John F. Kartje

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.

Gamma-Ray Variability of the BL Lacertae Object Markarian 421

We report on the γ-ray variability of Mrk 421 at Eγ > 300 GeV during the 1995 season, and concentrate on the results of an intense multiwavelength observing campaign in the period April 20 to May 5, which included >100 MeV γ-ray, X-ray, extreme-ultraviolet, optical, and radio observations, some of which show evidence for correlated behavior. Rapid variations in the TeV γ-ray light curve with doubling and decay times of 1 day require a compact emission region and significant Doppler boosting. The TeV data reveal that the γ-ray emission is best characterized by a succession of rapid flares with a relatively low baseline level of steady emission.

The Extreme Ultraviolet Spectrum of the BL Lacertae Object Markarian 421

We carried out a spectroscopic observation of the BL Lacertae object Mrk 421 with the Extreme Ultraviolet Explorer (EUVE) over an 11 day period in 1995 late April to early May (~242 ks useful time). During this period, the source underwent a flare that was detected also in X-rays and TeV γ-rays. The best continuous coverage of the flare was obtained by EUVE, which resolved the smooth rise and fall of the flux, measuring a variability of as much as a factor of ~1.5 over a span of ~2 days. The detected spectrum extended from ~65 to 100 A and could be fitted with a power law of energy spectral index αEUV ≈ 3.5 ± 0.8 for the measured Galactic hydrogen column density. The EUV spectrum is much steeper than the mean 1.5-7.5 keV X-ray spectrum, αX = 1.63 ± 0.02, measured simultaneously by the Advanced Satellite for Cosmology and Astrophysics. Furthermore, a simple power-law fit to the observed fluxes at 85 A and 1.5 keV (excluding the data from the first 4 days, the time of maximum variability) significantly overestimates the flux at the shortest detected EUV wavelengths. These two findings imply that strong absorption is occurring between ~65 and ~75 A. Such absorption is quite similar to that detected previously in our observation of the BL Lacertae object PKS 2155-304. We demonstrate that this absorption can be attributed to a superposition of Doppler-smeared absorption lines originating in high-velocity, QSO-type nuclear clouds of total column density ~5 × 1021 cm-2 that are ionized by the beamed continuum of the associated relativistic jet. We identify the lines as mostly L- and M-shell transitions of Mg and Ne. The data suggest that the velocity range spanned by the clouds is relatively small (from vi ≈ 0.05c to vf ≈ 0.1c). We find that such a range is consistent with a scenario in which the clouds are initially accelerated to vi by a magnetized outflow from a nuclear accretion disk, with radiation pressure further accelerating them to vf after they enter the beamed emission cone of the jet. We also compute the expected cloud absorption lines in the UV and soft X-ray regimes and use these results to constrain the physical parameters of the clouds and their chemical composition.

A portrait of the torus as a disk-driven hydromagnetic wind

Abstract Centrifugally driven winds originating from the surfaces of magnetized accretion disks in AGNs possess unique observational characteristics owing to their highly stratified structure and their exposure to the central engines radiation field. The obscuration and radiation properties of such winds may provide the physical basis for the dusty tori invoked in various AGN unification schemes. We discuss the angular-dependent optical and X-ray properties of such winds, their infrared emission, and their capacity to polarize the central source continuum spectrum via electron/dust scattering and dichroic extinction by magnetically aligned grains.

High energy emission from ultrarelativistic jets

We consider the interaction of an initially ultrarelativistic beam of electrons and protons with the radiation field near the accretion disk of an Active Galactic Nucleus (AGN). The particles in such a beam suffer a strong radiative drag, photoproducing off and upscattering the ambient radiation field. The upscattered radiation is also beamed and can carry away a significant fraction of the luminosity initially in the particle beam. Such a mechanism may be responsible for the strong γ‐ray emission seen by EGRET in objects like 3C279. We study the evolution of the beam in the anisotropic, inhomogeneous disk radiation field using an implicit kinetic code.When the Lorentz factors of the initial beam particles exceed ∼104, a photon‐e+e− cascade develops in the beam. The cascade effectively converts the energy of the initial particles into a large number of e+e− pairs that cool to form an outflow with terminal Lorentz factor γ∞∼10. The characteristic radiation spectrum formed in the cascade has α∼0.5 (Fν∝ν−α) ...

Water Maser Emission from Dusty Clouds in AGNs

A natural site for water maser emission in AGNs is provided by dusty gas with properties characteristic of broad line region (BLR) clouds. Radiation shielding by dust in the clouds is critical for allowing molecular gas to exist ≤ 1 pc from the central engine. Thus, the innermost radius at which such masers appear should correspond to the grain sublimation radius r sub . We suggest a dynamical model in which the masing clouds are embedded within a magnetized accretion disk wind.

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. 63 refs.

High energy spectra from the jet/disk interface

We investigated the dynamical interaction between ultrarelativistic particle beams and the local radiation fields of accretion disks in the central engines of AGNs. For electron‐proton or electron‐positron jets accelerated to initial Lorentz factors γi≲104, the particles mainly interact with the photons via Thomson scattering, which provides an effective means for both decelerating the relativistic beams and for producing the hard X‐ray spectra seen in many AGNs. For larger γi, scattering by the beam leads to the development of an e−e+ cascade, resulting from photon‐photon, photon‐electron, and photon‐photon pair production. We study the structure of these pair cascades and present preliminary calculations of the soft X‐ray through γ‐ray spectrum produced by the Comptonized disk photons.