4,356 research outputs found
Low radiative efficiency accretion at work in active galactic nuclei: the nuclear spectral energy distribution of NGC4565
We derive the spectral energy distribution (SED) of the nucleus of the
Seyfert galaxy NGC4565. Despite its classification as a Seyfert2, the nuclear
source is substantially unabsorbed. The absorption we find from Chandra data
(N_H=2.5 X 10^21 cm^-2) is consistent with that produced by material in the
galactic disk of the host galaxy. HST images show a nuclear unresolved source
in all of the available observations, from the near-IR H band to the optical U
band. The SED is completely different from that of Seyfert galaxies and QSO, as
it appears basically ``flat'' in the IR-optical region, with a small drop-off
in the U-band. The location of the object in diagnostic planes for low
luminosity AGNs excludes a jet origin for the optical nucleus, and its
extremely low Eddington ratio L_o/L_Edd indicates that the radiation we observe
is most likely produced in a radiatively inefficient accretion flow (RIAF).
This would make NGC4565 the first AGN in which an ADAF-like process is
identified in the optical. We find that the relatively high [OIII] flux
observed from the ground cannot be all produced in the nucleus. Therefore, an
extended NLR must exist in this object. This may be interpreted in the
framework of two different scenarios: i) the radiation from ADAFs is sufficient
to give rise to high ionization emission-line regions through photoionization,
or ii) the nuclear source has recently ``turned-off'', switching from a
high-efficiency accretion regime to the present low-efficiency state.Comment: 7 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Will Jets Identify the Progenitors of Type Ia Supernovae?
We use the fact that a Type Ia supernova has been serendipitously discovered
near the jet of the active galaxy 3C 78 to examine the question of whether jets
can enhance accretion onto white dwarfs. One interesting outcome of such a
jet-induced accretion process is an enhanced rate of novae in the vicinity of
jets. We present results of observations of the jet in M87 which appear to have
indeed discovered 11 novae in close proximity to the jet. We show that a
confirmation of the relation between jets and novae and Type Ia supernovae can
finally identify the elusive progenitors of Type Ia supernovae.Comment: 10 pages, 3 figure
Polarization Diagnostics for Cool Core Cluster Emission Lines
The nature of the interaction between low-excitation gas filaments at ~104 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~107 K in the centers of galaxy clusters remains a puzzle. The presence of a strong, empirical correlation between the two gas phases is indicative of a fundamental relationship between them, though as yet of undetermined cause. The cooler filaments, originally thought to have condensed from the hot gas, could also arise from a merger or the disturbance of cool circumnuclear gas by nuclear activity. Here, we have searched for intrinsic line emission polarization in cool core galaxy clusters as a diagnostic of fundamental transport processes. Drawing on developments in solar astrophysics, direct energetic particle impact induced polarization holds the promise to definitively determine the role of collisional processes such as thermal conduction in the ISM physics of galaxy clusters, while providing insight into other highly anisotropic excitation mechanisms such as shocks, intense radiation fields, and suprathermal particles. Under certain physical conditions, theoretical calculations predict of the order of 10% polarization. Our observations of the filaments in four nearby cool core clusters place stringent upper limits ( 0.1%) on the presence of emission line polarization, requiring that if thermal conduction is operative, the thermal gradients are not in the saturated regime. This limit is consistent with theoretical models of the thermal structure of filament interfacesPeer reviewe
Magnon Mediated Electric Current Drag Across a Ferromagnetic Insulator Layer
In a semiconductor hererostructure, the Coulomb interaction is responsible
for the electric current drag between two 2D electron gases across an electron
impenetrable insulator. For two metallic layers separated by a ferromagnetic
insulator (FI) layer, the electric current drag can be mediated by a
nonequilibrium magnon current of the FI. We determine the drag current by using
the semiclassical Boltzmann approach with proper boundary conditions of
electrons and magnons at the metal-FI interface.Comment: 13 pages, 2 figures: to appear in PR
HST emission-line images of nearby 3CR radio galaxies: two photoionization, accretion and feedback modes
We present HST/ACS narrow-band images of a low-z sample of 19 3C radio
galaxies to study the H and [OIII] emissions from the narrow-line
region (NLR). Based on nuclear emission line ratios, we divide the sample into
High and Low Excitation Galaxies (HEGs and LEGs). We observe different line
morphologies, extended line emission on kpc scale, large [OIII]/H
scatter across the galaxies, and a radio-line alignment. In general, HEGs show
more prominent emission line properties than LEGs: larger, more disturbed, more
luminous, and more massive regions of ionized gas with slightly larger covering
factors. We find evidence of correlations between line luminosities and (radio
and X-ray) nuclear luminosities. All these results point to a main common
origin, the active nucleus, which ionize the surrounding gas. However, the
contribution of additional photoionization mechanism (jet shocks and star
formation) are needed to account for the different line properties of the two
classes. A relationship between the accretion, photoionization and feedback
modes emerges from this study. For LEGs (hot-gas accretors), the synchrotron
emission from the jet represents the main source of ionizing photons. The lack
of cold gas and star formation in their hosts accounts for the moderate
ionized-gas masses and sizes. For HEGs (cold-gas accretors), an ionizing
continuum from a standard disk and shocks from the powerful jets are the main
sources of photoionization, with the contribution from star formation. These
components, combined with the large reservoir of cold/dust gas brought from a
recent merger, account for the properties of their extended emission-line
regions.Comment: accepted for publication on ApJ (22 pages, 12 figures
Anomalous cooling of the massive white dwarf in U Geminorum following a narrow dwarf nova outburst
We obtained Hubble GHRS medium resolution (G160M grating) phase-resolved
spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf
nova quiescence, 13 days and 61 days following the end of a narrow outburst.
