Gaussian polarizable-ion tight binding

To interpret ultrafast dynamics experiments on large molecules, computer simulation is required due to the complex response to the laser field. We present a method capable of efficiently computing the static electronic response of large systems to external electric fields. This is achieved by extending the density-functional tight binding method to include larger basis sets and by multipole expansion of the charge density into electrostatically interacting Gaussian distributions. Polarizabilities for a range of hydrocarbon molecules are computed for a multipole expansion up to quadrupole order, giving excellent agreement with experimental values, with average errors similar to those from density functional theory, but at a small fraction of the cost. We apply the model in conjunction with the polarizable-point-dipoles model to estimate the internal fields in amorphous poly(3-hexylthiophene-2,5-diyl)

Characteristics of enzymatic induction provoked by chlordane

The effects of various stresses, such as restraint and lowering or raising of environmental temperature, in mice pretreated with chlordane were investigated. (Chlordane is an inhibitor of protein synthesis.) It was found that restraint or exposure to a cold environment for three hours mobilized the chlordane stored in the adipose tissue of mice

A New Equilibrium for Accretion Disks Around Black Holes

Accretion disks around black holes in which the shear stress is proportional to the total pressure, the accretion rate is more than a small fraction of Eddington, and the matter is distributed smoothly are both thermally and viscously unstable in their inner portions. The nonlinear endstate of these instabilities is uncertain. Here a new inhomogeneous equilibrium is proposed which is both thermally and viscously stable. In this equilibrium the majority of the mass is in dense clumps, while a minority reaches temperatures $\sim 10^9$ K. The requirements of dynamical and thermal equilibrium completely determine the parameters of this system, and these are found to be in good agreement with the parameters derived from observations of accreting black holes, both in active galactic nuclei and in stellar binary systems.Comment: AAS LaTeX, accepted to Ap. J. Letter

Big Blue Bump and Transient Active Regions in Seyfert Galaxies

An important feature of the EUV spectrum (known as the Big Blue Bump, hereafter BBB) in Seyfert Galaxies is the narrow range in its cutoff energy E0 from source to source, even though the luminosity changes by 4 orders of magnitude. Here we show that if the BBB is due to accretion disk emission, then in order to account for this ``universality'' in the value of E0, the emission mechanism is probably optically thin bremsstrahlung. In addition, we demonstrate that the two-phase model with active regions localized on the surface of the cold disk is consistent with this constraint if the active regions are very compact and are highly transient, i.e., they evolve faster than one dynamical time scale.Comment: Accepted for publication in ApJL, uses aas2pp4.st

Detecting Compton Reflection and a Broad Iron Line in MCG-5-23-16 with the Rossi X-ray Timing Explorer

We report the detection with the Rossi X-ray Timing Explorer of a Compton reflection signature in the Seyfert galaxy MCG-5-23-16. RXTE also resolves the Fe K-alpha fluorescence line with FWHM ~48,000 km s^{-1}. This measurement provides the first independent confirmation of ASCA detections in Seyfert galaxies of broad Fe K-alpha lines that are thought to be the signature of emission from the inner regions of an accretion disk orbiting a black hole. Under the assumption that reflection arises from an isotropic source located above a neutral accretion disk, and using a theoretical model that accounts for the dependence of the reflected spectrum on inclination angle, we derive a 90% confidence range for the disk inclination of i = 50 to 81 degrees. The large inclination is consistent with that expected from the unified model for MCG-5-23-16 based on its Seyfert 1.9 classification. If we assume that the high-energy cutoff in the incident spectrum lies at energies larger than a few hundred keV, then the equivalent width of the Fe K-alpha line is much larger than predicted for the amount of reflection. This implies either an enhanced iron abundance, a covering factor of reflecting material > 0.5, or a cutoff in the incident spectrum at energies between ~60 and ~200 keV.Comment: Accepted for publication in ApJ, LaTeX. 14 pages including 3 figures, with 1 table as a separate postscript file. Typo corrected in abstrac

