Dimitris M. Christodoulou

THE INVARIANT TWIST OF MAGNETIC FIELDS IN THE RELATIVISTIC JETS OF ACTIVE GALACTIC NUCLEI

The origin of cosmic magnetic (B) fields remains an open question. It is generally believed that very weak primordial B fields are amplified by dynamo processes, but it appears unlikely that the amplification proceeds fast enough to account for the fields presently observed in galaxies and galaxy clusters. In an alternative scenario, cosmic B fields are generated near the inner edges of accretion disks in active galactic nuclei (AGNs) by azimuthal electric currents due to the difference between the plasma electron and ion velocities that arises when the electrons are retarded by interactions with photons. While dynamo processes show no preference for the polarity of the (presumably random) seed field that they amplify, this alternative mechanism uniquely relates the polarity of the poloidal B field to the angular velocity of the accretion disk, resulting in a unique direction for the toroidal B field induced by disk rotation. Observations of the toroidal fields of 29 AGN jets revealed by parsec-scale Faraday rotation measurements show a clear asymmetry that is consistent with this model, with the probability that this asymmetry came about by chance being less than 1%. This lends support to the hypothesis that the universe is seeded by B fields that are generated in AGNs via this mechanism and subsequently injected into intergalactic space by the jet outflows.

Three-dimensional Magnetohydrodynamic Simulations of Relativistic Jets Injected along a Magnetic Field

We present the first numerical simulations of moderately hot, supersonic jets propagating initially along the field lines of a denser magnetized background medium with Lorentz factor W = 4.56 and evolving in a four-dimensional spacetime. Compared with previous simulations in two spatial dimensions, the resulting structure and kinematics differ noticeably: the density of the Mach disk is lower, and the head speed is smaller. This is because the impacted ambient fluid and its embedded magnetic field make efficient use of the third spatial dimension as they are deflected circularly off of the head of the jet. As a result, a significant magnetic field component normal to the jet is created near the head. If the field is strong, backflow and field reversals are strongly suppressed; upstream, the field closes back on the surface of the beam and assists the collimation of the jet. If the field is weak, backflow and field reversals are more pronounced, although still not as extended as in the corresponding plane-parallel case. In all studied cases, the high-pressure region is localized near the jet head irrespective of the presence/strength of the magnetic field, and the head decelerates efficiently by transferring momentum to the background fluid that recedes along a thin bow shock in all directions. Furthermore, two oppositely directed currents circle near the surface of the cylindrical beam, and a third current circles on the bow shock. These preliminary results underline the importance of performing fully three-dimensional simulations to investigate the morphology and propagation of relativistic extragalactic jets.

The cosmic battery revisited

We reinvestigate the generation and accumulation of magnetic flux in optically thin accretion flows around active gravitating objects. The source of the magnetic field is the azimuthal electric current associated with the Poynting-Robertson drag on the electrons of the accreting plasma. This current generates magnetic field loops that open up because of the differential rotation of the flow. We show through simple numerical simulations that what regulates the generation and accumulation of magnetic flux near the center is the value of the plasma conductivity. Although the conductivity is usually considered to be effectively infinite for the fully ionized plasmas expected near the inner edge of accretion disks, the turbulence of those plasmas may actually render them much less conducting due to the presence of anomalous resistivity. We have discovered that if the resistivity is sufficiently high throughout the turbulent disk while it is suppressed inside its inner edge, an interesting steady state process is established: accretion carries and accumulates magnetic flux of one polarity inside the inner edge of the disk, whereas magnetic diffusion releases magnetic flux of the opposite polarity to large distances. In this scenario, magnetic flux of one polarity grows and accumulates at a steady rate in the region inside the inner edge and up to the point of equipartition when it becomes dynamically important. We argue that this inward growth and outward expulsion of oppositely directed magnetic fields that we propose could account for the ~30 minute cyclic variability observed in the Galactic microquasar GRS 1915+105.

STAR-FORMING PROCESSES FAR FROM THE GALACTIC DISK: INOPERATIVE OR INDOLENT WHERE OPERATIVE

Highly supersonic collisions between gaseous clouds may effectively trigger star formation in the disk of our Galaxy, but not in the diffuse environment of the Galactic halo. This is because the observed high-velocity clouds (HVCs) are not dominated by collisions: the characteristic time between cloudlet collisions inside an HVC at an assumed distance of 10 kpc is at least 1 Gyr for collective encounters and at least 10 Gyr if a particular cloudlet is considered. In agreement with this result, we also estimate that the observed cloudlets contain smaller masses than the nonmagnetic Jeans mass that signals favorable conditions for gravitational collapse and further fragmentation in the isothermal regime. The diffuse environment observed around the Magellanic Clouds (MCs) is more difficult to understand than HVCs. Six sparse blue associations and two young B-type stars have been observed in the H I bridge between the MCs, while no stars exist in the H I cloud complexes that make up the Magellanic Stream. We discuss the conditions under which spatially sporadic star formation took place in the Magellanic Bridge during the past 16-25 Myr and the reasons for the complete absence of star formation in the Stream during its entire lifetime. We also estimate the angular resolutions that need to be achieved by follow-up radio observations of these regions that could detect cold cloudlets embedded in the gas.

