Comptonization signatures in the rapid aperiodic variability of Galactic black-hole candidates

We investigate the effect of inverse-Compton scattering of flares of soft radiation in different geometries of a hot, Comptonizing region and a colder accretion disk around a solar-mass black hole. The photon-energy dependent light curves, their Fourier transforms, power spectra and Fourier-period dependent time lags of hard photons with respect to softer photons are discussed. On the basis of a comparison with existing data we find arguments against Comptonization of external soft radiation as well as Comptonization in a homogeneous medium as dominant mechanisms for the rapid aperiodic variability in Galactic black-hole candidates. Possible further observational tests for the influence of Comptonization on the rapid aperiodic variability of Galactic black-hole candidates are suggested.Comment: 32 pages, including 10 figures and 2 tables; uses epsf.sty, rotate.sty; submitted to Ap

Analyzing the Multiwavelength Spectrum and Variability of BL Lacertae During the July 1997 Outburst

The multiwavelength spectrum of BL Lacertae during its July 1997 outburst is analyzed in terms of different variations of the homogeneous leptonic jet model for the production of high-energy radiation from blazars. We find that a two-component gamma-ray spectrum, consisting of a synchrotron self-Compton and an external Compton component, is required in order to yield an acceptable fit to the broadband spectrum. Our analysis indicates that in BL Lac, unlike other BL Lac objects, the broad emission line region plays an important role for the high-energy emission. Several alternative blazar jet models are briefly discussed. In the appendix, we describe the formalism in which the process of Comptonization of reprocessed accretion disk photons is treated in the previously developed blazar jet simulation code which we use.Comment: Now accepted for publication in The Astronomical Journal. Significantly extended discussion w.r.t. original version. 3 Figures included using epsf.sty, rotate.st

Transformation Properties of External Radiation Fields, Energy-Loss Rates and Scattered Spectra, and a Model for Blazar Variability

We treat transformation properties of external radiation fields in the proper frame of a plasma moving with constant speed. The specific spectral energy densities of external isotropic and accretion-disk radiation fields are derived in the comoving frame of relativistic outflows, such as those thought to be found near black-hole jet and gamma-ray burst sources. Nonthermal electrons and positrons Compton-scatter this radiation field, and high-energy protons and ions interact with this field through photomeson and photopair production. We revisit the problem of the Compton-scattered spectrum associated with an external accretion-disk radiation field, and clarify a past treatment by the authors. Simple expressions for energy-loss rates and Thomson-scattered spectra are given for ambient soft photon fields consisting either of a surrounding external isotropic monochromatic radiation field, or of an azimuthally symmetric, geometrically thin accretion-disk radiation field. A model for blazar emission is presented that displays a characteristic spectral and variability behavior due to the presence of a direct accretion-disk component. The disk component and distinct flaring behavior can be bright enough to be detected from flat spectrum radio quasars with {\it GLAST}. Spectral states of blazars are characterized by the relative importance of the accretion-disk and scattered radiation fields and, in the extended jet, by the accretion disk, inner jet, and cosmic microwave background radiation fields.Comment: 43 pages, 12 figures, ApJ, in press; includes improvements in response to referee report, added references, section of detectability with GLAS

Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell

We discuss the space-and-time-dependent Monte Carlo code we have developed to simulate the relativistic radiation output from compact astrophysical objects, coupled to a Fokker-Planck code to determine the self-consistent lepton populations. We have applied this code to model the emission from a magnetized neutron star accretion shell near the Alfven radius, reprocessing the radiation from the neutron sar surface. We explore the parameter space defined by the accretion rate, stellar surface field and the level of wave turbulence in the shell. Our results are relevant to the emission from atoll sources, soft-X-ray transient X-ray binaries containing weakly magnetized neutron stars, and to recently suggested models of accretion-powered emission from anomalous X-ray pulsars.Comment: 24 pages, including 7 figures; uses epsf.sty. final version, accepted for publication in ApJ. Extended introduction and discussio

The redshift-dependence of gamma-ray absorption in the environments of strong-line AGN

The case of gamma-ray absorption due to photon-photon pair production of jet photons in the external photon environment like accretion disk and broad-line region radiation field of gamma-ray loud active galactic nuclei (AGN) that exhibit strong emission lines is considered. I demonstrate that this ''local opacity'', if detected, will almost unavoidably be redshift-dependent in the sub-TeV range. This introduces non-negligible biases, and complicates approaches for studying the evolution of the extragalactic background light with contemporary GeV instruments like e.g. the Gamma-ray Large Area Space Telescope (GLAST), etc., where the gamma-ray horizon is probed by means of statistical analysis of absorption features (e.g. Fazio-Stecker relation, etc.) in AGN spectra at various redshifts. It particularly applies to strong-line quasars where external photon fields are potentially involved in gamma-ray production.Comment: 19 pages, 5 figures; accepted for publication in Ap

