Radio Variability of Radio Quiet and Radio Loud Quasars

The majority of quasars are weak in their radio emission, with flux densities comparable to those in the optical, and energies far lower. A small fraction, about 10%, are hundreds to thousands of times stronger in the radio. Conventional wisdom holds that there are two classes of quasars, the radio quiets and radio louds, with a deficit of sources having intermediate power. Are there really two separate populations, and if so, is the physics of the radio emission fundamentally different between them? This paper addresses the second question, through a study of radio variability across the full range of radio power, from quiet to loud. The basic findings are that the root mean square amplitude of variability is independent of radio luminosity or radio-to-optical flux density ratio, and that fractionally large variations can occur on timescales of months or less in both radio quiet and radio loud quasars. Combining this with similarities in other indicators, such as radio spectral index and the presence of VLBI-scale components, leads to the suggestion that the physics of radio emission in the inner regions of all quasars is essentially the same, involving a compact, partially opaque core together with a beamed jet.Comment: 32 pages, 9 figures. Astrophysical Journal, in pres

The White Dwarfs within 20 Parsecs of the Sun: Kinematics and Statistics

We present the kinematical properties, distribution of spectroscopic subtypes, stellar population subcomponents of the white dwarfs within 20 pc of the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc sample of 129 white dwarfs nor is there convincing evidence of genuine thick disk subcomponent members within 20 parsecs. Virtually the entire 20 pc sample likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc sample is 1.6, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs. The addition of 5 new stars to the 20 pc sample yields a revised local space density of white dwarfs of $4.9\pm0.5 \times 10^{-3}$ M_{\sun}/yr and a corresponding mass density of $3.3\pm0.3 \times 10^{-3}$ M_{\sun}/pc$^{3}$. We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the DAZ and DZ stars) have photospheric metals that possibly originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, this suggests the possibility that the same percentage have planets or asteroid-like bodies orbiting them.Comment: Accepted for publication in The Astronomical Journa

A comparison of the optical properties of radio-loud and radio-quiet quasars

We have made radio observations of 87 optically selected quasars at 5 GHz with the VLA in order to measure the radio power for these objects and hence determine how the fraction of radio-loud quasars varies with redshift and optical luminosity. The sample has been selected from the recently completed Edinburgh Quasar Survey and covers a redshift range of 0.3 < z < 1.5 and an optical absolute magnitude range of -26.5 < M_{B} < -23.5 (h, q_{0} = 1/2). We have also matched up other existing surveys with the FIRST and NVSS radio catalogues and combined these data so that the optical luminosity-redshift plane is now far better sampled than previously. We have fitted a model to the probability of a quasar being radio-loud as a function of absolute magnitude and redshift and from this model infer the radio-loud and radio-quiet optical luminosity functions. The radio-loud optical luminosity function is featureless and flatter than the radio-quiet one. It evolves at a marginally slower rate if quasars evolve by density evolution, but the difference in the rate of evolutions of the two different classes is much less than was previously thought. We show, using Monte-Carlo simulations, that the observed difference in the shape of the optical luminosity functions can be partly accounted for by Doppler boosting of the optical continuum of the radio-loud quasars and explain how this can be tested in the future.Comment: 33 pages, 9 postscript figures, uses the AAS aaspp4 LaTeX style file, to appear in the 1 February 1999 issue of The Astrophysical Journa

Nova Cygni 2001/2 = V2275 Cyg

We present an analysis of low- and medium resolution spectra of the very fast nova, Nova Cygni 2001/2 (V2275 Cyg) obtained at nine epochs in August, September and October, 2001. The expansion velocity from hydrogen Balmer lines is found to be 2100 km/s, although early H-alpha profile showed a weak feature at -3500 km/s, too. The overall appearance of the optical spectrum is dominated by broad lines of H, He and N, therefore, the star belongs to the ``He/N'' subclass of novae defined by Williams (1992). Interstellar lines and bands, as well as BV photometry taken from the literature yielded to a fairly high reddening of E(B-V)=1.0+/-0.1 mag. The visual light curve was used to deduce M_V by the maximum magnitude versus rate of decline relationship. The resulting parameters are: t_0=2452141.4(+0.1)(-0.5), t_2=2.9+/-0.5 days, t_3=7+/-1 days, M_V=-9.7+/-0.7 mag. Adopting these parameters, the star lies between 3 kpc and 8 kpc from the Sun.Comment: 5 pages, 5 figures, accepted for publication in A&

