79 research outputs found
Structural anomalies in undoped Gallium Arsenide observed in high resolution diffraction imaging with monochromatic synchrotron radiation
Novel, streak-like disruption features restricted to the plane of diffraction have recently been observed in images obtained by synchrotron radiation diffraction from undoped, semi-insulating gallium arsenide crystals. These features were identified as ensembles of very thin platelets or interfaces lying in (110) planes, and a structural model consisting of antiphase domain boundaries was proposed. We report here the other principal features observed in high resolution monochromatic synchrotron radiation diffraction images: (quasi) cellular structure; linear, very low-angle subgrain boundaries in (110) directions, and surface stripes in a (110) direction. In addition, we report systematic differences in the acceptance angle for images involving various diffraction vectors. When these observations are considered together, a unifying picture emerges. The presence of ensembles of thin (110) antiphase platelet regions or boundaries is generally consistent not only with the streak-like diffraction features but with the other features reported here as well. For the formation of such regions we propose two mechanisms, operating in parallel, that appear to be consistent with the various defect features observed by a variety of techniques
Probing the Ionizing Continuum of Narrow-Line Seyfert 1 Galaxies. I.Observational Results
We present optical spectra and emission-line ratios of 12 Narrow-Line Seyfert
1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A
common feature in the EUV continuum of active galactic nuclei is the big blue
bump (BBB), generally associated with thermal accretion disk emission. While
Galactic absorption prevents direct access to the EUV range, it can be mapped
by measuring the strength of a variety of forbidden optical emission lines that
respond to different EUV continuum regions. We find that narrow emission-line
ratios involving [OII]3727, Hbeta, [OIII]5007, [OI]6300, Halpha,[NII]6583, and
[SII]6716,6731 indicate no significant difference between NLS1s and Broad-Line
Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy
distributions of their ionizing EUV - soft X-ray continua are similar. The
relative strength of important forbidden high ionization lines like [NeV]3426
compared to HeII4686 and the relative strength of [FeX]6374 appear to show the
same range as in BLS1 galaxies. However, a trend of weaker
F([OI]6300)/F(Halpha) emission-line ratios is indicated for NLS1s compared to
BLS1s. To recover the broad emission-line profiles we used Gaussian components.
This approach indicates that the broad Hbeta profile can be well described with
a broad component (FWHM = 3275 +- 800 km/s) and an intermediate broad component
(FWHM = 1200 +- 300 km/s). The width of the broad component is in the typical
range of normal BLS1s. The emission-line flux that is associated with the broad
component in these NLS1s amounts to at least 60% of the total flux. Thus it
dominates the total line flux, similar to BLS1 galaxies.Comment: 34 pages, 9 figures. accepted for publication in the
Astrophys.Journa
WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase. Detection of the little and big blue bumps
The blazar 3C 454.3 underwent an unprecedented optical outburst in spring
2005. This was first followed by a mm and then by a cm radio outburst, which
peaked in February 2006. We report on follow-up observations by the WEBT to
study the multiwavelength emission in the post-outburst phase. XMM-Newton
observations on July and December 2006 added information on the X-ray and UV
fluxes. The source was in a faint state. The radio flux at the higher
frequencies showed a fast decreasing trend, which represents the tail of the
big radio outburst. It was followed by a quiescent state, common at all radio
frequencies. In contrast, moderate activity characterized the NIR and optical
light curves, with a progressive increase of the variability amplitude with
increasing wavelength. We ascribe this redder-when-brighter behaviour to the
presence of a "little blue bump" due to line emission from the broad line
region, which is clearly visible in the source SED during faint states.
