Cavity spintronics recently heralded nonlocal magnonic signal transfer between magnetic samples. Here we show that by including superconductors in the cavity, we can make use of these principles to ...bring composite superconductor-ferromagnet systems to the macroscale. We analyze how a superconductor's ac conductivity influences the spin dynamics of a spatially separated magnet, and we discuss the potential impact on spintronic applications.
The aim of the present study is to explore how to disentangle energy-dependent time delays due to a possible Lorentz invariance violation (LIV) at Planck scale from intrinsic delays expected in ...standard blazar flares. We first characterised the intrinsic time delays in BL Lacs and flat-spectrum radio quasars in standard one-zone time-dependent synchrotron self-Compton or external Compton models, during flares produced by particle acceleration and cooling processes. We simulated families of flares with both intrinsic and external LIV-induced energy-dependent delays. Discrimination between intrinsic and LIV delays is then investigated in two different ways. A technique based on Euclidean distance calculation between delays obtained in the synchrotron and in the inverse-Compton spectral bumps is used to assess their degree of correlation. A complementary study is performed using spectral hardness versus intensity diagrams in both energy ranges. We show that the presence of non-negligible LIV effects, which essentially act only at very high energy (VHE), can drastically reduce the strong correlation expected between the X-ray and the VHE gamma-ray emission in leptonic scenarios. The LIV phenomenon can then be hinted at measuring the Euclidean distance $d_ E $ from simultaneous X-ray and gamma-ray flare monitoring. Large values of minimum distance $d_ E,min $ would directly indicate the influence of non-intrinsic time delays possibly due to LIV in SSC flares. LIV effects can also significantly modify the VHE hysteresis patterns in hardness-intensity diagrams and even change their direction of rotation compared to the X-ray behaviour. Both observables could be used to discriminate between LIV and intrinsic delays, provided high-quality flare observations are available.
Low-luminosity AGNs Istomin, Ya. N.; Sol, H.
Astronomy and astrophysics (Berlin),
03/2011, Letnik:
527
Journal Article
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Odprti dostop
Context. We propose that low-luminosity AGNs (LLAGNs), or some of them, are sources extracting their energy from the black hole rotation by the Blandford-Znajek mechanism. Aims. It is shown that ...almost all energy of the black hole rotation is converted to relativistic protons in a jet. Owing to the high magnetic-field magnitude near the black hole, required for the Blandford-Znajek mechanism, electrons are not strongly accelerated because of their high synchrotron losses. Conversely, protons gain energies on the order of (104 − 105)mpc2 when crossing the light cylinder surface. Protons are also accelerated in a disk by 2D turbulent motion of the disk matter. Methods. We calculate the luminosity of the synchrotron radiation by fast protons in the disk, the frequencies of this radiation being in the infrared band, and the luminosities corresponding to LLAGNs. We measure the very high energy (VHE) radiation luminosities from the disk and the jet, finding that VHE radiation is produced by collisions of accelerated protons with surrounding matter. Results. We predict a correlation between the infrared luminosity LIR and the VHE luminosity LVHE of the disk, $L_{\rm VHE}\propto L_{\rm IR}^{1/2}M^{3/2}$LVHE∝LIR1/2M3/2, where M is the mass of a black hole. Two low-luminosity sources Sgr A* and M 87, for which luminosities LIR and LVHE are known, appear to follow this scheme. Conclusions. The discovery of new bright VHE sources from LLAGNs could confirm our hypotheses that they are energy sources powered by the Blandford-Znajek mechanism.
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.
Recent results of the gamma-ray Cherenkov astronomy definitely prove the existence of fast variability in the very high energy (V.H.E.) gamma-ray flux of some active galactic nuclei. The BL Lac PKS ...2155-304 for instance showed variations down to a few minutes time scale. From standard light travel time argument, these variations put extremely strong constraints on the size of the TeV emitting zone, which has to be of the order of a few Schwarzschild radius, even for high values of the relativistic Doppler factor of the emitting jets. Such discovery is a challenge for particle acceleration scenarios, which have to imagine efficient acceleration processes at work in a very compact zone. Eventually, the immediate vicinity of the central black hole appears as the most conservative choice for the location of the TeV emission region of active galactic nuclei. In this paper, we propose a two-step mechanism for charged particle acceleration in the magnetosphere of a massive black hole surrounded by an accretion disk. Particles first gain energy by a stochastic process during the accretion phase. It is shown that effective proton acceleration up to energies 10
17
–10
19
eV is possible in a low-luminosity magnetized accretion disk with 2D turbulent motion. The distribution function of energetic protons over energies is a power law function with typical index ≃−1. Here electrons are not very efficiently accelerated because of their drastic losses by synchrotron radiation. In a second time, part of the fast particles escape from the disk and are then entrained by the magnetic structure above the disk, in the rotating black hole magnetosphere. They thus gain additional energy by direct centrifugal mechanism, up to about 10
20
eV for the protons and to 10–100 TeV for the electrons when they cross the light cylinder surface. Such energetic particles can further radiate in the TeV spectral range observed by Cherenkov experiments as HESS, MAGIC and VERITAS. Energetic protons can produce
γ
-radiation in the energy band 1 GeV–100 TeV and above mainly by nuclei collisions with the disk matter, clouds, or ambient low energy photons. Energetic electrons can also reach the required spectral range by inverse Compton emission. However their acceleration is less efficient due to heavy radiation losses, and only gained by centrifugal process during the second phase of the whole mechanism we describe. Our present analysis would therefore favor hadronic scenarios for TeV emission of active galactic nuclei. It is tempting to relate long term variability over years of TeV active galactic nuclei to the first stochastic acceleration phase, which also provides the needed power law particle distributions, while short term variability over minutes is more likely due to perturbations of the second fast direct acceleration phase.
