114 research outputs found
A BeppoSAX Observation of the IC1262 Galaxy Cluster
We present an analysis of BeppoSAX observations of the IC1262 galaxy cluster
and report the first temperature and abundance measurements, along with
preliminary indications of diffuse, nonthermal emission. By fitting a 6' (~360
h_50^-1 kpc) region with a single Mewe-Kaastra-Liedahl model with photoelectric
absorption, we find a temperature of 2.1 - 2.3 keV, and abundance of 0.45 -
0.77 (both 90% confidence). We find the addition of a power-law component
provides a statistically significant improvement (F-test = 90%) to the fit. The
addition of a second thermal component also improves the fit but we argue that
it is physically implausible. The power-law component has a photon index
(Gamma_X) of 0.4 - 2.8 and a nonthermal flux of (4.1 - 56.7) x 10-5 photons
cm^-2 s^-1 over the 1.5 - 10.5 keV range in the Medium Energy Concentrator
spectrometer detector. An unidentified X-ray source found in the ROSAT High
Resolution Imager observation (~0'.9 from the center of the cluster) is a
possible explanation for the nonthermal flux; however, additional evidence of
diffuse, nonthermal emission comes from the NRAO VLA Sky Survey and the
Westerbork Northern Sky Survey radio measurements, in which excess diffuse,
radio flux is observed after point-source subtraction. The radio excess can be
fitted to a simple power law with a spectral index of ~1.8, which is consistent
with the nonthermal X-ray emission spectral index. The steep spectrum is
typical of diffuse emission and the size of the radio source implies that it is
larger than the cD galaxy and not due to a discreet source
Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map
We present a near-infrared extinction study of the dark globule L694-2, a
starless core that shows strong evidence for inward motions in molecular line
profiles. The J,H, and K band data were taken using the European Southern
Observatory New Technology Telescope. The best fit simple spherical power law
model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled
in extinction. This power law slope is steeper than the value of p=2 for a
singular isothermal sphere, the initial condition of the inside-out model for
protostellar collapse. Including an additional extinction component along the
line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert
sphere results in a super-critical value of the dimensionless radius xi_max=25
+/- 3. The unstable configuration of material may be related to the observed
inward motions. The Bonnor-Ebert model matches the shape of the observed
profile, but significantly underestimates the amount of extinction (by a factor
of ~4). This discrepancy in normalization has also been found for the nearby
protostellar core B335 (Harvey et al. 2001). A cylindrical density model with
scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder
axis provides an equally good radial profile as a power law model, and
reproduces the asymmetry of the core remarkably well. In addition, this model
provides a basis for understanding the discrepancy in the normalization of the
Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full
extent (mass) of the core being missed by assuming symmetry between the
profiles in the plane of the sky and along the line-of-sight. If the core is
sufficiently magnetized then fragmentation may be avoided, and later evolution
might produce a protostar similar to B335.Comment: 38 pages, 7 figures, accepted to Astrophysical Journa
Dynamics of Accretion Flows Irradiated by a Quasar
We present the results from axisymmetric time-dependent HD calculations of gas flows which are under the influence of gravity of a black hole in quasars. We assume that the flows are non-rotating and exposed to quasar radiation. We take into account X-ray heating and the radiation force due to electron scattering and spectral lines. To compute the radiation field, we consider a standard accretion disk as a source of UV photons and a spherical central object as a source of X-rays. The gas temperature and ionization state in the flow are calculated self-consistently from the photoionization and heating rate of the central object. We find that for a 10e8 MSUN black hole with an accretion luminosity of 0.6 of the Eddington luminosity the flow settles into a steady state and has two components: (1) an equatorial inflow and (2) a bipolar inflow/outflow with the outflow leaving the system along the disk rotational axis. The inflow is a realization of a Bondi-like accretion flow. The second component is an example of a non-radial accretion flow which becomes an outflow once it is pushed close to the rotational axis where thermal expansion and the radiation pressure accelerate it outward. Our main result is that the existence of the above two flow components is robust to the outer boundary conditions and the geometry and spectral energy distribution of the radiation field. However, the flow properties are not robust. In particular, the outflow power and collimation is higher for the radiation dominated by the UV/disk emission than for the radiation dominated by the X-ray/central engine emission. Our most intriguing result is that a very narrow outflow driven by radiation pressure on lines can carry more energy and mass than a broad outflow driven by thermal expansion
Brief Communication: CATALYST - a multi-regional stakeholder think tank for fostering capacity development in disaster risk reduction and climate change adaptation
Abstract. This brief communication presents the work and objectives of the CATALYST project on "Capacity Development for Hazard Risk Reduction and Adaptation" funded by the European Commission (October 2011–September 2013). CATALYST set up a multi-regional think tank covering four regions (Central America and the Caribbean, East and West Africa, the European Mediterranean, and South and Southeast Asia), intending to strengthen capacity development for stakeholders involved in disaster risk reduction (DRR) and climate change adaptation, in the context of natural hazards. This communication concludes with a selection of recommendations for capacity development in DRR and climate change adaptation from the perspective of governance issues
Accretion of low angular momentum material onto black holes: 2D hydrodynamical inviscid case
We report on the first phase of our study of slightly rotating accretion
flows onto black holes. We consider inviscid accretion flows with a spherically
symmetric density distribution at the outer boundary, but with spherical
symmetry broken by the introduction of a small, latitude-dependent angular
momentum. We study accretion flows by means of numerical 2D, axisymmetric,
hydrodynamical simulations. Our main result is that the properties of the
accretion flow do not depend as much on the outer boundary conditions (i.e.,
the amount as well as distribution of the angular momentum) as on the geometry
of the non-accreting matter. The material that has too much angular momentum to
be accreted forms a thick torus near the equator. Consequently, the geometry of
the polar region, where material is accreted (the funnel), and the mass
accretion rate through it are constrained by the size and shape of the torus.
Our results show one way in which the mass accretion rate of slightly rotating
gas can be significantly reduced compared to the accretion of non-rotating gas
(i.e., the Bondi rate), and set the stage for calculations that will take into
account the transport of angular momentum and energy.Comment: LaTeX,to appear in Ap
Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity
© 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually, with a considerable fraction being sequestered in large deltas, home to over 500 million people. Most (more than 70 per cent) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping, and a sustainable supply of fluvial sediment is therefore critical to prevent deltas being 'drowned' by rising relative sea levels. Here we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones in transmitting suspended sediment to one of the world's great deltas. We demonstrate that spatial variations in the Mekong's suspended sediment load are correlated (r = 0.765, P < 0.1) with observed variations in tropical-cyclone climatology, and that a substantial portion (32 per cent) of the suspended sediment load reaching the delta is delivered by runoff generated by rainfall associated with tropical cyclones. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 megatonnes over recent years (1981-2005), of which 33.0 ± 7.1 megatonnes is due to a shift in tropical-cyclone climatology. Consequently, tropical cyclones have a key role in controlling the magnitude of, and variability in, transmission of suspended sediment to the coast. It is likely that anthropogenic sediment trapping in upstream reservoirs is a dominant factor in explaining past, and anticipating future, declines in suspended sediment loads reaching the world's major deltas. However, our study shows that changes in tropical-cyclone climatology affect trends in fluvial suspended sediment loads and thus are also key to fully assessing the risk posed to vulnerable coastal systems
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