Moreover, the data from the XMM-Newton OM reveal a rise of the SED in the UV,
suggesting the existence of a "big blue bump" due to thermal emission from the
accretion disc. The X-ray spectra are well fitted with a power-law model with
photoelectric absorption, possibly larger than the Galactic one. However, the
comparison with previous X-ray observations would imply that the amount of
absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum
presents a curvature, which may depend on the X-ray brightness. In this case,
two scenarios are possible.Comment: 9 pages, 7 figures, accepted for publication in A&
AGILE detection of extreme gamma-ray activity from the blazar PKS 1510-089 during March 2009. Multifrequency analysis
We report on the extreme gamma-ray activity from the FSRQ PKS 1510-089
observed by AGILE in March 2009. In the same period a radio-to-optical
monitoring of the source was provided by the GASP-WEBT and REM. Moreover,
several Swift ToO observations were triggered, adding important information on
the source behaviour from optical/UV to hard X-rays. We paid particular
attention to the calibration of the Swift/UVOT data to make it suitable to the
blazars spectra. Simultaneous observations from radio to gamma rays allowed us
to study in detail the correlation among the emission variability at different
frequencies and to investigate the mechanisms at work. In the period 9-30 March
2009, AGILE detected an average gamma-ray flux of (311+/-21)x10^-8 ph cm^-2
s^-1 for E>100 MeV, and a peak level of (702+/-131)x10^-8 ph cm^-2 s^-1 on
daily integration. The gamma-ray activity occurred during a period of
increasing activity from near-IR to UV, with a flaring episode detected on
26-27 March 2009, suggesting that a single mechanism is responsible for the
flux enhancement observed from near-IR to UV. By contrast, Swift/XRT
observations seem to show no clear correlation of the X-ray fluxes with the
optical and gamma-ray ones. However, the X-ray observations show a harder
photon index (1.3-1.6) with respect to most FSRQs and a hint of
harder-when-brighter behaviour, indicating the possible presence of a second
emission component at soft X-ray energies. Moreover, the broad band spectrum
from radio-to-UV confirmed the evidence of thermal features in the optical/UV
spectrum of PKS 1510-089 also during high gamma-ray state. On the other hand,
during 25-26 March 2009 a flat spectrum in the optical/UV energy band was
observed, suggesting an important contribution of the synchrotron emission in
this part of the spectrum during the brightest gamma-ray flare, therefore a
significant shift of the synchrotron peak.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in Astronomy
and Astrophysic
Gravitating discs around black holes
Fluid discs and tori around black holes are discussed within different
approaches and with the emphasis on the role of disc gravity. First reviewed
are the prospects of investigating the gravitational field of a black
hole--disc system by analytical solutions of stationary, axially symmetric
Einstein's equations. Then, more detailed considerations are focused to middle
and outer parts of extended disc-like configurations where relativistic effects
are small and the Newtonian description is adequate.
Within general relativity, only a static case has been analysed in detail.
Results are often very inspiring, however, simplifying assumptions must be
imposed: ad hoc profiles of the disc density are commonly assumed and the
effects of frame-dragging and completely lacking. Astrophysical discs (e.g.
accretion discs in active galactic nuclei) typically extend far beyond the
relativistic domain and are fairly diluted. However, self-gravity is still
essential for their structure and evolution, as well as for their radiation
emission and the impact on the environment around. For example, a nuclear star
cluster in a galactic centre may bear various imprints of mutual star--disc
interactions, which can be recognised in observational properties, such as the
relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be
available from http://www.iop.org/EJ/journal/CQ
ROBO: a Model and a Code for the Study of the Interstellar Medium
We present ROBO, a model and its companion code for the study of the
interstellar medium (ISM). The aim is to provide an accurate description of the
physical evolution of the ISM and to set the ground for an ancillary tool to be
inserted in NBody-Tree-SPH (NB-TSPH) simulations of large scale structures in
cosmological context or of the formation and evolution of individual galaxies.
The ISM model consists of gas and dust. The gas chemical composition is
regulated by a network of reactions that includes a large number of species
(hydrogen and deuterium based molecules, helium, and metals). New reaction
rates for the charge transfer in and collisions are
presented. The dust contains the standard mixture of carbonaceous grains
(graphite grains and PAHs) and silicates of which the model follows the
formation and destruction by several processes. The model takes into account an
accurate treatment of the cooling process, based on several physical
mechanisms, and cooling functions recently reported in the literature. The
model is applied to a wide range of the input parameters and the results for
important quantities describing the physical state of the gas and dust are
presented. The results are organized in a database suited to the artificial
neural networks (ANNs). Once trained, the ANNs yield the same results obtained
by ROBO, with great accuracy. We plan to develop ANNs suitably tailored for
applications to NB-TSPH simulations of cosmological structures and/or galaxies.Comment: accepted for publication in section 15. Numerical methods and codes
of Astronomy and Astrophysic
High energy radiation from Centaurus A
We calculate for the nearest active galactic nucleus (AGN), Centaurus A, the
flux of high energy cosmic rays and of accompanying secondary photons and