We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic jet and an unmagnetized sheath plasma. We have examined the strong magnetic ...fields generated by kinetic shear (Kelvin–Helmholtz) instabilities. Compared to the previous studies using counter-streaming performed by Alves et al. (2012), the structure of the kinetic Kelvin–Helmholtz instability (KKHI) of our jet-sheath configuration is slightly different, even for the global evolution of the strong transverse magnetic field. In our simulations the major components of growing modes are the electric field Ez, perpendicular to the flow boundary, and the magnetic field By, transverse to the flow direction. After the By component is excited, an induced electric field Ex, parallel to the flow direction, becomes significant. However, other field components remain small. We find that the structure and growth rate of KKHI with mass ratios mi/me = 1836 and mi/me = 20 are similar. In our simulations saturation in the nonlinear stage is not as clear as in counter-streaming cases. The growth rate for a mildly-relativistic jet case (γj = 1.5) is larger than for a relativistic jet case (γj = 15).
In a conventional ultrasonic polar scan (UPS) experiment, the amplitude or time-of-flight-diffraction (TOFD) values of the transmitted and/or reflected acoustic pulse are recorded for a wide range of ...incidence angles, in view of gaining knowledge about the elastic properties at the insonified material spot. Here we apply the pulsed UPS technique and investigate the backscattered signal, resulting in the ultrasonic backscatter polar scan (UBPS). It is shown that a UPBS contains a blueprint of geometrically related features of the insonified material spot which can be of particular interest for various industrial applications. We applied the UBPS for (i) the determination of the stacking sequence of a cross-ply composite laminate, (ii) the semi-quantification of a multidirectional microscopic surface corrugation, (iii) the detection of corrosion in an early stage as well as (iv) the detection and the localization of a closed surface breaking crack.
Railway-induced vibrations are a growing matter of environmental concern. The rapid development of transportation, the increase of vehicle speeds and vehicle weights have resulted in higher vibration ...levels. In the meantime vibrations that were tolerated in the past are now considered to be a nuisance. Numerous solutions have been proposed to remedy these problems. The majority only acts on a specific part of the dynamic behaviour of the track. This paper presents a possible solution to reduce the noise generated by the ‘pinned–pinned’ frequencies. Pinned–pinned frequencies correspond with standing waves whose nodes are positioned exactly at the sleeper supports. The two first pinned–pinned frequencies are situated approximately at 950 and 2200
Hz (UIC60-rail and sleeper spacing of 0.60
m). To attenuate these vibrations, the Department of MEMC at the VUB has developed a dynamic vibration absorber called the Double Tuned Rail Damper (DTRD). The DTRD is mounted between two sleepers on the rail and is powered by the motion of the rail. The DTRD consists of two major parts: a steel plate which is connected to the rail with an interface of an elastic layer, and a rubber mass. The two first resonance frequencies of the steel plate coincide with the targeted pinned–pinned frequencies of the rail. The rubber mass acts as a motion controller and energy absorber. Measurements at a test track of the French railway company (SNCF) have shown considerable attenuation of the envisaged pinned–pinned frequencies. The attenuation rate surpasses 5
dB/m at certain frequency bands.
Untethered soft actuators are usually based on 3D engineered special polymers such as liquid crystal networks or hydrogels that require complex fabrication methods. Here, an easy‐to‐process, ...anisotropic composite soft actuator based on a simple photothermal dye‐doped polyethylene film is presented. The 3D anisotropic polymer films are prepared by solution‐casting and subsequent thermal solid‐stretching. The resulting soft actuators exhibit large and controllable bending in response to light (UV and/or near‐infrared (NIR)) and are able to act as grippers picking up cargo. Additionally, the films are capable of realizing “diving” and “surfing” locomotion in and over a liquid via the photothermal induced Marangoni effect, yielding a NIR light‐fueled transporter able to pick up cargo. The results open up new possibilities of using commodity polymers in a broad range of applications including untethered soft actuators and robotic devices.
3D anisotropic single‐layer polyethylene composite films are fabricated. Such films can be used to fabricate light‐responsive actuators, grippers, surfers, and transporters. This work opens up innovative possibilities to use commodity polymers in a broad range of applications such as untethered soft actuators and robotic devices.
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