Formation and destruction of jets in X-ray binaries

Context. Neutron-star and black-hole X- ray binaries (XRBs) exhibit radio jets, whose properties depend on the X-ray spectral state e.nd history of the source. In particular, black-hole XRBs emit compact,· 8teady radio jets when they are in the so-called hard state. These jets become eruptive as the sources move toward the soft state, disappear in the soft state, and then re-appear when the sources return to the hard state. The jets from neutron-star X-ray binaries are typically weaker radio emitters than the black-hole ones at the same X-ray lumin06ity and in some cases radio emission is detected in the soft state. Aims. Significant phenomenology has been developed to describe the spectral states of neutron-star and black-hole XRBs, and there is general agreement about the type of the accretion disk around the compact object in the various spectral states. We investigate whether the phenomenology describing the X-ray emission on one hand and the jet appearance and disappearance on the other can be put together in a consistent physical picture. Methods. We consider the so-called Poynting-Robertson cosmic battery (PReB), which has been shown to explain in a natural way the formation of magnetic fields in the disks of AGNs and the ejection of jets. We investigate whether the PRCB can also explain the [ormation, destruction, and variability or jets in XRBs. Results. We find excellent agreement between the conditions under which the PRCB is efficient (Le., the type of the accretion disk) and the emission or destruction of the r.adio jet. Conclusions. The disk-jet connection in XRBs can be explained in a natural way using the PReB.

The stability of accretion tori. IV: Fission and fragmentation of slender, self-gravitating annuli

We present results on the dynamical evolution of two-dimensional, geometrically slender, differentially rotating, self-gravitating annuli orbiting around a central mass. These are idealized models of thick (pressure supported) accretion disks. The initial axisymmetric equilibrium models we have studied are compressible with polytropic index n=1 and have constant specific angular momentum. The linear growth rates of the different modes are calculated through a solution of the linearized fluid equations. Some model evolutions have also been carried into the nonlinear regime using a two-dimensional, Eulerian hydro code that is second-order accurate in both space and time

Revisiting the dynamical case for a massive black hole in IC10 X-1

The relative phasing of the X-ray eclipse ephemeris and optical radial velocity (RV) curve for the X-ray binary IC10 X-1 suggests the He[

A photometric and dynamical study of the Helix galaxy NGC 2685

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The crucial role of cooling in the making of molecular clouds and stars

4686] emission-line originates in a shadowed sector of the stellar wind that avoids ionization by X-rays from the compact object. The line attains maximum blueshift when the wind is directly toward us at mid X-ray eclipse, as is also seen in Cygnus X-3. If the RV curve is unrelated to stellar motion, evidence for a massive black hole evaporates because the mass function of the binary is unknown. The reported X-ray luminosity, spectrum, slow QPO, and broad eclipses caused by absorption/scattering in the WR wind are all consistent with either a low-stellar-mass BH or a NS. For a NS, the centre of mass lies inside the WR envelope whose motion is then far below the observed 370 km/s RV amplitude, while the velocity of the compact object is as high as 600 km/s. The resulting 0.4\% doppler variation of X-ray spectral lines could be confirmed by missions in development. These arguments also apply to other putative BH binaries whose RV and eclipse curves are not yet phase-connected. Theories of BH formation and predicted rates of gravitational wave sources may need revision.

Dominance of outflowing electric currents on decaparsec to kiloparsec scales in extragalactic jets

We present optical and near-infrared surface photometry of the Helix galaxy NGC 2685 and its two rings. We detected the inner (polar) ring in the near-infrared J-band and found that its colors are consistent with a normal initial mass function and an age of at most 5-6 Gyr. The inner ring is almost as red as the central galaxy suggesting that it is a long-lived structure. The central galaxy may be either an S0 or an E6/E7 galaxy and has a reddened nucleus. We have constructed gas-dynamical models of the inner self-gravitating ring in a variety of potentials and searched for long-lived models of both rings. These models indicate that the overall potential is either generally oblate-like triaxial or prolate-like triaxial in shape