2,284 research outputs found
The existence of warm and optically thick dissipative coronae above accretion disks
In the past years, several observations of AGN and X-ray binaries have
suggested the existence of a warm T around 0.5-1 keV and optically thick, \tau
~ 10-20, corona covering the inner parts of the accretion disk. These
properties are directly derived from spectral fitting in UV to soft-X-rays
using Comptonization models. However, whether such a medium can be both in
radiative and hydrostatic equilibrium with an accretion disk is still
uncertain. We investigate the properties of such warm, optically thick coronae
and put constraints on their existence. We solve the radiative transfer
equation for grey atmosphere analytically in a pure scattering medium,
including local dissipation as an additional heating term in the warm corona.
The temperature profile of the warm corona is calculated assuming it is cooled
by Compton scattering, with the underlying dissipative disk providing photons
to the corona. Our analytic calculations show that a dissipative thick,
(\tau_{cor} ~ 10-12) corona on the top of a standard accretion disk can reach
temperatures of the order of 0.5-1 keV in its upper layers provided that the
disk is passive. But, in absence of strong magnetic fields, the requirement of
a Compton cooled corona in hydrostatic equilibrium in the vertical direction
sets an upper limit on the Thomson optical depth \tau_{cor} < 5 . We show this
value cannot be exceeded independently of the accretion disk parameters.
However, magnetic pressure can extend this result to larger optical depths.
Namely, a dissipative corona might have an optical depth up to ~ 20 when the
magnetic pressure is 100 times higher that the gas pressure. The observation of
warm coronae with Thomson depth larger than ~ 5 puts tights constraints on the
physics of the accretion disk/corona systems and requires either strong
magnetic fields or vertical outflows to stabilize the system.Comment: 9 pages 6 figure, submitted to A&A, comments are welcom
Scaling of the electron dissipation range of solar wind turbulence
Electron scale solar wind turbulence has attracted great interest in recent
years. Clear evidences have been given from the Cluster data that turbulence is
not fully dissipated near the proton scale but continues cascading down to the
electron scales. However, the scaling of the energy spectra as well as the
nature of the plasma modes involved at those small scales are still not fully
determined. Here we survey 10 years of the Cluster search-coil magnetometer
(SCM) waveforms measured in the solar wind and perform a statistical study of
the magnetic energy spectra in the frequency range []Hz. We show that a
large fraction of the spectra exhibit clear breakpoints near the electon
gyroscale , followed by steeper power-law like spectra. We show that
the scaling below the electron breakpoint cannot be determined unambiguously
due to instrumental limitations that will be discussed in detail. We compare
our results to recent ones reported in other studies and discuss their
implication on the physical mechanisms and the theoretical modeling of energy
dissipation in the SW.Comment: 10 pages, submitte
Absorption lines from magnetically-driven winds in X-ray binaries
High resolution X-ray spectra of black hole X-ray binaries (BHBs) show
blueshifted absorption lines from disk winds which seem to be equatorial. Winds
occur in the Softer (disk-dominated) states of the outburst and are less
prominent or absent in the Harder (power-law dominated) states. We use
self-similar magneto-hydrodynamic (MHD) accretion-ejection models to explain
the disk winds in BHBs. In our models, the density at the base of the outflow
from the accretion disk is not a free parameter, but is determined by solving
the full set of dynamical MHD equations. Thus the physical properties of the
outflow are controlled by the global structure of the disk. We studied
different MHD solutions characterized by different values of (a) the disk
aspect ratio () and (b) the ejection efficiency (). We use two
kinds of MHD solutions depending on the absence (cold solution) or presence
(warm solution) of heating at the disk surface. Such heating could be from e.g.
dissipation of energy due to MHD turbulence in the disk or from illumination.
We use each of these MHD solutions to predict the physical parameters of an
outflow; put limits on the ionization parameter (), column density and
timescales, motivated by observational results; and thus select regions within
the outflow which are consistent with the observed winds. The cold MHD
solutions cannot account for winds due to their low ejection efficiency. But
warm solutions can explain the observed physical quantities in the wind because
they can have sufficiently high values of (, implying larger
mass loading at the base of the outflow). Further from our thermodynamic
equilibrium curve analysis for the outflowing gas, we found that in the Hard
state a range of is thermodynamically unstable, and had to be excluded.
