Search

We selected a sample of 24 XMM-Newton light curves (LCs) of four high energy peaked blazars, PKS 0548 - 322, ON 231, 1ES 1426+428, and PKS 2155 - 304. These data comprise continuous LCs of 7.67-18.97 hr in length. We searched for possible quasi-periodic oscillations (QPOs) and intra-day variability (IDV) timescales in the LCs of these blazars. We found a likely QPO in one LC of PKS 2155 - 304 which was reported elsewhere. In the remaining 23 LCs we found hints of possible weak QPOs in one LC of each ON 231 and PKS 2155 - 304, but neither is statistically significant. We found IDV timescales that ranged from 15.7 to 46.8 ks in eight LCs. In 13 LCs any variability timescales were longer than the length of the data. Assuming that the possible weak QPO periods in the blazars PKS 2155 - 304 and ON 231 are real and are associated with the innermost portions of their accretion disk, we can estimate that their central black hole masses exceed 1.2 x 107 M . Emission models for radio-loud active galactic nuclei that could explain our results are briefly discussed.

We have performed a structure function analysis of the Rossi X-Ray Timing Explorer All-Sky Monitor data to search for variability in 24 blazars using data trains that each exceed 12 yr. Although 20 of them show nominal periods through this technique, the great majority of these "periods" are clearly related to yearly variations arising from the instrument. Nonetheless, an apparently real periodic component of about 17 days was detected for the blazar AO 0235+164 and it was confirmed by discrete correlation function and periodogram analyses. For 1ES 2321+419, a component of variability with a near periodicity of about 420 days was detected by all of these methods. We discuss several possible explanations for these nearly periodic components and conclude that they most likely arise from the intersections of a shock propagating down a relativistic jet that possesses a helical structure.

We present the results of a periodicity search of 20 intra-day variable optical light curves of the blazar S5 0716+714, selected from a database of 102 light curves spanning over three years. We use a wavelet analysis technique along with a randomization test and find strong candidates for nearly periodic variations in eight light curves, with probabilities ranging from 95% to >99%. This is the first good evidence for periodic, or more precisely, quasi-periodic, components in the optical intra-day variable light curves of any blazar. Such periodic flux changes support the idea that some active galactic nuclei variability, even in blazars, is based on accretion disk fluctuations or oscillations. These intra-day variability timescales are used to estimate that the central black hole of the blazar S5 0716+714 has a mass >2.5 x 106 M . As we did not find any correlations between the flux levels and intra-day variability timescales, it appears that more than one emission mechanism is at work in this blazar.

We investigate aspects of the cosmological evolution of FR II radio galaxies, focusing first on the abilities of models to match data for linear sizes, radio powers, redshifts, and spectral indices. Here we consider modifications to the theoretical models we had treated earlier, primarily by accounting for the growth of the radius of hot spots with source size. Better fits to the distributions of most of the data in three low-frequency surveys can be found with sensible choices of model parameters, but no model yet considered gives a good match to all of the survey data simultaneously, nor does any do a good job of producing the spectral index distributions. The observational data sets are too small to completely discriminate among the models. We calculate the volume fraction of the "relevant universe" cumulatively occupied by the expanding radio galaxy lobes over the quasar era, when these powerful radio galaxies were much more common, and when they have been argued to play an important role in triggering galaxy formation and spreading magnetic fields and metals. We found the cumulative relevant volume filling factor of radio galaxies to be ~5%, so we conclude that these impacts are smaller than previously estimated but that they are still significant.

We have studied three-dimensional hydrodynamic interactions of relativistic extragalactic jets with two-phase ambient media. These jets propagate through a denser homogeneous gas and then impact clouds with densities 100 to 1000 times higher than the initial beam density. The deflection angle of the jet is influenced more by the density contrast of the cloud than by the beam Mach number of the jet. A relativistic jet with low relativistic beam Mach number can eventually be slightly bent after it crosses the dense cloud; however, we have not seen permanently bent structures in the interaction of a high relativistic beam Mach number jet with a cloud. The relativistic jet impacts on dense clouds do not necessarily destroy the clouds completely, and much of the cloud body can survive as a coherent blob. This enhancement of cloud durability is partly due to the geometric influence of the off-axis collisions we consider and also arises from the lower rate of cloud fragmentation through the Kelvin-Helmholtz instability for relativistic jets. To compare our simulations with observed extragalactic radio jets, we have computed the approximate surface distributions of synchrotron emission at different viewing angles. These surface intensity maps show that relativistic jets interacting with clouds can produce synchrotron emission knots similar to structures observed in many VLBI-scale radio sources. We find that the synchrotron emission increases steeply at the moment of impact and the emission peaks right before the jet passes through the cloud.

