4,458 research outputs found
Systematic Errors in the Estimation of Black Hole Masses by Reverberation Mapping
The mass of the central black hole in many active galactic nuclei has been
estimated on the basis of the assumption that the dynamics of the broad
emission line gas are dominated by the gravity of the black hole. The most
commonly-employed method is to estimate a characteristic size-scale from
reverberation mapping experiments and combine it with a characteristic velocity
taken from the line profiles; the inferred mass is then estimated by . We critically discuss the evidence supporting the assumption of
gravitational dynamics and find that the arguments are still inconclusive. We
then explore the range of possible systematic error if the assumption of
gravitational dynamics is granted. Inclination relative to a flattened system
may cause a systematic underestimate of the central mass by a factor , where is the aspect ratio of the flattening. The coupled
effects of a broad radial emissivity distribution, an unknown angular radiation
pattern of line emission, and sub-optimal sampling in the reverberation
experiment can cause additional systematic errors as large as a factor of 3 or
more in either direction.Comment: 19 pages, 4 figures, AASLaTeX, accepted by Ap
The analysis of solar models: Neutrinos and oscillations
Tests of solar neutrino flux and solar oscillation frequencies were used to assess standard stellar structure theory. Standard and non-standard solar models are enumerated and discussed. The field of solar seismology, wherein the solar interior is studied from the measurement of solar oscillations, is introduced
Molecular footprint of drug-selective pressure in a human immunodeficiency virus transmission chain
Known human immunodeficiency virus (HIV) transmission histories are invaluable models for investigating the evolutionary and transmission dynamics of the virus and to assess the accuracy of phylogenetic reconstructions. Here we have characterized an HIV-1 transmission chain consisting of nine infected patients, almost all of whom were treated with antiviral drugs at later stages of infection. Partial pol and env gp41 regions of the HIV genome were directly sequenced from plasma viral RNA for at least one sample from each patient. Phylogenetic analyses in pol using likelihood methods inferred an evolutionary history not fully compatible with the known transmission history. This could be attributed to parallel evolution of drug resistance mutations resulting in the incorrect clustering of multidrug-resistant virus. On the other hand, a fully compatible phylogenetic tree was reconstructed from the env sequences. We were able to identify and quantify the molecular footprint of drug-selective pressure in pol using maximum likelihood inference under different codon substitution models. An increased fixation rate of mutations in the HIV population of the multidrug-resistant patient was demonstrated using molecular clock modeling. We show that molecular evolutionary analyses, guided by a known transmission history, can reveal the presence of confounding factors like natural selection and caution should be taken when accurate descriptions of HIV evolution are required.status: publishe
The End of the Lines for OX 169: No Binary Broad-Line Region
We show that unusual Balmer emission line profiles of the quasar OX 169,
frequently described as either self-absorbed or double peaked, are actually
neither. The effect is an illusion resulting from two coincidences. First, the
forbidden lines are quite strong and broad. Consequently, the [N II]6583 line
and the associated narrow-line component of H-alpha present the appearance of
twin H-alpha peaks. Second, the redshift of 0.2110 brings H-beta into
coincidence with Na I D at zero redshift, and ISM absorption in Na I D divides
the H-beta emission line. In spectra obtained over the past decade, we see no
substantial change in the character of the line profiles, and no indication of
intrinsic double-peaked structure. The H-gamma, Mg II, and Ly-alpha emission
lines are single peaked, and all of the emission-line redshifts are consistent
once they are correctly attributed to their permitted and forbidden-line
identifications. A systematic shift of up to 700 km/s between broad and narrow
lines is seen, but such differences are common, and could be due to
gravitational and transverse redshift in a low-inclination disk. Stockton &
Farnham (1991) had called attention to an apparent tidal tail in the host
galaxy of OX 169, and speculated that a recent merger had supplied the nucleus
with a coalescing pair of black holes which was now revealing its existence in
the form of two physically distinct broad-line regions. Although there is no
longer any evidence for two broad emission-line regions in OX 169, binary black
holes should form frequently in galaxy mergers, and it is still worthwhile to
monitor the radial velocities of emission lines which could supply evidence of
their existence in certain objects.Comment: 19 pages, 5 figures, accepted for publication in Ap.
