMBH measurement methods and its accuracy depends only on the ISM being close to hydrostatic, which is supported by the smooth X-ray isophotes of the galaxies. Combined with results from our recent study of the elliptical galaxy NGC 4649, this brings the number of galaxies with SMBHs measured in this way to four. Of these, three already have mass determinations from the kinematics of either the stars or a central gas disk, and hence join only a handful of galaxies with M BH measured by more than one technique. We find good agreement between the different methods, providing support for the assumptions implicit in both the hydrostatic and the dynamical models. The stellar mass-to-light ratios for each galaxy inferred by our technique are in agreement with the predictions of stellar population synthesis models assuming a Kroupa initial mass function (IMF). This concurrence implies that no more than ~10%-20% of the ISM pressure is nonthermal, unless there is a conspiracy between the shape of the IMF and nonthermal pressure. Finally, we compute Bondi accretion rates , finding that the two galaxies with the highest exhibit little evidence of X-ray cavities, suggesting that the correlation with the active galactic nuclei jet power takes time to be established.

New prediction of extragalactic GeV γ-ray emission from radio lobes of young AGN jets

Abstract We present a new prediction of GeV γ-ray emission from radio lobes of young active galactic nuclei jets. In the previous work of Kino, Kawakatu &amp; Ito, MeV γ-ray bremsstrahlung emission was predicted from young cocoons/radio lobes in the regime of no coolings. In this study, we include cooling effects of bremsstrahlung emission and adiabatic loss. With the initial conditions determined by observed young radio lobes, we solve a set of equations describing the expanding lobe evolution. Then, we find that the lobes initially have electron temperature of ∼GeV, and they cool down to ∼MeV by the adiabatic loss. Correspondingly, the lobes initially yield bright bremsstrahlung luminosity in ∼GeV range and they fade out. We estimate these γ-ray emissions and show that nearby young radio lobes could be detected with Fermi Gamma-ray Space Telescope.

Three-dimensional magnetic field structure of six parsec-scale active galactic nuclei jets

The parsec-scale Faraday rotation measure (RM) distribution of six 'blazars' (0954+658, 1156+295, 1418+546, 1749+096, 2007+777 and 2200+420) are investigated using multifrequency (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22 and 43 GHz) polarization observations with the American Very Long Baseline Array taken on 2006 July 2. Analysis of the RM provides the direction of the line-of-sight (LoS) magnetic field component, as well as the intrinsic (unrotated) two-dimensional polarization distribution on the plane of the sky. Our results show that the magnitude of the core RM increases systematically with frequency, and is well described by a power law, where |RM core | ∝ v a . Our measured values of a vary from 0.9 to 3.8, providing information on the assumed power-law fall-off in the electron density (n e ) with distance from the central engine (r) for each source. RM gradients were detected across the jets of 0954+658, 1156+295 and 1418+546, supporting the presence of helical magnetic fields in a sheath or boundary layer surrounding their jets. We find a bi-modal distribution of the intrinsic jet polarization orientation, with the polarization angles either aligned or orthogonal to the jet direction. The polarization of 2200+420 displays a continuous structure, with the polarization angles remaining aligned with the jet direction even as it bends. This indicates that the magnetic field structure in the synchrotron emitting plasma is dominated by an ordered transverse component. A helical magnetic field geometry can neatly explain both the bi-model distribution of the jet polarization orientation and the ordered polarization structure on these scales. For 0954+658, 1418+546 and 2200+420, we find that the core RM changes sign with distance from the central engine. We provide an explanation for this by considering a boundary layer of Faraday rotating material threaded by a helical magnetic field, where bends in the relativistic jet or accelerating/decelerating flows give rise to changes in the dominant LoS components of the magnetic field, which in turn gives rise to different signs of the RM.

A RELATIVISTIC ELECTRON-POSITRON OUTFLOW FROM A TEPID FIREBALL

Through detailed numerical simulations, we demonstrate that relativistic outflows (Lorentz factor $\Gamma \sim 7$) of electron-positron pairs can be produced by radiative acceleration even when the flow starts from a nearly pair equilibrium state at subrelativistic temperatures. Contrary to the expectation that pairs annihilate during an expansion stage for such low temperatures, we find that most pairs can survive for the situations obtained in our previous work. This is because in the outflow-generating region the dynamical timescale is short enough even though the fireball is optically thick to scattering. Several problems that should be solved to apply to actual active galactic nucleus jets are discussed.

Bootstrap energization of relativistic electrons in magnetized plasmas

AbstractIn situ and remote observations indicate that relativistic or ultra relativistic electrons are formed at various magnetized configurations. It is suggested that a specific bootstrap mechanism operates in some of these environments. The mechanism applies to (a) relativistic electrons observed on localized field lines in outer radiation belt - through a process initiated at a distant substorm injection; (b) relativistic electrons observed at the interplanetary medium - through a process initiated via coronal injection, at large distances from flares or propagating CME; (c) ultra-relativistic electrons deduced at the galactic jets - through a process initiated via local injection at the small-scale magnetic field. The injected nonisotropic electrons excite whistler waves which boost efficiently the tail of the electron distribution.

ON THE X-RAY/TEV CONNECTION IN GALACTIC JET SOURCES

There are three Galactic jet sources from which TeV emission has been detected: LS 5039, LS I +61 303 and Cygnus X-1. These three sources show power-law tails in X-rays and soft gamma-rays that could indicate a nonthermal origin of this radiation. In addition, all three sources apparently show correlated and complex behavior at X-ray and TeV energies. In some cases, this complex behavior is related to the orbital motion (e.g. LS 5039, LS I +61 303), and in some others it is related to some transient events occurring in the system (e.g. Cygnus X-1, and likely also LS I +61 303 and LS 5039). Based on modeling results or on energetic grounds, it seems difficult to explain the emission in the X-/soft gamma-ray and the TeV bands as coming from the same (i.e. one-zone) region. We also stress the importance of the pair creation phenomena in these systems, which harbor a massive and hot star, for the radio and the X-ray emission, as a secondary pair radiation component may be significant in these energy ranges. Finally, we point out that the presence of the star can indeed have a strong impact on both the nonthermal radiation production and the jet dynamics.

X‐raying a galactic jet set

Science NewsVolume 173, Issue 3 p. 38-38 This Week X-raying a galactic jet set Ron Cowen, Ron CowenSearch for more papers by this author Ron Cowen, Ron CowenSearch for more papers by this author First published: 30 September 2009 https://doi.org/10.1002/scin.2008.5591730309AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume173, Issue319 January 2008Pages 38-38 RelatedInformation