neutrinos expected from hadronic interactions in the source. We use as two
basic models for the generation of ultrahigh energy cosmic rays (UHECR) shock
acceleration in the radio jet and acceleration in the regular electromagnetic
field close to the core of the AGN. While scattering on photons dominates in
scenarios with acceleration close to the core, scattering on gas becomes more
important if acceleration takes place along the jet. Normalizing the UHECR flux
from Centaurus A to the observations of the Auger experiment, the neutrino flux
may be marginally observable in a 1 km neutrino telescope, if a steep UHECR
flux \d N/\d E\propto E^{-\alpha} with extends down to
eV. The associated photon flux is close to or exceeds the observational data of
atmospheric Cherenkov and -ray telescopes for \alpha\gsim 2. In
particular, we find that already present data favour either a softer UHECR
injection spectrum than for Centaurus A or a lower UHECR flux than
expected from the normalization to the Auger observations.Comment: 12 pages, 6 figures; v2: revised version to appear in a special issue
of New Journal for Physic
A Ly{\alpha} blob and zabs {\approx} zem damped Ly{\alpha} absorber in the dark matter halo of the binary quasar Q 0151+048
Q0151+048 is a physical QSO pair at z ~ 1.929 with a separation of 3.3 arcsec
on the sky. In the spectrum of Q0151+048A (qA), a DLA is observed at a higher
redshift. We have previously detected the host galaxies of both QSOs, as well
as a Lya blob. We performed low-resolution spectroscopy with the slit aligned
with the extended emission. We also observed the system using the
medium-resolution VLT/X-shooter spectrograph and the slit aligned with the two
QSOs. We measure systemic redshifts of zem(A)=1.92924{\pm}0.00036 and
zem(B)=1.92863{\pm}0.00042 from the H{\beta} and H{\alpha} emission lines,
respectively. We estimate the masses of the black holes of the two QSOs to be
10^9.33 M{\odot} and 10^8.38 M{\odot} for qA and qB, respectively. From this we
infered the mass of the dark matter halos hosting the two QSOs: 10^13.74
M{\odot} and 10^13.13 M{\odot} for qA and qB, respectively. We observe a
velocity gradient along the major axis of the Lya blob consistent with the
rotation curve of a large disk galaxy, but it may also be caused by gas inflow
or outflow. We detect residual continuum in the DLA trough which we interpret
as emission from the host galaxy of qA. The derived H0 column density of the
DLA is log NH0 = 20.34 {\pm} 0.02. Metal column densities results in an overall
metallicity of 0.01 Z{\odot}. We detect CII* which allows us to make a physical
model of the DLA cloud. From the systemic redshifts of the QSOs, we conclude
that the Lya blob is associated with qA rather than with the DLA. The DLA must
be located in front of both the Lya blob and qA at a distance larger than 30
kpc. The two QSOs accrete at normal eddington ratios. The DM halo of this
double quasar will grow to the mass of our local super-cluster at z=0. We point
out that those objects therefore form an ideal laboratory to study the physical
interactions in a z=2 pre-cursor of our local super-cluster.Comment: 12 pages, 11 figures, accepted for publication in A&
Androgenic dependence of exophytic tumor growth in a transgenic mouse model of bladder cancer: a role for thrombospondin-1
<p>Abstract</p> <p>Background</p> <p>Steroid hormones influence mitogenic signaling pathways, apoptosis, and cell cycle checkpoints, and it has long been known that incidence of bladder cancer (BC) in men is several times greater than in women, a difference that cannot be attributed to environmental or lifestyle factors alone. Castration reduces incidence of chemically-induced BC in rodents. It is unclear if this effect is due to hormonal influences on activation/deactivation of carcinogens or a direct effect on urothelial cell proliferation or other malignant processes. We examined the effect of castration on BC growth in UPII-SV40T transgenic mice, which express SV40 T antigen specifically in urothelium and reliably develop BC. Furthermore, because BC growth in UPII-SV40T mice is exophytic, we speculated BC growth was dependent on angiogenesis and angiogenesis was, in turn, androgen responsive.</p> <p>Methods</p> <p>Flat panel detector-based cone beam computed tomography (FPDCT) was used to longitudinally measure exophytic BC growth in UPII-SV40T male mice sham-operated, castrated, or castrated and supplemented with dihydrotestosterone (DHT). Human normal bladder and BC biopsies and mouse bladder were examined quantitatively for thrombospondin-1 (TSP1) protein expression.</p> <p>Results</p> <p>Mice castrated at 24 weeks of age had decreased BC volumes at 32 weeks compared to intact mice (p = 0.0071) and castrated mice administered DHT (p = 0.0233; one-way ANOVA, JMP 6.0.3, SAS Institute, Inc.). Bladder cancer cell lines responded to DHT treatment with increased proliferation, regardless of androgen receptor expression levels. TSP1, an anti-angiogenic factor whose expression is inhibited by androgens, had decreased expression in bladders of UPII-SV40T mice compared to wild-type. Castration increased TSP1 levels in UPII-SV40T mice compared to intact mice. TSP1 protein expression was higher in 8 of 10 human bladder biopsies of normal versus malignant tissue from the same patients.</p> <p>Conclusion</p> <p>FPDCT allows longitudinal monitoring of exophytic tumor growth in the UPII-SV40T model of BC that bypasses need for chemical carcinogens, which confound analysis of androgen effects. Androgens increase tumor cell growth <it>in vitro </it>and <it>in vivo </it>and decrease TSP1 expression, possibly explaining the therapeutic effect of castration. This effect may, in part, explain gender differences in BC incidence and implies anti-androgenic therapies may be effective in preventing and treating BC.</p
Effects of nitrogen availability on polymalic acid biosynthesis in the yeast-like fungus Aureobasidium pullulans
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