Three-dimensional simulations of hydrodynamic jets are computed at rather high resolution using the ZEUS-3D code. The parameters we employ are suitable for moderate-to-high-power radio jets emerging through a galactic atmosphere or halo and eventually crossing a tilted pressure-matched interface with a hotter intracluster medium. Before they cross this interface, these simulations aim the jets so that they hit massive clouds within the galactic halo, with densities 10 or more times higher than the ambient atmospheric density and hundreds to thousands times the jet density. Such clouds are set up with radii several times that of the jet and could correspond to giant molecular cloud complexes or small cannibalized galaxies. We find that while powerful jets eventually disperse the clouds, for off-center collisions, nonaxisymmetric instabilities are induced in those jets. Those instabilities grow faster for lower Mach number jets and can disrupt the jets substantially sooner than occurs for similar simulations of jets not hitting clouds. Such interactions could be related to some compact steep-spectrum source morphologies. Weak jets can be effectively halted or destroyed by reasonably massive clouds, and this type of interaction may have relevance for the paucity of extended radio jets in spiral galaxies. Slow, dense jets may be bent yet remain stable for fairly extended times, and such interactions can be responsible for some of the wide-angle tail and most of the "dogleg" radio source morphologies.

We present evidence for a striking new feature of powerful radio galaxies: gigantic disklike structures of thermal gas and dust oriented roughly perpendicular to the radio jets and situated around the elliptical host galaxy. These superdisks, or fat pancakes, appear to have a typical diameter of at least 75 kpc and a width of ~25 kpc. Quite plausibly, they are tidally stretched remnants of gas rich disk galaxies, or damped Lyman-a clouds, captured by the massive elliptical, and heated by its hot coronal gas. Observational manifestations of the superdisks include the sharp, quasi-linear, edges of the radio lobes on the side facing the central elliptical, detected in at least a dozen well mapped radio galaxies at low to moderate redshifts. For radio galaxies with z > 1.8, evidence for superdisks is based on the apparent asymmetry of diffuse Lya emission associated with the radio lobes, which is a very sensitive tracer of dust. This asymmetry can be understood if the brighter Lya emission is associated with the radio lobe on the near side of the nucleus and hence not obscured by the dust in the superdisk. Superdisks can provide more consistent explanations for some of the best established correlations among the radio source properties: namely, the Laing-Garrington effect and the correlated radio-optical asymmetry. They are also likely to be responsible for part of the low-frequency flux variability of compact radio sources. We argue they may provide an improved explanation of Lya absorption dips in the emission profiles of distant radio galaxies. Additional methods of searching for superdisks and testing this scenario are proposed.

Models for active galactic nuclei and X-ray binaries where "cold" rotating matter is illuminated by a nonthermal source above it have been widely studied, and they successfully explain the main spectral features of black hole candidates, including reflection humps above 10 keV and the iron line at 6.4 keV. Making the reasonable assumptions that the innermost part of an accretion disk around a black hole is unstable and clumpy and that it is illuminated by X-ray sources originating above it, we compute the X-ray variability induced by both a clump of matter moving relativistically around the black hole and the nonthermal source moving above the disk. One interesting new result we find is that even for a face-on geometry, rotation-induced variability can still be observed as long as the nonthermal source is not exactly at the symmetry axis; this is true even if the accretion disk is smooth. We also show that the reflected X-ray component, peaked around 10 or several tens of keV, can vary more than other spectral components, such as the thermal emission from the disk and the external power-law flux, and that it carries richer information on the nonthermal sources. This is because the reflected component experiences a double path through the gravitational field of the central mass, i.e., from the source to the disk and then from the disk to the observer. Thus our study is most relevant to X-ray variability of black hole candidates where relativistic effects are strong. These results may provide a partial explanation of why some Seyfert 1 galaxies exhibit rapid X-ray variability with timescales comparable to the fastest orbital periods, and why soft X-rays and UV emission generally show slower variability than do the hard X-rays.

