1,303 research outputs found
Comparative needs in child abuse education and resources: perceptions from three medical specialties
A 15.65 solar mass black hole in an eclipsing binary in the nearby spiral galaxy Messier 33
Stellar-mass black holes are discovered in X-ray emitting binary systems,
where their mass can be determined from the dynamics of their companion stars.
Models of stellar evolution have difficulty producing black holes in close
binaries with masses >10 solar masses, which is consistent with the fact that
the most massive stellar black holes known so all have masses within 1 sigma of
10 solar masses. Here we report a mass of 15.65 +/- 1.45 solar masses for the
black hole in the recently discovered system M33 X-7, which is located in the
nearby galaxy Messier 33 (M33) and is the only known black hole that is in an
eclipsing binary. In order to produce such a massive black hole, the progenitor
star must have retained much of its outer envelope until after helium fusion in
the core was completed. On the other hand, in order for the black hole to be in
its present 3.45 day orbit about its 70.0 +/- 6.9 solar mass companion, there
must have been a ``common envelope'' phase of evolution in which a significant
amount of mass was lost from the system. We find the common envelope phase
could not have occured in M33 X-7 unless the amount of mass lost from the
progenitor during its evolution was an order of magnitude less than what is
usually assumed in evolutionary models of massive stars.Comment: To appear in Nature October 18, 2007. Four figures (one color figure
degraded). Differs slightly from published version. Supplementary Information
follows in a separate postin
Ultrahard carbon film from epitaxial two-layer graphene
Atomically thin graphene exhibits fascinating mechanical properties, although
its hardness and transverse stiffness are inferior to those of diamond. To
date, there hasn't been any practical demonstration of the transformation of
multi-layer graphene into diamond-like ultra-hard structures. Here we show that
at room temperature and after nano-indentation, two-layer graphene on SiC(0001)
exhibits a transverse stiffness and hardness comparable to diamond, resisting
to perforation with a diamond indenter, and showing a reversible drop in
electrical conductivity upon indentation. Density functional theory
calculations suggest that upon compression, the two-layer graphene film
transforms into a diamond-like film, producing both elastic deformations and
sp2-to-sp3 chemical changes. Experiments and calculations show that this
reversible phase change is not observed for a single buffer layer on SiC or
graphene films thicker than 3 to 5 layers. Indeed, calculations show that
whereas in two-layer graphene layer-stacking configuration controls the
conformation of the diamond-like film, in a multilayer film it hinders the
phase transformation.Comment: Published online on Nature Nanotechnology on December 18, 201
Foundations of Black Hole Accretion Disk Theory
This review covers the main aspects of black hole accretion disk theory. We
begin with the view that one of the main goals of the theory is to better
understand the nature of black holes themselves. In this light we discuss how
accretion disks might reveal some of the unique signatures of strong gravity:
the event horizon, the innermost stable circular orbit, and the ergosphere. We
then review, from a first-principles perspective, the physical processes at
play in accretion disks. This leads us to the four primary accretion disk
models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin)
disks, slim disks, and advection-dominated accretion flows (ADAFs). After
presenting the models we discuss issues of stability, oscillations, and jets.
Following our review of the analytic work, we take a parallel approach in
reviewing numerical studies of black hole accretion disks. We finish with a few
select applications that highlight particular astrophysical applications:
measurements of black hole mass and spin, black hole vs. neutron star accretion
disks, black hole accretion disk spectral states, and quasi-periodic
oscillations (QPOs).Comment: 91 pages, 23 figures, final published version available at
http://www.livingreviews.org/lrr-2013-
The Galactic Center Black Hole Laboratory
The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*)
shows flare emission from the millimeter to the X-ray domain. A detailed
analysis of the infrared light curves allows us to address the accretion
phenomenon in a statistical way. The analysis shows that the near-infrared
flare amplitudes are dominated by a single state power law, with the low states
in SgrA* limited by confusion through the unresolved stellar background. There
are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO
is one of them. Its nature is unclear. It may be comparable to similar stellar
dusty sources in the region or may consist predominantly of gas and dust. In
this case a particularly enhanced accretion activity onto SgrA* may be expected
in the near future. Here the interpretation of recent data and ongoing
observations are discussed.Comment: 30 pages - 7 figures - accepted for publication by Springer's
"Fundamental Theories of Physics" series; summarizing GC contributions of 2
conferences: 'Equations of Motion in Relativistic Gravity' at the
Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the
COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov.
