423 research outputs found
X-ray and optical counterparts of hard X-ray selected sources from the SHEEP survey: first results
We present followup observations of five hard X-ray sources from the ASCA
5-10 keV SHEEP survey, which has a limiting flux of erg
cm s. Chandra data have been obtained to improve the X-ray
positions from a few arcmin to , which allows unambiguous optical
identification. While the objects almost certainly house AGN based on their
X-ray luminosity, optical spectroscopy reveals a variety of properties. The
identifications indicate that the SHEEP survey samples the same populations as
deeper surveys which probe the origin of the X-ray background, but because the
SHEEP sources are far brighter, they are more amenable to detailed followup
work. We find a variety of classifications and properties, including a type II
QSO, a galaxy undergoing star formation, and a broad-line AGN which has a very
hard X-ray spectrum, indicating substantial absorption in the X-ray but none in
the optical. Two objects have X-ray/optical flux ratios which, were they at an
X-ray flux level typical of objects in Chandra deep surveys, would place them
in the ``optically faint'' category. They are both identified with broad line
QSOs at z. Clearly this survey - which is relatively unbiased against
obscured objects - is revealing a set of remarkable objects quite different to
the familiar classes of AGN found in previous optical and soft X-ray surveys.Comment: 5 pages, 3 figures. MNRAS, in pres
The influence of initial impurities and irradiation conditions on defect production and annealing in silicon for particle detectors
Silicon detectors in particle physics experiments at the new accelerators or
in space missions for physics goals will be exposed to extreme radiation
conditions. The principal obstacles to long-term operation in these
environments are the changes in detector parameters, consequence of the
modifications in material properties after irradiation. The phenomenological
model developed in the present paper is able to explain quantitatively, without
free parameters, the production of primary defects in silicon after particle
irradiation and their evolution toward equilibrium, for a large range of
generation rates of primary defects. Vacancy-interstitial annihilation,
interstitial migration to sinks, divacancy and vacancy-impurity complex (VP,
VO, V2O, CiOi and CiCs) formation are taken into account. The effects of
different initial impurity concentrations of phosphorus, oxygen and carbon, as
well as of irradiation conditions are systematically studied. The correlation
between the rate of defect production, the temperature and the time evolution
of defect concentrations is also investigated.Comment: 14 pages, 8 figures, submitted to Nucl. Instrum. Meth. Phys. Res.
Peptidylarginine Deiminase Inhibitors Reduce Bacterial Membrane Vesicle Release and Sensitize Bacteria to Antibiotic Treatment
Outer membrane and membrane vesicles (OMV/MV) are released from bacteria and participate in cell communication, biofilm formation and host-pathogen interactions. Peptidylarginine deiminases (PADs) are phylogenetically conserved enzymes that catalyze post-translational deimination/citrullination of proteins, causing structural and functional changes in target proteins. PADs also play major roles in the regulation of eukaryotic extracellular vesicle release. Here we show phylogenetically conserved pathways of PAD-mediated OMV/MV release in bacteria and describe deiminated/citrullinated proteins in E. coli and their derived OMV/MVs. Furthermore, we show that PAD inhibitors can be used to effectively reduce OMV/MV release, both in Gram-negative and Gram-positive bacteria. Importantly, this resulted in enhanced antibiotic sensitivity of both E. coli and S. aureus to a range of antibiotics tested. Our findings reveal novel strategies for applying pharmacological OMV/MV-inhibition to reduce antibiotic resistance
X-ray narrow line region variability as a geometry probe: The case of NGC 5548
We study the long time scale variability of the gas responsible for the X-ray
narrow emission lines in the Seyfert 1 galaxy NGC 5548, in order to constrain
the location and geometry of the emitting gas. Using X-ray spectra taken with
the Chandra-LETGS and HETGS instruments and with XMM-Newton RGS and combining
them with long-term monitoring observations of the Rossi X-ray Timing Explorer
(RXTE), we perform a correlation analysis in order to try constrain the time
scale on which the narrow line emitting gas responds to variations of the
continuum flux. With the inclusion of the 2007 Chandra-LETGS observation we
have an additional observation at an historically low flux level. We conclude
that the NLR in NGC 5548 is in the form of an ionization cone, compact in size,
and located between 1 and 15 pc from the central source, depending on the exact
geometry of the NLR.Comment: 6 pages, 4 figures, accepted for publication in A&
Active Galactic Nuclei and the Truncation of Star Formation in K+A Galaxies
We have searched for active galactic nuclei (AGNs) in K+A galaxies, using
multiwavelength imaging and spectroscopy in the Bootes field of the NOAO Deep
Wide-Field Survey. The K+A galaxies, which have had their star formation
rapidly truncated, are selected via their strong Balmer absorption lines and
weak H-alpha emission. Our sample consists of 24 K+A galaxies selected from
6594 0.10<z<0.35 galaxies brighter than I=20 with optical spectroscopy from the
AGN and Galaxy Evolution Survey. Two thirds of the K+A galaxies are likely
ongoing galaxy mergers, with nearby companion galaxies or tidal tails. Galaxy
mergers may be responsible for the truncation of star formation, or we are
observing the aftermath of merger triggered starbursts. As expected, the
optical colors of K+A galaxies largely fall between blue galaxies with ongoing
star formation and red passive galaxies. However, only 1% of the galaxies with
colors between the red and blue populations are K+A galaxies, and we conclude
that the truncation of star formation in K+A galaxies must have been unusually
