2,542 research outputs found
The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic Nuclei
A combined analysis of the profiles of the main broad quasar emission lines
in both Hubble Space Telescope and optical spectra shows that while the
profiles of the strong UV lines are quite similar, there is frequently a strong
increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that
the suggestion that the high velocity gas is optically-thin presents many
problems. We show that the relative strengths of the high velocity wings arise
naturally in an optically-thick BLR component. An optically-thick model
successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha
ratios and flatter Balmer decrements in the line wings, the strengths of CIII]
and the lambda 1400 blend, and the strong variability of high-velocity,
high-ionization lines (especially HeII and HeI).Comment: 34 pages in AASTeX, including 10 pages of figures. Submitted to
Astrophysical Journa
Measurement techniques and instruments suitable for life-prediction testing of photovoltaic arrays
Array failure modes, relevant materials property changes, and primary degradation mechanisms are discussed as a prerequisite to identifying suitable measurement techniques and instruments. Candidate techniques and instruments are identified on the basis of extensive reviews of published and unpublished information. These methods are organized in six measurement categories - chemical, electrical, optical, thermal, mechanical, and other physicals. Using specified evaluation criteria, the most promising techniques and instruments for use in life prediction tests of arrays were selected
Electrostatic Conveyer for Excitons
We report on the study of indirect excitons in moving lattices - conveyers
created by a set of AC voltages applied to the electrodes on the sample
surface. The wavelength of this moving lattice is set by the electrode
periodicity, the amplitude is controlled by the applied voltage, and the
velocity is controlled by the AC frequency. We observed the dynamical
localization-delocalization transition for excitons in the conveyers and
measured its dependence on the exciton density and conveyer amplitude and
velocity. We considered a model for exciton transport via conveyers. The
theoretical simulations are in agreement with the experimental data.Comment: 4 pages, 4 figures + supplemental material including two video
The BeppoSAX view of the X-ray active nucleus of NGC4258
BeppoSAX observed NGC4258 on December 1998, when its 2-10 keV luminosity was
\~1E41 erg/s. 100% variability is observed in the 3-10 keV band on timescales
of a half a day while 20% variability is observed on timescales of one hour.
The nuclear component is visible above 2 keV only, being obscured by a column
density of (9.5+/-1.2)E22 cm-2; this component is detected up to 70 keV with
S/N>=3 and with the steep power law energy index of 1.11+/-0.14. Bremsstrahlung
emission for the 2-70 keV X-ray luminosity, as expected in ADAF models with
strong winds, is ruled out by the data. The ratio between the nuclear radio and
the X-ray luminosities is <=1E-5, similar to that of radio quiet AGN. X-ray
variability and spectral shape, radio to X-ray and NIR to X-ray luminosity
ratios suggest that the nucleus of NGC4258 could be a scaled-down version of a
Seyfert nucleus. The soft (E<=2keV) X-ray emission is complex. There are at
least two thermal-like components, with T1=0.6+/-0.1 keV and T2>=1.3 keV. The
cooler (L(0.1-2.4keV)=1E40 erg/s) component is probably associated with the
jet, resolved in X-rays by the ROSAT HRI. The second component, which can be
modeled equally well by an unobscured power law model, has L(0.1-2.4keV)~7E39
erg/s, consistent with that expected from discrete X-ray sources in the host
galaxy. NGC4258 and other maser AGNs show strong nuclear X-ray absorption. We
propose that this large column of gas might be responsible for shielding the
regions of maser emission from X-ray illumination. So a large column density
gas may be a necessary property of masing AGNs.Comment: ApJ in pres
Optical Polarization of 52 Radio-Loud QSOs and BL Lac Objects
Polarization measurements are presented for 52 radio-loud QSOs and BL Lac
objects. For 9 highly polarized (p >3%) AGN, these are the first published
polarization measurements. Of these 9, 7 are highly-polarized QSOs (HPQs), one
is a BL Lac object and another is a likely BL Lac object. Polarization
variability is confirmed for some of these new and previously known
highly-polarized AGN. While 6 of the HPQs have flat radio spectra are almost
certainly blazars, PKS 1452-217 is probably a new member of the rare class of
radio-loud QSOs that show high polarization by scattering, and is therefore
important for testing orientation Unified Schemes. In competition for the
highest redshift HPQ are the well-observed QSO PKS 0438-43 at z = 2.85, with
maximum p = 4.7%, and PKS 0046-315 at z = 2.72, for which we find p = 13%.Comment: 6 pages. Astronomical Journal, in pres
The UV Properties of the Narrow Line Quasar I Zwicky 1
I Zw 1 is the prototype narrow line quasar. We report here the results of our
study of the UV emission of I Zw 1 using a high S/N (50-120) spectrum obtained
with the HST FOS. The following main new results are obtained: 1. The Mg II and
Al III doublets are partially/fully resolved. The measured doublet ratios
verify theoretical predictions that the lines are thermalized in the BLR. 2. A
weak associated UV absorption system is detected in N~V, and possibly also in C
IV and Lya, suggesting an outflow with a velocity of 1870 km/s and velocity
dispersion <300 km/s. 3. Lines from ions of increasing ionization level show
increasing excess blue wing flux, and an increasing line peak velocity shift,
reaching a maximum blueshift of about 2000 km/s for He II 1640. This may
indicate an out-flowing component in the BLR, where the ionization level
increases with velocity, and which is visible only in the approaching
direction. The highest velocity part of this outflow may produce the associated
UV absorption system. 4. The small C III] 1909 EW, and the small C III]
1909/Lya and C III] 1909/Si III] 1892 flux ratios indicate a typical BLR
density of 10^11, i.e. about an order of magnitude larger than implied by C
III] 1909 in most quasars. A BLR component of a higher density is implied by
the EW and doublet ratio of the Al III 1857 doublet. 5. Prominent Fe II UV 191
emission is seen, together with weaker line emission at 1294 and 1871 A. These
three features have been proposed as evidence for significant Lya pumping of
the 8-10 eV levels of Fe II. 6. Significant Fe III emission is present. The Fe
III UV 34 and UV 48 multiplets are clearly resolved, and Fe III UV 1, UV 47, UV
50, and UV 68 may also be present. (Shortened version)Comment: 28 pages, 1 table and 7 figures included. Uses aas2pp4.sty. Scheduled
for the Astrophysical Journal November 10, 1997 issue, Vol. 48
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