Plutonium isotopic determination from gamma-ray spectra

The use of low- and medium-resolution room-temperature detectors for the nondestructive assay of nuclear materials has widespread applications to the safeguarding of nuclear materials. The challenge to using these detectors is the inherent difficulty of the spectral analysis to determine the amount of specific nuclear materials in the measured samples. This is especially true for extracting plutonium isotopic content from low- and medium-resolution spectral lines that are not well resolved. In this paper, neural networks trained by stochastic and singular value decomposition algorithms are applied to retrieve the plutonium isotopic content from a simulated NaI spectra. The simulated sample consists of isotopes {sup 238}Pu, {sup 239}Pu, {sup 240}Pu, {sup 241}Pu, {sup 242}Pu, and {sup 241}Am. It is demonstrated that the neutral network optimized by singular value decomposition (SVD) and stochastic training algorithms is capable of estimating plutonium content consistently resulting in an average error much smaller than the error previously reported

Mechanism of Magnetic Flux Loss in Molecular Clouds

We investigate the detailed processes working in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities $n_{\rm H} < 10^4 {\rm cm}^{-3}$, and charged grains contribute more than 90% at $n_{\rm H} > 10^6 {\rm cm}^{-3}$. Thus grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time $t_B$ by a power law $t_B \propto B^{-\gamma}$, where $B$ is the mean field strength in the cloud, we find $\gamma \approx 2$, characteristic to ambipolar diffusion, only at $n_{\rm H} < 10^7 {\rm cm}^{-3}$. At higher densities, $\gamma$ decreases steeply with $n_{\rm H}$, and finally at $n_{\rm H} \approx n_{\rm dec} \approx {\rm a few} \times 10^{11} {\rm cm}^{-3}$, where magnetic fields effectively decouple from the gas, $\gamma << 1$ is attained, reminiscent of Ohmic dissipation, though flux loss occurs about 10 times faster than by Ohmic dissipation. Ohmic dissipation is dominant only at $n_{\rm H} > 1 \times 10^{12} {\rm cm}^{-3}$. While ions and electrons drift in the direction of magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, where grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities as $n_{\rm H} \approx n_{\rm dec}$, the process going on at high densities is quite different from ambipolar diffusion in which particles of opposite charges are supposed to drift as one unit.Comment: 34 pages including 9 postscript figures, LaTex, accepted by Astrophysical Journal (vol.573, No.1, July 1, 2002

Emission Line Flickering from the Secondary Star in Cataclysmic Variables? A study of V3885 Sagitarii

Spectrophotometric observations of H-alpha and He I 6678 emission lines of the nova-like Cataclysmic Variable V3885 Sgr are presented and analyzed. The binary orbital period was determined as P = 0.20716071(22) days. Doppler Tomography was performed with both H-alpha and He I lines. Disc radial emissivity profiles were also computed. The tomography mapping of flickering sources was performed using the H-alpha line, from which we concluded that the flickering is not uniformly distributed on the disc. The observed tomogram of the flickering was compared with simulations, suggesting that the most intense flickering source in the H-alpha is not located in the accretion disc. It is proposed that the main line flickering source may be associated with the illuminated secondary star.Comment: 19 pages, 10 figures, 1 table. Accepted to be published on AJ. to donwload high resolution figures: http://www.astro.iag.usp.br/~fabiola/V3885_hires.pd

Disclosing the Radio Loudness Distribution Dichotomy in Quasars: An Unbiased Monte Carlo Approach Applied to the SDSS-FIRST Quasar Sample

We investigate the dichotomy in the radio loudness distribution of quasars by modelling their radio emission and various selection effects using a Monte Carlo approach. The existence of two physically distinct quasar populations, the radio-loud and radio-quiet quasars, is controversial and over the last decade a bimodal distribution of radio loudness of quasars has been both affirmed and disputed. We model the quasar radio luminosity distribution with simple unimodal and bimodal distribution functions. The resulting simulated samples are compared to a fiducial sample of 8,300 quasars drawn from the SDSS DR7 Quasar Catalog and combined with radio observations from the FIRST survey. Our results indicate that the SDSS-FIRST sample is best described by a radio loudness distribution which consists of two components, with 12+/-1 % of sources in the radio-loud component. On the other hand, the evidence for a local minimum in the loudness distribution (bimodality) is not strong and we find that previous claims for its existence were probably affected by the incompleteness of the FIRST survey close to its faint limit. We also investigate the redshift and luminosity dependence of the radio loudness distribution and find tentative evidence that at high redshift radio-loud quasars were rarer, on average "louder", and exhibited a smaller range in radio loudness. In agreement with other recent work, we conclude that the SDSS-FIRST sample strongly suggests that the radio loudness distribution of quasars is not a universal function, and that more complex models than presented here are needed to fully explain available observations.Comment: Accepted for publication in ApJ; 13 pages, 10 figure