Magnetohydrodynamic Production of Relativistic Jets Meier, David L.; Koide, Shinji; Uchida, Yutaka
Science (American Association for the Advancement of Science),
01/2001, Letnik:
291, Številka:
5501
Journal Article
Recenzirano
A number of astronomical systems have been discovered that generate collimated flows of plasma with velocities close to the speed of light. In all cases, the central object is probably a neutron star ...or black hole and is either accreting material from other stars or is in the initial violent stages of formation. Supercomputer simulations of the production of relativistic jets have been based on a magnetohydrodynamic model, in which differential rotation in the system creates a magnetic coil that simultaneously expels and pinches some of the infalling material. The model may explain the basic features of observed jets, including their speed and amount of collimation, and some of the details in the behavior and statistics of different jet-producing sources.
ABSTRACT We study the kinematics of ridge lines on the parsec-scale jet of the active galactic nucleus BL Lacertae. We show that the ridge lines display transverse patterns that move superluminally ...downstream, and that the moving patterns are analogous to waves on a whip. Their apparent speeds βapp (units of c) range from 3.9 to 13.5, corresponding to in the galaxy frame. We show that the magnetic field in the jet is well ordered with a strong transverse component, and assume that it is helical and that the transverse patterns are Alfvén waves propagating downstream on the longitudinal component of the magnetic field. The wave-induced transverse speed of the jet is non-relativistic ( ). In 2010 the wave activity subsided and the jet then displayed a mild wiggle that had a complex oscillatory behavior. The Alfvén waves appear to be excited by changes in the position angle of the recollimation shock, in analogy to exciting a wave on a whip by shaking the handle. A simple model of the system with plasma sound speed βs = 0.3 and apparent speed of a slow MHD wave βapp, S = 4 yields Lorentz factor of the beam Γbeam ∼ 4.5, pitch angle of the helix (in the beam frame) ∼ 67°, Alfvén speed βA ∼ 0.64, and magnetosonic Mach number Mms ∼ 4.7. This describes a plasma in which the magnetic field is dominant and in a rather tight helix, and Alfvén waves are responsible for the moving transverse patterns.
•The strength and dynamics of magnetoelastic coupling through the paramagnetic (PM) – antiferromagnetic (AFM) – ferrimagnetic (FIM) transitions in multiferroic hexagonal ErMnO3 are revealed by ...Resonant Ultrasound Spectroscopy.•Elastic stiffening by up to 2% below the PM – AFM transition at 80 K arises from biquadratic coupling between strain and the magnetic order parameter with relaxation times longer than ∼ 10-6 s for the response of spins to changes in strain.•A peak in acoustic loss in the vicinity of 250 K is attributed to strain-mediated pinning/freezing of some aspect of the domain microstructure with an activation energy of ∼ 0.25–0.3 eV.•A peak in acoustic loss in the vicinity of 250 K is attributed to strain-mediated pinning/freezing of some aspect of the domain microstructure with an activation energy of ∼ 0.25–0.3 eV.•Subtle variations in magnetoelastic coupling behaviour in ErMnO3 relate to both the magnetic order parameters and magnetic domain structures.
The strength and dynamics of magnetoelastic coupling through the paramagnetic (PM) – antiferromagnetic (AFM) – ferrimagnetic (FIM) transitions in multiferroic hexagonal ErMnO3 have been investigated by Resonant Ultrasound Spectroscopy. Elastic stiffening by up to 2% below the PM – AFM transition at 80 K arises from biquadratic coupling between strain and the magnetic order parameter with relaxation times longer than ∼ 10-6 s for the response of spins to changes in strain. In contrast with YMnO3, the PM – AFM transition in ErMnO3 is accompanied by a peak in acoustic loss immediately below the Néel point which is interpreted in terms of strain relaxation accompanying ordering of spins of Er3+ at 4b sites. Changes in the magnetic ordering scheme at the AFM – FIM transition near 3 K are accompanied by elastic softening of ∼ 0.03 %. During poling of the low temperature ferrimagnetic structure round magnetic hysteresis loops, small changes in elastic stiffness which arise due to the contribution of piezomagnetic and/or piezoelectric moduli are detected. Contributions of piezoelectric moduli to acoustic resonance frequencies also permit changes in the configuration of ferroelectric domains to be detected in response both to cycling through this transition and to application of a magnetic field. A peak in acoustic loss in the vicinity of 250 K is attributed to strain-mediated pinning/freezing of some aspect of the domain microstructure with an activation energy of ∼ 0.25–0.3 eV. A return to the original elastic properties on heating to temperatures above ∼ 250 K is interpreted in terms of backswitching of domains to the configuration they had at the start. These observations confirm the existence of subtle variations in magnetoelastic coupling behaviour relating to both the magnetic order parameters and magnetic domain structures.