abrupt (<100 Myr). We examined the AGN content of K+A galaxies with both
optical emission-line ratios (BPT diagrams) and Chandra X-ray imaging. At least
half of all K+A galaxies display the optical emission-line ratios of AGNs, and
a third of M_R<-22 K+A galaxies host AGNs with X-ray luminosities of 10^{42}
erg/s. The faintest K+A galaxies do not show clear evidence for hosting AGNs,
having emission-line ratios consistent with photoionization by massive stars
and few X-ray detections. We speculate that two mechanisms may be responsible
for the truncation of star formation in K+A galaxies, with AGN feedback only
playing a role in M_R<-20.5 galaxies.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 8
figure
The \u3cem\u3eHubble Space Telescope\u3c/em\u3e Sample of Radio-loud Quasars: The Ly\u3cem\u3eα\u3c/em\u3e/H\u3cem\u3eβ\u3c/em\u3e Ratio
We have used the first Hubble Space Telescope Faint Object Spectrograph spectra of our sample of radio- loud quasars, and quasi-simultaneous ground-based spectrophotometry, to investigate the intensity ratio Lyα/Hβ, whose small observed values are one of the outstanding problems of active galactic nuclei research. The present sample of 20 quasars with complete flux and profile data shows the first significant correlations of this ratio with other observed properties. The strongest correlations are with various continuum slope indicators: we find smaller Lyα/Hβ ratios in quasars whose continua rise more steeply into the red. The long-wavelength continuum slope (1909-4861 Å) is strongly correlated with Lyα/Hβ, but the short-wavelength continuum slope (1215-1909 Å) is not. A separation into line components shows that the above correlations arise mostly from the red wings of the lines. The core-to-wing flux ratio is also correlated with the slope. The correlation of Lyα/Hβ with continuum slope is consistent with line and continuum reddening by an external dust screen with Galactic-type extinction of up to E\u3eB\u3e\u3csub\u3e-\u3c/sub\u3e\u3csub\u3eV\u3e = 0.3. In this case the intrinsic Lyα/Hβ ratio is ˜20. However, other trends expected if dust were the sole factor are not seen. There are indications that core-dominated and lobe-dominated sources differ in their Lyα/Hβ and continuum slope dependence. We calculate a grid of theoretical hydrogen line ratios and use it to investigate reddening and alternative explanations, such as dependence upon ionizing flux. We suggest that several different mechanisms are operating
The Narrow-Line Region of High-Luminosity Active Galactic Nuclei
We have made high signal-to-noise spectroscopic observations of seven radio-loud quasars with the Faint Object Spectrograph on the Hubble Space Telescope and from the ground at McDonald Observatory and at Kitt Peak National Observatory. The resolution is 300-400 km s-1 over the wavelength range 1000-8500 Ã…, enabling us to separate the broad and narrow components of the emission lines. This is the first study of the optical and UV narrow lines in such high-luminosity active galactic nuclei (AGNs).
The most important and striking observational result is the relative weakness of the narrow ultraviolet lines, assuming that they have the same widths as the narrow [0 III] λ5007 emission lines. We do not have a single definite detection of a narrow UV line in any of the seven quasars. We have measured all the strong optical narrow lines and have derived upper limits to the strengths of narrow lines in the UV. The latter are much weaker than predicted by dust-free photoionization models and also weaker than those typically seen in Seyfert 2 galaxies and narrow-line radio galaxies. There is direct evidence for the presence of dust with significant reddening, typically E(B-V) ~ 0.5. A comparison of our sample with the previously observed Seyfert 2s (Kinney et al.) shows that two explanations for the reddening are needed. One is simple foreground reddening, presumably in the host galaxy. However some narrow-line spectra show an unusually weak Lyα/Hβ intensity ratio, but apparently case B Hα/Hβ. We interpret this as the result of dust inside the narrow line clouds and show model calculations to support this claim. These and other calculations presented in this paper are used to argue that much of the narrow-line region in the high-luminosity objects lies well beyond the nucleus
Quasars and the Big Blue Bump
We investigate the ultraviolet-to-optical spectral energy distributions
(SEDs) of 17 active galactic nuclei (AGNs) using quasi-simultaneous
spectrophotometry spanning 900-9000 Angstrom (rest frame). We employ data from
the Far Ultraviolet Spectroscopic Explorer (FUSE), the Hubble Space Telescope
(HST), and the 2.1-meter telescope at Kitt Peak National Observatory (KPNO).
Taking advantage of the short-wavelength coverage, we are able to study the
so-called "big blue bump," the region where the energy output peaks, in detail.
Most objects exhibit a spectral break around 1100 Angstrom. Although this
result is formally associated with large uncertainty for some objects, there is
strong evidence in the data that the far-ultraviolet spectral region is below
the extrapolation of the near-ultraviolet-optical slope, indicating a spectral
break around 1100 Angstrom. We compare the behavior of our sample to those of
non-LTE thin-disk models covering a range in black-hole mass, Eddington ratio,
disk inclination, and other parameters. The distribution of ultraviolet-optical
spectral indices redward of the break, and far-ultraviolet indices shortward of
the break, are in rough agreement with the models. However, we do not see a
correlation between the far-ultraviolet spectral index and the black hole mass,
as seen in some accretion disk models. We argue that the observed spectral
break is intrinsic to AGNs, although intrinsic reddening as well as
Comptonization can strongly affect the far-ultraviolet spectral index. We make
our data available online in digital format.Comment: 32 pages (10pt), 12 figures. Accepted for publication in Ap
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