The spectral wavelength ranges were centered upon three different line regions:
N V (1238\AA, 1242\AA), Si III (1300\AA) and He II (1640\AA). All of the
quiescent spectra at both epochs are dominated by absorption lines and show no
emission features. The Si III and He II absorption line velocities versus
orbital phase trace the orbital motion of the white dwarf but the N~V
absorption velocities appear to deviate from the white dwarf motion. We confirm
our previously reported low white dwarf rotational velocity, V sin i= 100 km/s.
We obtain a white dwarf orbital velocity semi-amplitude K1=107 km/s. Using the
gamma-velocity of Wade (1981) we obtain an Einstein redshift of 80.4 km/s and
hence a carbon core white dwarf mass of ~1.1 Msun. We report the first subsolar
chemical abundances of C and Si for U Gem with C down by 0.05 with respect to
the Sun, almost certainly a result of C depletion due to thermonuclear
processing. This C-depletion is discussed within the framework of a weak TNR,
contamination of the secondary during the common envelope phase, and mixing of
C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova
event. Remarkably the Teff of the white dwarf 13 days after outburst is only
32,000K, anomalously cooler than previous early post-outburst measurements.
Extensive cooling during an extraordinarily long (210 days) quiescence followed
by accretion onto an out-of-equilibrium cooled degenerate could explain the
lower Teff.Comment: 16 pages AAS-Latex, 4 Figures, accepted for publication in Ap
Weakly Coupled Motion of Individual Layers in Ferromagnetic Resonance
We demonstrate a layer- and time-resolved measurement of ferromagnetic
resonance (FMR) in a Ni81Fe19 / Cu / Co93Zr7 trilayer structure. Time-resolved
x-ray magnetic circular dichroism has been developed in transmission, with
resonant field excitation at a FMR frequency of 2.3 GHz. Small-angle (to 0.2
degree), time-domain magnetization precession could be observed directly, and
resolved to individual layers through elemental contrast at Ni, Fe, and Co
edges. The phase sensitivity allowed direct measurement of relative phase lags
in the precession oscillations of individual elements and layers. A weak
ferromagnetic coupling, difficult to ascertain in conventional FMR
measurements, is revealed in the phase and amplitude response of individual
layers across resonance.Comment: 22 pages, 6 figures submitted to Physical Review
Vitamin D Deficiency in Cystic Fibrosis
Cystic Fibrosis is the most common inherited genetic respiratory disorder in the Western World. Hypovitaminosis D is almost universal in CF patients, likely due to a combination of inadequate absorption, impaired metabolism, and lack of sun exposure. Inadequate levels are associated with the high prevalence of bone disease or osteoporosis in CF patients, which is associated with increased morbidity including fractures, kyphosis, and worsening pulmonary status. Treatment goals include regular monitoring 25 hydroxyvitamin D (25OHD) levels with aggressive treatment for those with levels <75 nmol/L (<30 ng/mL). More research is needed to determine optimal supplementation goals and strategies
Temporal behavior of the inverse spin Hall voltage in a magnetic insulator-nonmagnetic metal structure
It is demonstrated that upon pulsed microwave excitation, the temporal
behavior of a spin-wave induced inverse spin Hall voltage in a magnetic
insulator-nonmagnetic metal structure is distinctly different from the temporal
evolution of the directly excited spin-wave mode from which it originates. The
difference in temporal behavior is attributed to the excitation of long-lived
secondary spin-wave modes localized at the insulator-metal interface
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