19 - 22, 2013
Pressure is not a state function for generic active fluids
Pressure is the mechanical force per unit area that a confined system exerts
on its container. In thermal equilibrium, it depends only on bulk properties
(density, temperature, etc.) through an equation of state. Here we show that in
a wide class of active systems the pressure depends on the precise interactions
between the active particles and the confining walls. In general, therefore,
active fluids have no equation of state, their mechanical pressures exhibit
anomalous properties that defy the familiar thermodynamic reasoning that holds
in equilibrium. The pressure remains a function of state, however, in some
specific and well-studied active models that tacitly restrict the character of
the particle-wall and/or particle-particle interactions.Comment: 8 pages + 9 SI pages, Nature Physics (2015
X-ray Absorption and Reflection in Active Galactic Nuclei
X-ray spectroscopy offers an opportunity to study the complex mixture of
emitting and absorbing components in the circumnuclear regions of active
galactic nuclei, and to learn about the accretion process that fuels AGN and
the feedback of material to their host galaxies. We describe the spectral
signatures that may be studied and review the X-ray spectra and spectral
variability of active galaxies, concentrating on progress from recent Chandra,
XMM-Newton and Suzaku data for local type 1 AGN. We describe the evidence for
absorption covering a wide range of column densities, ionization and dynamics,
and discuss the growing evidence for partial-covering absorption from data at
energies > 10 keV. Such absorption can also explain the observed X-ray spectral
curvature and variability in AGN at lower energies and is likely an important
factor in shaping the observed properties of this class of source.
Consideration of self-consistent models for local AGN indicates that X-ray
spectra likely comprise a combination of absorption and reflection effects from
material originating within a few light days of the black hole as well as on
larger scales. It is likely that AGN X-ray spectra may be strongly affected by
the presence of disk-wind outflows that are expected in systems with high
accretion rates, and we describe models that attempt to predict the effects of
radiative transfer through such winds, and discuss the prospects for new data
to test and address these ideas.Comment: Accepted for publication in the Astronomy and Astrophysics Review. 58
pages, 9 figures. V2 has fixed an error in footnote
Population specificity of the DNAI1 gene mutation spectrum in primary ciliary dyskinesia (PCD)
<p>Abstract</p> <p>Background</p> <p>Mutations in the <it>DNAI1 </it>gene, encoding a component of outer dynein arms of the ciliary apparatus, are the second most important genetic cause of primary ciliary dyskinesia (PCD), the genetically heterogeneous recessive disorder with the prevalence of ~1/20,000. The estimates of the <it>DNAI1 </it>involvement in PCD pathogenesis differ among the reported studies, ranging from 4% to 10%.</p> <p>Methods</p> <p>The coding sequence of <it>DNAI1 </it>was screened (SSCP analysis and direct sequencing) in a group of PCD patients (157 families, 185 affected individuals), the first ever studied large cohort of PCD patients of Slavic origin (mostly Polish); multiplex ligation-dependent probe amplification (MLPA) analysis was performed in a subset of ~80 families.</p> <p>Results</p> <p>Three previously reported mutations (IVS1+2-3insT, L513P and A538T) and two novel missense substitutions (C388Y and G515S) were identified in 12 families (i.e. ~8% of non-related Polish PCD patients). The structure of background SNP haplotypes indicated common origin of each of the two most frequent mutations, IVS1+2-3insT and A538T. MLPA analysis did not reveal any significant differences between patients and control samples. The Polish cohort was compared with all the previously studied PCD groups (a total of 487 families): IVS1+2-3insT remained the most prevalent pathogenetic change in <it>DNAI1 </it>(54% of the mutations identified worldwide), and the increased global prevalence of A538T (14%) was due to the contribution of the Polish cohort.</p> <p>Conclusions</p> <p>The worldwide involvement of <it>DNAI1 </it>mutations in PCD pathogenesis in families not preselected for ODA defects ranges from 7 to 10%; this global estimate as well as the mutation profile differs in specific populations. Analysis of the background SNP haplotypes suggests that the increased frequency of chromosomes carrying A538T mutations in Polish patients may reflects local (Polish or Slavic) founder effect. Results of the MLPA analysis indicate that no large exonic deletions are involved in PCD pathogenesis.</p
- …