Making the usual assumption that the relatively cold matter within the central engine of an active galactic nucleus (or galactic black hole candidate) is in the form of a relativistic accretion disk, we compute the composite spectrum of the original disk plus a primary X-ray power-law source illuminating it from above, as well as the reflected emission from the disk. All special and general relativistic effects on both infalling photons and outgoing photons are considered in a Schwarzschild geometry. The strength, shape, and broadening of the reflected spectrum depend on the direction of the X-ray source relative to the disk and the observer's viewing angle. The reflected photons extract energy and angular momentum from the relativistically rotating accretion disk and are beamed in the direction of the disk velocity. The reflection hump could essentially disappear if viewed far from the symmetry axis because the X-ray photons are affected by gravity both approaching and leaving the disk. This may produce a difference between X-ray spectra for Seyfert 1 and Seyfert 2 galaxies. For a given observation angle, the reflection hump is most sensitive to the inclination of the source relative to the accretion disk. Thus the spectral shape may also shed light on the location of the primary X-ray source, which is probably either in a jet or in a corona; however, additional computations involving distributed sources will be necessary before detailed comparisons with observations are feasible.

In the innermost part of an accretion disk around a black hole, electron scattering could provide the dominant opacity, and so the X-ray radiation originating from this hot region is expected to be partially linearly polarized. If strong magnetic fields are present on or above the disk, then synchrotron radiation from electrons may also contribute to the polarization, although different orientations of the magnetic field and Faraday rotation might reduce the effect. Both observational and theoretical studies suggest that the inner disk region is unstable and could appear "clumpy." In this paper we investigate polarization features due to polarized orbiting clumps around a black hole. It is found that, in contrast to the Newtonian case, rapid polarization variability can be produced by those regions emitting extra radiation, and that the variability amplitudes of both the degree of polarization and the angle of the polarization plane are energy dependent, i.e., the polarization variability amplitudes are larger at higher energy. This feature will not appear if the central object is not gravitationally strong, even when polarized clumps rotate around the object, and this trend depends only weakly on the local physics, such as the specific polarization mechanism or optical depth of the sources. Energy-dependent polarization variability is a direct result of near-field bending of light rays by the central black hole, and it is unique to black hole systems involving accretion disks. Since accretion disks around black holes are expected in both active galactic nuclei and X-ray binaries, we look for future X-ray polarimetry missions to confirm our prediction of this phenomenon.

Three-dimensional numerical simulations of light supersonic hydrodynamic jets have been performed to quantify the crucial roles of the interstellar medium (ISM) and intracluster medium (ICM) in defining the gross morphologies of powerful radio galaxies. Such a jet emerges through a power-law atmosphere (ISM) of its host galaxy and then crosses into a hotter, but less dense, ICM. Our eight medium-resolution simulations are followed to lengths of 45 initial jet radii. Simulations with different jet velocities, jet densities, extensions of the ISM (along the jet's axis), and inclination angles of the ISM/ICM interface are compared.

A general relativistic formula for the flux density ratio of a nuclear jet to its counterjet is obtained. The key general relativistic effect, i.e., light bending by the black hole, favors the visibility of the counterjet. The formula is significantly different from the one that considers only Doppler effects when the angle between a line of sight and the jet, i, is small, say, i < 10deg. It is found that gravitational lensing plays a vital role if i ~ 1deg, although the probability of this effect dominating is small. It is possible for a counterjet to be visible even if the jet has a high value of Lorentz factor, g. Lensed counterjets will only be visible for subluminal sources. We apply the more general formula to the recently observed counterjet in NGC 1275 and show that the value of i is slightly smaller than when derived by the traditional formula. For reasonable assumptions about other parameters, we set limits on the viewing angle of i > 16deg and on the spectral index of a < 1.6.

The bright spot model explains in a natural way the variability coherence observed in black hole candidates at different X-ray energies.