6,962 research outputs found
Broad iron emission lines in Seyfert Galaxies - re-condensation of gas onto an inner disk below the ADAF
Recent observations of Seyfert 1 AGN with Chandra, XMM-Newton and Suzaku
revealed broad iron K_alpha emission lines, some relativistically blurred. For
galactic black hole X-ray binaries XMM-Newton spectra during hard state also
reveal the presence of a relativistic iron emission line and a thermal
component, interpreted as an indication for a weak inner cool accretion disk
underneath a hot corona. These thermal components were found after the
transition from soft to hard spectral state and can be understood as sustained
by re-condensation of gas from an advection-dominated flow (ADAF) onto the
disk. In view of the similarity of accretion flows around stellar mass and
supermassive black holes we discuss whether the broad iron emission lines in
Seyfert 1 AGN can be understood as arising from a similar accretion flow
geometry. We derive Eddington-scaled accretion rates for Seyfert galaxies with
strong lines in samples of Miller (2007) and Nandra et al. (2007). For the
evaluation we use the observed X-ray luminosity, bolometric corrections and
black hole masses from literature, most values taken from Fabian and Vasudevan
(2009). Rates derived are less than 0.1 of the Eddington rate for more than
half of the sources. For 10^7 to 10^8 solar mass black holes in Seyfert 1 AGN
this limit corresponds to 0.01 to 0.2 solar masses per year. Our investigation
shows that for quite a number of Seyfert AGN in hard spectral state iron
emission lines can arise from an inner weak disk surrounded by an ADAF as
predicted by the re-condensation model. Some of the remaining sources with
higher accretion rates may be in a spectral state comparable to the "very high"
state of LMXBs. (abridged)Comment: 9 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
On the Origin of Ultraviolet Emission and the Accretion Model of Low-luminosity AGNs
Low-luminosity active galactic nuclei (LLAGNs) are generally believed to be
powered by an inner radiatively inefficient, advection-dominated accretion flow
(ADAF), an outer truncated thin disk, and a jet. Maoz (2007) recently
challenged this picture based on the observation that the strength of
ultraviolet emission relative to the X-ray and radio bands does not depart from
empirical trends defined by more luminous sources. He advocates that AGNs
across all luminosities have essentially the same accretion and radiative
processes, which in luminous sources are described by a standard optically
thick, geometrically thin disk. We calculate ADAF models and demonstrate that
they can successfully fit the observed spectral energy distributions of the
LLAGNs in Maoz's sample. Our model naturally accommodates the radio and X-ray
emission, and the ultraviolet flux is well explained by a combination of the
first-order Compton scattering in the ADAF, synchrotron emission in the jet,
and black body emission in the truncated thin disk. It is premature to dismiss
the ADAF model for LLAGNs. The UV data can be fit equally well using a standard
thin disk, but an additional corona and jet would be required to account for
the X-ray and radio emission. We argue that there are strong theoretical
reasons to prefer the ADAF model over the thin disk scenario. We discuss
testable predictions that can potentially discriminate between the two
accretion models.Comment: 20 pages, 3 figures; ApJ in pres
Revisiting the "Fundamental Plane" of Black Hole Activity at Extremely Low Luminosities
We investigate the origin of the X-ray emission in low-luminosity AGNs
(LLAGNs). Yuan & Cui (2005) predicted that the X-ray emission should originate
from jets rather than from an advection-dominated accretion flow (ADAF) when
the X-ray luminosity of the source is below a critical value of
. This prediction implies that the
X-ray spectrum in such sources should be fitted by jets rather than ADAFs.