Influence of s,p-d and s-p exchange couplings on exciton splitting in (Zn,Mn)O
This work presents results of near-band gap magnetooptical studies on
(Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity and
photoluminescence, that shift towards higher energies when the Mn concentration
increases and split nonlinearly under the magnetic field. Excitonic shifts are
determined by the s,p-d exchange coupling to magnetic ions, by the
electron-hole s-p exchange, and the spin-orbit interactions. A quantitative
description of the magnetoreflectivity findings indicates that the free
excitons A and B are associated with the Gamma_7 and Gamma_9 valence bands,
respectively, the order reversed as compared to wurtzite GaN. Furthermore, our
results show that the magnitude of the giant exciton splittings, specific to
dilute magnetic semiconductors, is unusual: the magnetoreflectivity data is
described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, what
points to small and positive N_0 beta. It is shown that both the increase of
the gap with x and the small positive value of the exchange energy N_0 beta
corroborate recent theory describing the exchange splitting of the valence band
in a non-perturbative way, suitable for the case of a strong p-d hybridization.Comment: 8 pages, 8 figure
Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548
We present a new scheme for modeling the broad line region in active galactic
nuclei (AGNs). It involves photoionization calculations of a large number of
clouds, in several pre-determined geometries, and a comparison of the
calculated line intensities with observed emission line light curves. Fitting
several observed light curves simultaneously provides strong constraints on
model parameters such as the run of density and column density across the
nucleus, the shape of the ionizing continuum, and the radial distribution of
the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC
5548, we were able to reconstruct the light curves of four ultraviolet
emission-lines, in time and in absolute flux. This has not been achieved by any
previous work. We argue that the Balmer lines light curves, and possibly also
the MgII2798 light curve, cannot be tested in this scheme because of the
limitations of present-day photoionization codes. Our fit procedure can be used
to rule out models where the particle density scales as r^{-2}, where r is the
distance from the central source. The best models are those where the density
scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density
at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the
particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We
have also tested the idea that the spectral energy distribution (SED) of the
ionizing continuum is changing with continuum luminosity. None of the
variable-shape SED tried resulted in real improvement over a constant SED case
although models with harder continuum during phases of higher luminosity seem
to fit better the observed spectrum. Reddening and/or different composition
seem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication
in Ap
Los hongos de colombia - vii: leotiaceae - iv: hymenoscyphus caudatus and related species from colombia and adjacent regions
The present paper redescribes and illustrates five common species of Hymenoscyphus, H. caudatus, H. serotinus, H. affin. scutulus, H. Iasiopodtum and amp; H.sclerogenus, from Colombia and adjacent regions in South America. Emphasis is given to discussions of anatomical and morphological variability and host specificity of the species included. Inoperculate Discomycetes are generally small Ascomycetes producing an apothecium as their sexual fruiting structure. The largest order, the Helotiales, contains 5 families: Dermateaceae, Hyaloscyphaceae, Orbiliaceae, Sclerotiniaceae and Leotiaceae, In recent years there have been several monographic and submonographic studies on selected genera within these families, but no definitive studies on the principal family, Leotiaceae (previously referred to as the Helotiaceae), has been prepared. These studies have resulted in a better understanding of some of the family limits, but most of the generic limits still remain vague and confused. This is especially true for Hymenoscyphus (previously referred to by most workers as Helotium), which for decades has been the depository for small Discomycetes of uncertain affinities. White (1942, 1942-a, 1943, 1944) began to work systematically through the species of Hymenoscyphus and to bring order out of the existing chaos. His carefully prepared descriptions, meticulously drawn illustrations, and long synonymies were the real important contribution to the understanding of the genus. The most recent attempts to studyHymenoscyphus have been by Dennis, who summarized his works in 1964 (Dennis, 1964). However, as Dennis himself points out, he was unable to come to a sound understanding and arrangement of the species because of the extent of the problems, and the resulting treatment is rather artificial. He relied heavily upon substrate and geographical relations, rather than true morphological similarities. We agree with Dennis, that this has resulted in the separation of closely related taxa and an "unnecessary proliferation of names". Although Dennis contributions were extraordinary in terms of the number of described species he examined and in his literature searches, he did not have at his disposal necessary tropical collections for a complete comprehension of the individual species and/or the entire genus. Further, as pointed out by Dumont (1976), because of limited collections Dennis was unable to understand the full morphological and geographical variation of certain species and recognized several taxa, when in the final analysis only one existed. Many of these deficiencies are apparent in his monumental work on "The Fungus Flora of Venezuela and Adjacent Countries" (Dennis, 1970). InHymenoscyphus for example, he included only seven species (in his 1964 treatment he included more than 70 species for the world), and of these three, H. crocatus , H. caracassensis and H.atrosubiculatum have been removed from the genus by Dumont (1974) and Dumont and Pal (1978). Arendholz (1979) has recently provided a very important contribution to the understanding of several species of leaf inhabiting species ofHymenoscyphus. Again, as with previous workers, his work stressed temperate collections and temperate species. The purpose of the present study is to report the findings of some of our tropical collecting, to expand the understanding of five species of Hymenoscyphus which appear to be common in northern South America, to discuss the morphological variability observed, and to comment on the host or substrate specificity (or lack of it) within certain taxa. The materials and methods used during these studies are the same as those reported by Dumont (1972). The following is a key to the five species reported herein.The present paper redescribes