X-ray Spectral Signatures of the Photon Bubble Model for Ultraluminous X-ray Sources

The nature of ultraluminous X-ray sources in nearby galaxies is one of the major open questions in modern X-ray astrophysics. One possible explanation for these objects is an inhomogeneous, radiation dominated accretion disk around a $\sim 10 M_{\odot}$ black hole -- the so-called ``photon bubble'' model. While previous studies of this model have focused primarily on its radiation-hydrodynamics aspects, in this paper, we provide an analysis of its X-ray spectral (continuum and possible edge and line) characteristics. Compton reflection between high and low density regions in the disk may provide the key to distinguishing this model from others, such as accretion onto an intermediate mass black hole. We couple a Monte Carlo/Fokker-Planck radiation transport code with the XSTAR code for reflection to simulate the photon spectra produced in a photon bubble model for ULXs. We find that reflection components tend to be very weak and in most cases not observable, and make predictions for the shape of the high-energy Comptonizing spectra. In many cases the Comptonization dominates the spectra even down to $\sim$ a few keV. In one simulation, a \sim 9 \kev feature was found, which may be considered a signature of photon bubbles in ULXs; furthermore, we make predictions of high energy power-laws which may be observed by future instruments.Comment: Accepted for publication in the Astrophysical Journa

Effects of geometric anisotropy on local field distribution: Ewald-Kornfeld formulation

We have applied the Ewald-Kornfeld formulation to a tetragonal lattice of point dipoles, in an attempt to examine the effects of geometric anisotropy on the local field distribution. The various problems encountered in the computation of the conditionally convergent summation of the near field are addressed and the methods of overcoming them are discussed. The results show that the geometric anisotropy has a significant impact on the local field distribution. The change in the local field can lead to a generalized Clausius-Mossotti equation for the anisotropic case.Comment: Accepted for publications, Journal of Physics: Condensed Matte

Time-Dependent Synchrotron and Compton Spectra from Jets of Microquasars

Jet models for the high-energy emission of Galactic X-ray binary sources have regained significant interest with detailed spectral and timing studies of the X-ray emission from microquasars, the recent detection by the HESS collaboration of very-high-energy gamma-rays from the microquasar LS~5039, and the earlier suggestion of jet models for ultraluminous X-ray sources observed in many nearby galaxies. Here we study the synchrotron and Compton signatures of time-dependent electron injection and acceleration, adiabatic and radiative cooling, and different jet geometries in the jets of Galactic microquasars. Synchrotron, synchrotron-self-Compton, and external-Compton radiation processes with soft photons provided by the companion star and the accretion disk are treated. An analytical solution is presented to the electron kinetic equation for general power-law geometries of the jets for Compton scattering in the Thomson regime. We pay particular attention to predictions concerning the rapid flux and spectral variability signatures expected in a variety of scenarios, making specific predictions concerning possible spectral hysteresis, similar to what has been observed in several TeV blazars. Such predictions should be testable with dedicated monitoring observations of Galactic microquasars and ultraluminous X-ray sources using Chandra and/or XMM-Newton.Comment: Accepted for publication in ApJ; 37 manuscript pages, including 10 eps figures; uses AASTeX macro

The seasonal detection of strawberry viruses in Victoria, Australia

AbstractPCR tests were adopted from international, peer-reviewed literature and developed for the detection of Strawberry mottle sadwavirus (SMoV), Strawberry crinkle cytorhabdovirus (SCV), Strawberry mild yellow edge potexvirus (SMYEV), Strawberry vein banding caulimovirus (SVBV), Beet pseudos yellows crinivirus (BPYV), and Strawberry pallidosis associated crinivirus (SPaV) in Victoria, Australia. The PCR tests were applied to 23 positive control plants infected with one or more viruses and these plants have been tested monthly from May 2005 to April 2007. Our results have indicated that the viruses were most reliably detected by PCR during May-October. In November, December and January of each year a decline in the number of positive PCR results for BPYV, SVBV and SPaV was observed. Twelve positive control plants maintained at the AQIS post entry quarantine screenhouse at Knoxfield, Victoria, and also infected with one or more viruses, were tested monthly from August 2006 to April 2007. A similar decline in the ability to detect SMoV, SVBV and SPaV in the AQIS positive control plants was observed in 2006/07 and November was the least reliable month for detection of strawberry viruses in these plants. These results indicate that spring and autumn may be the optimal times for PCR detection of strawberry viruses in south east Australia.Keywords: Strawberry mottle sadwavirus; Strawberry crinkle cytorhabdovirus; Strawberry mild yellow edge potexvirus; Strawberry vein banding caulimovirus; Beet pseudos yellows crinivirus; Strawberry pallidosis associated crinivirus; detection; polymerase chain reaction; PCR; certificatio