Abstract
Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While 12CO has been targeted extensively, isotopologues such as 13CO have the advantage of ...being less optically thick and observations have recently become accessible across full galaxy discs. We present a comprehensive new data set of 13CO(1–0) observations with the IRAM 30-m telescope of the full discs of nine nearby spiral galaxies from the EMPIRE survey at a spatial resolution of ∼1.5 kpc. 13CO(1–0) is mapped out to 0.7 − 1 r25 and detected at high signal-to-noise ratio throughout our maps. We analyse the 12CO(1–0)-to-13CO(1–0) ratio (ℜ) as a function of galactocentric radius and other parameters such as the 12CO(2–1)-to-12CO(1–0) intensity ratio, the 70-to-160 μm flux density ratio, the star formation rate surface density, the star formation efficiency, and the CO-to-H2 conversion factor. We find that ℜ varies by a factor of 2 at most within and amongst galaxies, with a median value of 11 and larger variations in the galaxy centres than in the discs. We argue that optical depth effects, most likely due to changes in the mixture of diffuse/dense gas, are favoured explanations for the observed ℜ variations, while abundance changes may also be at play. We calculate a spatially resolved 13CO(1–0)-to-H2 conversion factor and find an average value of 1.0 × 1021 cm−2 (K km s−1)−1 over our sample with a standard deviation of a factor of 2. We find that 13CO(1–0) does not appear to be a good predictor of the bulk molecular gas mass in normal galaxy discs due to the presence of a large diffuse phase, but it may be a better tracer of the mass than 12CO(1–0) in the galaxy centres where the fraction of dense gas is larger.
The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new 2 − 4″ (∼75 − 150 pc at 7.7 Mpc) ...observations at 2 and 3 mm covering the central 50″ (∼1.9 kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO
+
, HNC, CS, HC
3
N, N
2
H
+
, C
2
H, CH
3
OH, and H
2
CO. We complemented these with published 1 mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density Σ
SFR
on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 – the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO
+
, and HNC). We find that HCO
+
correlates best with Σ
SFR
, but the dense gas fraction (
f
dense
) and star formation efficiency of the dense gas (SFE
dense
) fits show different behaviours than expected from large-scale disc observations. The SBE has a higher Σ
SFR
,
f
dense
, and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2−1)/CO(1−0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO
+
/HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies.
Attributing awareness to oneself and to others Kelly, Yin T.; Webb, Taylor W.; Meier, Jeffrey D. ...
Proceedings of the National Academy of Sciences - PNAS,
04/2014, Letnik:
111, Številka:
13
Journal Article
Recenzirano
Odprti dostop
This study tested the possible relationship between reported visual awareness (“I see a visual stimulus in front of me”) and the social attribution of awareness to someone else (“That person is aware ...of an object next to him”). Subjects were tested in two steps. First, in an fMRI experiment, subjects were asked to attribute states of awareness to a cartoon face. Activity associated with this task was found bilaterally within the temporoparietal junction (TPJ) among other areas. Second, the TPJ was transiently disrupted using single-pulse transcranial magnetic stimulation (TMS). When the TMS was targeted to the same cortical sites that had become active during the social attribution task, the subjects showed symptoms of visual neglect in that their detection of visual stimuli was significantly affected. In control trials, when TMS was targeted to nearby cortical sites that had not become active during the social attribution task, no significant effect on visual detection was found. These results suggest that there may be at least some partial overlap in brain mechanisms that participate in the social attribution of sensory awareness to other people and in attributing sensory awareness to oneself.
The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ...ratio of the reflectivity is measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be (0.6±0.1) μB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02 μB per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.
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