1,344 research outputs found
Quasar Parallax: a Method for Determining Direct Geometrical Distances to Quasars
We describe a novel method to determine direct geometrical distances to
quasars that can measure the cosmological constant, Lambda, with minimal
assumptions. This method is equivalent to geometric parallax, with the
`standard length' being the size of the quasar broad emission line region
(BELR) as determined from the light travel time measurements of reverberation
mapping. The effect of non-zero Lambda on angular diameter is large, 40% at
z=2, so mapping angular diameter distances vs. redshift will give Lambda with
(relative) ease. In principle these measurements could be made in the UV,
optical, near infrared or even X-ray bands. Interferometers with a resolution
of 0.01mas are needed to measure the size of the BELR in z=2 quasars, which
appear plausible given reasonable short term extrapolations of current
technology.Comment: 13 pages, with 3 figures. ApJ Letters, in press (Dec 20, 2002
A view of the narrow-line region in the infrared: active galactic nuclei with resolved fine-structure lines in the Spitzer archive
We queried the Spitzer archive for high-resolution observations with the
Infrared Spectrograph of optically selected active galactic nuclei (AGN) for
the purpose of identifying sources with resolved fine-structure lines that
would enable studies of the narrow-line region (NLR) at mid-infrared
wavelengths. By combining 298 Spitzer spectra with 6 Infrared Space Observatory
spectra, we present kinematic information of the NLR for 81 z<=0.3 AGN. We used
the [NeV], [OIV], [NeIII], and [SIV] lines, whose fluxes correlate well with
each other, to probe gas photoionized by the AGN. We found that the widths of
the lines are, on average, increasing with the ionization potential of the
species that emit them. No correlation of the line width with the critical
density of the corresponding transition was found. The velocity dispersion of
the gas, sigma, is systematically higher than that of the stars, sigma_*, in
the AGN host galaxy, and it scales with the mass of the central black hole,
M_BH. Further correlations between the line widths and luminosities L, and
between L and M_BH, are suggestive of a three dimensional plane connecting
log(M_BH) to a linear combination of log(sigma) and log(L). Such a plane can be
understood within the context of gas motions that are driven by AGN feedback
mechanisms, or virialized gas motions with a power-law dependence of the NLR
radius on the AGN luminosity. The M_BH estimates obtained for 35 type 2 AGN
from this plane are consistent with those obtained from the M_BH-sigma_*
relation.Comment: ApJ, revised to match the print versio
Reversible Eu<sup>2+</sup> â Eu<sup>3+</sup> transitions at EuâSi interfaces
Valence switching at EuâSi interfaces is demonstrated by resonant photoemission during repeated oxidationâreduction cycles performed by roomâtemperature O2 exposure and mild heating. The Eu2+ â Eu3+ transitions are accompanied by Fermi level switching associated with changes in the stoichiometry of the surface heterostructure. The ability to cycle between two wellâdefined magnetic states at a surface may be attractive in technological applications
XMM-Newton RGS observation of the warm absorber in Mrk 279
The Seyfert 1 galaxy Mrk 279 was observed by XMM-Newton in November 2005 in
three consecutive orbits, showing significant short-scale variability (average
soft band variation in flux ~20%). The source is known to host a two-component
warm absorber with distinct ionisation states from a previous Chandra
observation. We aim to study the warm absorber in Mrk 279 and investigate any
possible response to the short-term variations of the ionising flux, and to
assess whether it has varied on a long-term time scale with respect to the
Chandra observation. We find no significant changes in the warm absorber on
neither short time scales (~2 days) nor at longer time scales (two and a half
years), as the variations in the ionic column densities of the most relevant
elements are below the 90% confidence level. The variations could still be
present but are statistically undetected given the signal-to-noise ratio of the
data. Starting from reasonable standard assumptions we estimate the location of
the absorbing gas, which is likely to be associated with the putative dusty
torus rather than with the Broad Line Region if the outflowing gas is moving at
the escape velocity or larger.Comment: 10 pages, 9 figures, 6 tables. Accepted for publication in Astronomy
& Astrophysic
High-ionization mid-infrared lines as black hole mass and bolometric luminosity indicators in active galactic nuclei
We present relations of the black hole mass and the optical luminosity with
the velocity dispersion and the luminosity of the [Ne V] and the [O IV]
high-ionization lines in the mid-infrared (MIR) for 28 reverberation-mapped
active galactic nuclei. We used high-resolution Spitzer Infrared Spectrograph
and Infrared Space Observatory Short Wavelength Spectrometer data to fit the
profiles of these MIR emission lines that originate from the narrow-line region
of the nucleus. We find that the lines are often resolved and that the velocity
dispersion of [Ne V] and [O IV] follows a relation similar to that between the
black hole mass and the bulge stellar velocity dispersion found for local
galaxies. The luminosity of the [Ne V] and the [O IV] lines in these sources is
correlated with that of the optical 5100A continuum and with the black hole
mass. Our results provide a means to derive black hole properties in various
types of active galactic nuclei, including highly obscured systems.Comment: accepted for publication in ApJ
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
The Relationship Between Luminosity and Broad-Line Region Size in Active Galactic Nuclei
We reinvestigate the relationship between the characteristic broad-line
region size (R_blr) and the Balmer emission-line, X-ray, UV, and optical
continuum luminosities. Our study makes use of the best available
determinations of R_blr for a large number of active galactic nuclei (AGNs)
from Peterson et al. Using their determinations of R_blr for a large sample of
AGNs and two different regression methods, we investigate the robustness of our
correlation results as a function of data sub-sample and regression technique.
Though small systematic differences were found depending on the method of
analysis, our results are generally consistent. Assuming a power-law relation
R_blr \propto L^\alpha, we find the mean best-fitting \alpha is about
0.67+/-0.05 for the optical continuum and the broad H\beta luminosity, about
0.56+/-0.05 for the UV continuum luminosity, and about 0.70+/-0.14 for the
X-ray luminosity. We also find an intrinsic scatter of about 40% in these
relations. The disagreement of our results with the theoretical expected slope
of 0.5 indicates that the simple assumption of all AGNs having on average same
ionization parameter, BLR density, column density, and ionizing spectral energy
distribution, is not valid and there is likely some evolution of a few of these
characteristics along the luminosity scale.Comment: 11 pages, 2 figures, emulateapj, accepted for publication in The
Astrophysical Journa
- âŠ