Long-Term X-ray Spectral Variability in Seyfert 1 Galaxies

Direct time-resolved spectral fitting has been performed on continuous RXTE monitoring of seven Seyfert 1 galaxies in order to study their broadband spectral variability and Fe K alpha variability characteristics on time scales of days to years. Variability in the Fe K alpha line is not detected in some objects but is present in others, e.g., in NGC 3516, NGC 4151 and NGC 5548 there are systematic decreases in line flux by factors of ~2-5 over 3-4 years. The Fe K alpha line varies less strongly than the broadband continuum, but, like the continuum, exhibits stronger variability towards longer time scales. Relatively less model-dependent broadband fractional variability amplitude (Fvar) spectra also show weaker line variability compared to the continuum variability. Comparable systematic long-term decreases in the line and continuum are present in NGC 5548. Overall, however, there is no evidence for correlated variability between the line and continuum, severely challenging models in which the line tracks continuum variations modified only by a light-travel time delay. Local effects such as the formation of an ionized skin at the site of line emission may be relevant. The spectral fitting and Fvar spectra both support spectral softening as continuum flux increases.Comment: Accepted for publication in ApJ. 29 page

Discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an historical low X-ray flux state

We report the discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an extremely low X-ray state. A comparison of Swift observations obtained in May and June/July 2007 with all previous X-ray observations between 1971 to 2006 show the AGN to have diminished in flux by a factor of more than 30, the lowest X-ray flux Mkn 335 has ever been observed in. The Swift observations show an extremely hard X-ray spectrum at energies above 2 keV. Possible interpretations include partial covering absorption or X-ray reflection from the disk. In this letter we consider the partial covering interpretation. The Swift observations can be well fit by a strong partial covering absorber with varying absorption column density N_H= 1-4 x 10^{23} cm-2 and a covering fraction f_c=0.9 - 1. When corrected for intrinsic absorption, the X-ray flux of Mkn 335 varies by only factors of 4-6. In the UV Mkn 335 shows variability in the order of 0.2 mag. We discuss the similarity of Mkn 335 with the highly variable NLS1 WPVS007, and speculate about a possible link between NLS1 galaxies and broad-absorption line quasars.Comment: ApJ Letter accepted; 8 pages, 2 figures; The new version has three more sentences in the introduction and three references added to the discussio

Quasi-thermal Comptonization and gamma-ray bursts

Quasi-thermal Comptonization in internal shocks formed between relativistic shells can account for the high energy emission of gamma-ray bursts. This is in fact the dominant cooling mechanism if the typical energy of the emitting particles is achieved either through the balance between heating and cooling or as a result of electron-positron pair production. Both processes yield sub or mildly relativistic energies. In this case the synchrotron spectrum is self-absorbed, providing the seed soft photons for the Comptonization process, whose spectrum is flat [F(v) ~ const], ending either in an exponential cutoff or a Wien peak, depending on the scattering optical depth of the emitting particles. Self-consistent particle energy and optical depth are estimated and found in agreement with the observed spectra.Comment: 10 pages, ApJ Letters, accepted for publicatio

Flash-Heating of Circumstellar Clouds by Gamma Ray Bursts

The blast-wave model for gamma-ray bursts (GRBs) has been called into question by observations of spectra from GRBs that are harder than can be produced through optically thin synchrotron emission. If GRBs originate from the collapse of massive stars, then circumstellar clouds near burst sources will be illuminated by intense gamma radiation, and the electrons in these clouds will be rapidly scattered to energies as large as several hundred keV. Low-energy photons that subsequently pass through the hot plasma will be scattered to higher energies, hardening the intrisic spectrum. This effect resolves the "line-of-death" objection to the synchrotron shock model. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electron-positron pairs that can be detected within ~ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy would reveal past GRB explosions.Comment: 9 pages, 1 figure, submitted to ApJ Letter

Self-Organized Criticality in Compact Plasmas

Compact plasmas, that exist near black-hole candidates and in gamma ray burst sources, commonly exhibit self-organized non-linear behavior. A model that simulates the non-linear behavior of compact radiative plasmas is constructed directly from the observed luminosity and variability. The simulation shows that such plasmas self organize, and that the degree of non-linearity as well as the slope of the power density spectrum increase with compactness. The simulation is based on a cellular automaton table that includes the properties of the hot (relativistic) plasmas, and the magnitude of the energy perturbations. The plasmas cool or heat up, depending on whether they release more or less than the energy of a single perturbation. The energy release depends on the plasmas densities and temperatures, and the perturbations energy. Strong perturbations may cool the previously heated plasma through shocks and/or pair creation. New observations of some active galactic nuclei and gamma ray bursters are consistent with the simulationComment: 9 pages, 5 figures, AASTeX, Submitted to ApJ