Furthermore, below the correlation between radio ()
and X-ray () luminosities and the black hole mass ()--the
so-called fundamental plane of black hole activity--should deviate from the
general correlation obtained by Merloni, Heinz & Di Matteo (2003) and become
steeper. The Merloni et al. correlation is described by , while the predicted correlation is
. We collect
data from the literature to check the validity of these two expectations. We
find that among the 16 LLAGNs with good X-ray and radio spectra, 13 are
consistent with the Yuan & Cui prediction. For the 22 LLAGNs with , the fundamental plane correlation is described by , also in excellent
agreement with the prediction.Comment: 24 pages, 2 tables, 3 figures; accepted by Ap
Long surf
Theoretical investigations by the authors on surf due to long swell are summarized and presented here in a form unencumbered by mathematical technicalities. The conclusions derived thus far are mainly qualitative, but in some respects the r esults are quite detailed and differ radically from results of earlier investigations; some new observations are also presented...
Possible Evidence for Truncated Thin Disks in the Low-Luminosity Active Galactic Nuclei M81 and NGC 4579
M81 and NGC 4579 are two of the few low-luminosity active galactic nuclei
which have an estimated mass for the central black hole, detected hard X-ray
emission, and detected optical/UV emission. In contrast to the canonical ``big
blue bump,'' both have optical/UV spectra which decrease with increasing
frequency in a plot. Barring significant reddening by dust and/or
large errors in the black hole mass estimates, the optical/UV spectra of these
systems require that the inner edge of a geometrically thin, optically thick,
accretion disk lies at roughly 100 Schwarzschild radii. The observed X-ray
radiation can be explained by an optically thin, two temperature,
advection-dominated accretion flow at smaller radii.Comment: emulateapj.sty, to appear in ApJ Letter
The hard to soft spectral transition in LMXBs - affected by recondensation of gas into an inner disk
Soft and hard spectral states of X-ray transient sources reflect two modes of
accretion, accretion via a geometrically thin, optically thick disk or an
advection-dominated accretion flow (ADAF). The luminosity at transition between
these two states seems to vary from source to source, or even for the same
source during different outbursts, as observed for GX 339-4. We investigate how
the existence of an inner weak disk in the hard state affects the transition
luminosity. We evaluate the structure of the corona above an outer truncated
disk and the resulting disk evaporation rate for different irradiation. In some
cases, recent observations of X-ray transients indicate the presence of an
inner cool disk during the hard state. Such a disk can remain during quiescence
after the last outburst as long as the luminosity does not drop to very low
values (10^-4 to 10^-3 of the Eddington luminosity). Consequently, as part of
the matter accretes via the inner disk, the hard irradiation is reduced. The
hard irradiation is further reduced, occulted and partly reflected by the inner
disk. This leads to a hard-soft transition at a lower luminosity if an inner
disk exists below the ADAF. This seems to be supported by observations for GX
339-4.Comment: 9 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Differentiation and Three-dimensional Organization of Retinal Ganglion Cells using Human Induced Pluripotent Stem Cells
poster abstractRetinal Ganglion Cells (RGCs) are a type of neuron which function to relay visual messages between the retina and brain, and are characterized by their long axons which form part of the optic nerve. Dysfunction in this communication pathway is highly implicated in degenerative blinding disorders such as glaucoma. Unique applications using human induced pluripotent stem cells (hiPSCs) offer the ability to model human diseases, and potentially develop novel therapeutic approaches to rescue or replace damaged cells. In order to better understand the progression of degenerative eye diseases, a remaining challenge is to precisely identify the sequence of events which contribute to the diseased state, and how their features differ from non-diseased cells. Efforts were therefore undertaken to visually document the maturation of RGCs by analyzing their morphology and three-dimension organization at varying stages of development. Induced retinal cells were harvested at six different stages of development and fixed in 4% paraformaldehyde (PFA) solution to arrest their development. Cells were then cryoprotected in combinations of sucrose and Optimal Cutting Temperature (OCT) solutions, and frozen using powered dry ice. Following cryostat sectioning, samples were subject to immunocytochemistry staining to visualize for retinal-like organization of cells. Preliminary results have indicated the presence of the RGC marker Brn3, as well as markers for other retinal cell types. Future tests intend to characterize these retinal cell types according to their morphology and three-dimensional organization
The Role of FDI Motives in the Link between Institutional Distance and Subsidiary Ownership Choice by Emerging Market Multinational Enterprises
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