and illustrates five common species of Hymenoscyphus, H. caudatus, H. serotinus, H. affin. scutulus, H. Iasiopodtum and amp; H. sclerogenus, from Colombia and adjacent regions in South America. Emphasis is given to discussions of anatomical and morphological variability and host specificity of the species included. Inoperculate Discomycetes are generally small Ascomycetes producing an apothecium as their sexual fruiting structure. The largest order, the Helotiales, contains 5 families: Dermateaceae, Hyaloscyphaceae, Orbiliaceae, Sclerotiniaceae and Leotiaceae, In recent years there have been several monographic and submonographic studies on selected genera within these families, but no definitive studies on the principal family, Leotiaceae (previously referred to as the Helotiaceae), has been prepared. These studies have resulted in a better understanding of some of the family limits, but most of the generic limits still remain vague and confused. This is especially true for Hymenoscyphus (previously referred to by most workers as Helotium), which for decades has been the depository for small Discomycetes of uncertain affinities. White (1942, 1942-a, 1943, 1944) began to work systematically through the species of Hymenoscyphus and to bring order out of the existing chaos. His carefully prepared descriptions, meticulously drawn illustrations, and long synonymies were the real important contribution to the understanding of the genus. The most recent attempts to study Hymenoscyphus have been by Dennis, who summarized his works in 1964 (Dennis, 1964). However, as Dennis himself points out, he was unable to come to a sound understanding and arrangement of the species because of the extent of the problems, and the resulting treatment is rather artificial. He relied heavily upon substrate and geographical relations, rather than true morphological similarities. We agree with Dennis, that this has resulted in the separation of closely related taxa and an "unnecessary proliferation of names". Although Dennis contributions were extraordinary in terms of the number of described species he examined and in his literature searches, he did not have at his disposal necessary tropical collections for a complete comprehension of the individual species and/or the entire genus. Further, as pointed out by Dumont (1976), because of limited collections Dennis was unable to understand the full morphological and geographical variation of certain species and recognized several taxa, when in the final analysis only one existed. Many of these deficiencies are apparent in his monumental work on "The Fungus Flora of Venezuela and Adjacent Countries" (Dennis, 1970). In Hymenoscyphus for example, he included only seven species (in his 1964 treatment he included more than 70 species for the world), and of these three, H. crocatus , H. caracassensis and H. atrosubiculatum have been removed from the genus by Dumont (1974) and Dumont and Pal (1978). Arendholz (1979) has recently provided a very important contribution to the understanding of several species of leaf inhabiting species of Hymenoscyphus. Again, as with previous workers, his work stressed temperate collections and temperate species. The purpose of the present study is to report the findings of some of our tropical collecting, to expand the understanding of five species of Hymenoscyphus which appear to be common in northern South America, to discuss the morphological variability observed, and to comment on the host or substrate specificity (or lack of it) within certain taxa. The materials and methods used during these studies are the same as those reported by Dumont (1972). The following is a key to the five species reported herein
Anisotropic Scaling in Layered Aperiodic Ising Systems
The influence of a layered aperiodic modulation of the couplings on the
critical behaviour of the two-dimensional Ising model is studied in the case of
marginal perturbations. The aperiodicity is found to induce anisotropic
scaling. The anisotropy exponent z, given by the sum of the surface
magnetization scaling dimensions, depends continuously on the modulation
amplitude. Thus these systems are scale invariant but not conformally invariant
at the critical point.Comment: 7 pages, 2 eps-figures, Plain TeX and epsf, minor correction
The Circumstellar Disk of HD 141569 Imaged with NICMOS
Coronagraphic imaging with the Near Infrared Camera and Multi Object
Spectrometer on the Hubble Space Telescope reveals a large, ~400 AU (4'')
radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected
light image at 1.1 micron shows the disk oriented at a position angle of 356
+/- 5 deg and inclined to our line of sight by 51 +/- 3 deg; the intrinsic
scattering function of the dust in the disk makes the side inclined toward us,
the eastern side, brighter. The disk flux density peaks 185 AU (1.''85) from
the star and falls off to both larger and smaller radii. A region of depleted
material, or a gap, in the disk is centered 250 AU from the star. The dynamical
effect of one or more planets may be necessary to explain this morphology.Comment: 4 pages, LaTeX with emulateapj.sty and epsfig.sty, 4 postscript
figures, Accepted to ApJ Letter
Long-Term Profile Variability of Double Peaked Emission Lines in Active Galactic Nuclei
We present up to two decades of spectroscopic monitoring of the double-peaked
broad Halpha emission lines of seven broad-line radio galaxies. These extremely
broad, double-peaked lines are successfully modeled by emission from gas in the
outer accretion disk, and their profiles vary on timescales of months to years.
This variability is a valuable probe for understanding the structure and
dynamics of the accretion disk. We characterize the long-term broad-line
profile variability in detail, and compare the evolution of the line profiles
with predictions from a few simple physical models for dynamical processes in
the accretion disk. We find no evidence for persistent, periodic variability
that would be indicative of a precessing elliptical disk, or a circular disk
with a long-lived, single-armed spiral or warp. We do find transient, periodic
variability on the orbital timescale, possibly resulting from shocks induced by
tidal perturbations, and variability in the wings of the profile which suggests
changes in the emissivity of the inner accretion flow. Dramatic but localized
profile changes are observed during flares and high-states of broad-line flux.
In 3C 332, these changes can be explained by a slow, smooth, secular change in
disk illumination. In Arp 102B and 3C 390.3, however, a simple disk model
cannot explain the asymmetric profile changes, suggesting a need for more
sophisticated models. We also observe sharp, transient features that appear in
the blue peak of the objects, which require a well-organized velocity field.Comment: 76 pages, 52 figures, 14 tables, Accepted for publication in ApJS,
figures in Appendix are low resolutio
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