Engineering topics

Konigl, Arieh

Publications and source records attributed to Konigl, Arieh.

10 records

NASA NTRS1994

Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes

Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within approximately 10(M(sub 8)) pc of the central 10(exp 8)(M(sub 8)) solar mass black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (greater than or approximately equal to 10(exp 5) K) and hot (greater than or approximately equal to 10(exp 6) K) gas components that have been inferred to exist on scales less than or approximately equal to 10(exp 2) pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Finally, we discuss the application of this model to the interpretation of the approximately 0.6 keV X-ray absorption feature reported in several BLOs.

Konigl, Arieh↗

NASA NTRS1994

A protostellar jet model for the water masers in W49N

Observations by Gwinn, Moran, & Reid of the proper motions of water masers in W49N show that they have elongated distribution expanding from a common center. Features with high space velocity only occur far from the center, while low-velocity features occur at all distances. We propose that these observations can be interpreted in terms of a shell of shocked molecular gas that is driven by the expanding cocoon of a high-velocity protostellar jet. We present three-dimensional numerical simulations in support of this interpretation and argue that this source provides a unique oppurtunity for a detailed study of jet-driven cocoons.

Low, Mordecai-Mark Mac↗

NASA NTRS1994

Extreme ultraviolet spectroscopy of PKS 2155-304

We present the extreme ultraviolet (75-110 A) spectrum of the BL Lac object PKS 2155-304, the first spectrum of an extragalactic source obtained with the Extreme Ultraviolet Explorer (EUVE). The spectrum shows a generally smooth continuum, which can be modeled by a single power law plus interstellar absorption, and possibly an absorption feature at approximately 80 A. The best fit to the data suggests that the EUV spectrum can be interpreted as a simple extrapolation of the X-ray continuum, with an energy index alpha is approximately = 1.6; however, shallower or steeper power laws with indices between -0.4 and 2.7 cannot be ruled out by the existing EUV data alone. The data provide strong constraints on the interstellar neutral H and He along the line of sight. Using a column density of 1.36 x 10(exp 20)/sq cm for the Galactic neutral hydrogen along the PKS 2155-304 line of sight, the neutral helium column density is constrained to be 9%-10% of the hydrogen amount.

Fruscione, Antonella↗

NASA NTRS1991

Molecular line emission models of Herbig-Haro objects. I - H2 emission

A comprehensive model for molecular hydrogen emssion in Herbig-Haro objects that are associated with the heads of radiative stellar jets is presented by using a simple representation of the jet head as a comprising a leading bow shock and a trailing jet shock, separated by a dense layer of cool shocked gas. Attention is given to collisional excitation in a nondissociative shock and formation pumping in the molecular reformation zone behind a dissociative shock, employing detailed shock and photodissociation-region emission models that incorporate most of the relevant atomic physics and chemistry. The conditions under which each of these excitation mechanisms may be expected to contribute to the observed emission are discussed, and a general diagnostic scheme for discriminating among them is constructed. Applying this scheme to the HH 1-2 system, strong evidence for excitation by the radiation field of a fast shock is found. It is inferred that FUV pumping contributes a significant fraction of the H2 line emission, and it is shown that this can occur only if the UV pump lines are not strongly self-shielded.

Wolfire, Mark G.↗

NASA NTRS1991

Scattering-induced optical polarization in thick accretion disks

A general formalism for calculating the linear polarization induced by scattering within the central funnel of a thick accretion disk is presented, and it is shown that multiple photon reflections off the funnel walls can produce polarization values of up to about 10 percent, with the polarization position angle aligned parallel to the disk symmetry axis. It is suggested that this process is responsible for the observed optical polarization levels in X-ray-selected BL Lac objects (XBLs), which generally show linear polarization percentages P less than about 10 percent. According to this interpretation, XBLs with high optical polarization are viewed at an angle of less than about 60 deg to the funnel axis and their projected polarization vectors should be preferentially aligned with the associated radio jets. The possible relevance of this model to Seyfer 1 galaxies and quasars is also discussed.

Kartje, John F.↗

NASA NTRS1990

The structure and evolution of radiatively cooling jets

The two-dimensional simulations presently used to characterize the structure and evolution of radiatively cooling supersonic jets reveal that cooling jet morphologies resemble those of adiabatic outflows, but with the fundamental difference that a dense, cold shell will condense out of the shocked gas at the head of the jet when the cooling distance behind either of the two principal shocks is smaller than the jet radius. For very high cooling rates, the material that accumulates at the head of the jet forms an extended plug of cold gas resembling the 'nose cone' observed in numerical simulations of strongly magnetized adiabatic jets. An investigation is made of the dependence of jet properties on the density ratio between the beam and the ambient medium, as well as on the strength of radiative cooling.

Blondin, John M.↗

NASA NTRS1989

The radiative deceleration of ultrarelativistic jets in active galactic nuclei

A detailed study of the dynamical interaction between a highly relativistic jet and the thermal radiation field from an AGN accretion disk is reported, and the Comptonized spectrum arising from this interaction is self-consistently determined. A simple model that captures the essential radiative and geometrical features of realistic disk configurations is presented, and the disk radiation field is calculated. The results confirm Phinney's (1987) suggestion that the thermal radiation field produced by accretion in an AGN could be very effective in decelerating ultrarelativistic jets that are accreted by electromagnetic or hydromagnetic forces closer to the central black hole. Terminal Lorentz factors are consistent with the values inferred in superluminal radio sources are readily produced in this model for plausible disk and jet parameters without additional acceleration in the interaction zone. A new interpretation of the hard X-ray component detected in BL Lac spectra is proposed.

Melia, Fulvio↗

NASA NTRS1989

Herbig-Haro objects as the heads of radiative jets

The interpretation of certain HH objects as the heads of nonadiabatic supersonic jets is examined using two-dimensional numerical simulations. It is found that radiative jets develop a dense shell between the jet shock and the leading bow shock when the cooling distance behind either one of these shocks is smaller than the jet radius. It is proposed that the radiatively cooling shell may account for the variable emission pattern from objects like HH 1. Also, it is suggested that HH objects with measured space velocities that exceed the spectroscopically inferred shock velocities could correspond to heavy jets in which the bow shock is effectively adiabatic. Low-excitation objects in which these velocities are comparable may represent light jets where the jet shock is nonradiative.

Blondin, John M.↗

NASA NTRS1987

Magnetic braking in weakly ionized media

The combined magnetic braking-ambipolar diffusion problem in weakly ionized, rigidly rotating disks is studied. An analytical solution is presented for a disk whose angular velocity and magnetic yield vectors are aligned with the symmetry axis, illustrating the effects of the relative azimuthal drift of neutrals and ions. The effects of radial drift are added, commenting on the ratio of the characteristic ambipolar diffusion and magnetic braking time scales in high-mass and low-mass disks. A numerical calculation is used to show the combined action of these two processes.

Konigl, Arieh↗

NASA NTRS1986

Magnetic energy dissipation in force-free jets

It is shown that a magnetic pressure-dominated, supersonic jet which expands or contracts in response to variations in the confining external pressure can dissipate magnetic energy through field-line reconnection as it relaxes to a minimum-energy configuration. In order for a continuous dissipation to occur, the effective reconnection time must be a fraction of the expansion time. The dissipation rate for the axisymmetric minimum-energy field configuration is analytically derived. The results indicate that the field relaxation process could be a viable mechanism for powering the synchrotron emission in extragalactic jets if the reconnection time is substantially shorter than the nominal resistive tearing time in the jet.

Choudhuri, Arnab Rai↗