This constrain made it impossible to have any wind at all, in the Hard state.Comment: 16 Pages, 10 figures in the main body and 4 figures in the appendix.
Accepted for publication in A&
Properties of AGN coronae in the NuSTAR era â II. Hybrid plasma
The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous active galactic nuclei. The continuum has a high-energy turnover, typically in the range of one to several 100 keV and is suggestive of Comptonization by thermal electrons. We are studying hard X-ray spectra of AGN obtained with NuSTAR after correction for X-ray reflection and under the assumption that coronae are compact, being only a few gravitational radii in size as indicated by reflection and reverberation modelling. Compact coronae raise the possibility that the temperature is limited and indeed controlled by electronâpositron pair production, as explored earlier (Paper I). Here, we examine hybrid plasmas in which a mixture of thermal and non-thermal particles is present. Pair production from the non-thermal component reduces the temperature leading to a wider temperature range more consistent with observations
Simulated Dynamics of Mixed Versus Uniform Grain Size Sediment Pulses in a Gravel-Bedded River
Mountain rivers often receive sediment in the form of episodic, discrete pulses from a variety of natural and anthropogenic processes. Once emplaced in the river, the movement of this sediment depends on flow, grain size distribution, and channel and network geometry. Here, we simulate downstream bed elevation changes that result from discrete inputs of sediment (10,000 m3), differing in volume and grain size distribution, under medium and high flow conditions. We specifically focus on comparing bed responses between mixed and uniform grain size sediment pulses. This work builds on a Lagrangian, bed-material sediment transport model and applies it to a 27 km reach of the mainstem Nisqually River, Washington, USA. We compare observed bed elevation change and accumulation rates in a downstream lake to simulation results. Then we investigate the magnitude, timing, and persistence of downstream changes due to the introduction of synthetic sediment pulses by comparing the results against a baseline condition (without pulse). Our findings suggest that bed response is primarily influenced by the sediment-pulse grain size and distribution. Intermediate mixed-size pulses (~50% of the median bed gravel size) are likely to have the largest downstream impact because finer sizes translate quickly and coarser sizes (median bed gravel size and larger) disperse slowly. Furthermore, a mixed-size pulse, with a smaller median grain size than the bed, increases bed mobility more than a uniform-size pulse. This work has important implications for river management, as it allows us to better understand fluvial geomorphic responses to variations in sediment supply
Timescale Dependence in River Channel Migration Measurements
Accurately measuring river meander migration over time is critical for sediment budgets and understanding how rivers respond to changes in hydrology or sediment supply. However, estimates of meander migration rates or streambank contributions to sediment budgets using repeat aerial imagery, maps, or topographic data will be underestimated without proper accounting for channel reversal. Furthermore, comparing channel planform adjustment measured over dissimilar timescales are biased because shortâ and longâterm measurements are disproportionately affected by temporary rate variability, longâterm hiatuses, and channel reversals. We evaluate the role of timescale dependence for the Root River, a single threaded meandering sandâ and gravelâbedded river in southeastern Minnesota, USA, with 76 years of aerial photographs spanning an era of landscape changes that have drastically altered flows.
Empirical data and results from a statistical river migration model both confirm a temporal measurementâscale dependence, illustrated by systematic underestimations (2â15% at 50 years) and convergence of migration rates measured over sufficiently long timescales (\u3e 40 years). Frequency of channel reversals exerts primary control on measurement bias for longer time intervals by erasing the record of observable migration. We conclude that using longâterm measurements of channel migration for sediment remobilization projections, streambank contributions to sediment budgets, sediment flux estimates, and perceptions of fluvial change will necessarily underestimate such calculations. © 2019 John Wiley & Sons, Ltd
Searching for TeV DM evidence from Dwarf Irregular Galaxies with the HAWC Observatory
The dynamics of dwarf irregular (dIrr) galaxies are observed to be dominated by dark matter (DM). Recently, the DM density distribution has been studied for 31 dIrrs. Their extended DM halo (Burket type profile) makes these objects good candidates for DM searches. Located in Puebla (Mexico), the High Altitude Water Cherenkov (HAWC) Observatory is an optimal instrument to perform such DM searches, because of its large sky coverage (8.4 sr per day). We analyzed a set of two years of HAWC data and we found no significant DM signal from dIrr galaxies. We present the upper limits for DM annihilation cross-section with dIrr galaxies
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