92 research outputs found
Heavy coronal ions in the heliosphere. II. Expected fluxes of energetic neutral He atoms from the heliosheath
Aims. A model of heliosheath density and energy spectra of alpha-particles
and He+ ions carried by the solar wind is developed. Neutralization of
heliosheath He+ ions, mainly by charge exchange (CX) with neutral interstellar
H and He atoms, gives rise to ~0.2 - ~100 keV fluxes of energetic neutral He
atoms (He ENA). Such fluxes, if observed, would give information about plasmas
in the heliosheath and heliospheric tail. Methods. Helium ions crossing the
termination shock (TS) constitute suprathermal (test) particles convected by
(locally also diffusing through) hydrodynamically calculated background plasma
flows (three versions of flows are employed). The He ions proceed from the TS
towards heliopause (HP) and finally to the heliospheric tail (HT). Calculations
of the evolution of alpha- and He+ particle densities and energy spectra
include binary interactions with background plasma and interstellar atoms,
adiabatic heating (cooling) resulting from flow compression (rarefaction), and
Coulomb scattering on background plasma. Results. Neutralization of
suprathermal He ions leads to the emergence of He ENA fluxes with energy
spectra modified by the Compton-Getting effect at emission and ENA loss during
flight to the Sun. Energy-integrated He ENA intensities are in the range ~0.05
- ~50 cm^-2 s^-1 sr^-1 depending on spectra at the TS (assumed
kappa-distributions), background plasma model, and look direction. The
tail/apex intensity ratio varies between ~1.8 and ~800 depending on model
assumptions. Energy spectra are broad with maxima in the ~0.2 - ~3 keV range
depending on the look direction and model. Conclusions. Expected heliosheath He
ENA fluxes may be measurable based on the capabilities of the IBEX spacecraft.
Data could offer insight into the heliosheath structure and improve
understanding of the post-TS solar wind plasmas. HT direction and extent could
be assessed.Comment: 11 pages, 8 figures. Accepted Astronomy & Astrophysic
Chaotic and stochastic processes in the accretion flows of the black hole X-ray binaries revealed by recurrence analysis
The black hole candidates exhibit fast variability of their X-ray emission on
a wide range of timescales. The short, coherent variations, with frequencies
above 1 Hz, are referred to as quasi-periodic oscillations, and are generally
explained by resonant effects in the black hole accretion flow. The purely
stochastic variability that occurs due to turbulent conditions in the plasma,
is quantified by the power density spectra and appears practically in all types
of sources and their spectral states. The specific kind of quasi-periodic
flares is expected, when the global structure of the accretion flow, governed
by the nonlinear hydrodynamics, induces fluctuations around a fixed point
solution. These conditions, which occur at high accretion rates, lead to the
variability in the sense of deterministic chaos. We study the nonlinear
behaviour of X-ray sources using the recurrence analysis method. We estimate
quantitatively the indications for deterministic chaos, such as the Renyi's
entropy, for the observed time series, and we compare them with the surrogate
data. Using the data collected by RXTE satellite, we reveal the oscillations
pattern and the observable properties of six black hole systems. We confirm
that both the well known microquasar GRS 1915+105, as well as its recently
discovered analogue, IGR J17091-3624, exhibit variability characteristic to
deterministic chaotic system. We conclude therefore that the underlying nature
of the process must be intrinsically connected in these sources with the
accretion flow instability, that leads to the limit cycle oscillations around a
fixed point. Furthermore, we studied four other black hole candidates with high
accretion rates. For GX 339-4, GRO J1655-40 and XTE J1550-564 we obtained
significant results for some observations, whereas for XTE J1650-500 no such
data set was found.Comment: 18 pages, 17 figures, 3 tables, accepted for publication in A&A, the
structure of the paper changed, section about testing removed, results
unchange
Interplay between heartbeat oscillations and wind outflow in microquasar IGR J17091-3624
During the bright outburst in 2011, the black hole candidate IGR J17091-3624
exhibited strong quasi-periodic flare-like events (on timescales of tens of
seconds) in some characteristic states, the so-called heartbeat state. From the
theoretical point of view, these oscillations may be modeled by the process of
accretion disk instability, driven by the dominant radiation pressure and
enhanced heating of the plasma. Although the mean accretion rate in this source
is probably below the Eddington limit, the oscillations will still have large
amplitudes. As the observations show, the source can exhibit strong wind
outflow during the soft state. This wind may help to partially or even
completely stabilize the heartbeat. Using our hydrodynamical code GLADIS, we
modeled the evolution of an accretion disk responsible for X-ray emission of
the source. We accounted for a variable wind outflow from the disk surface. We
examined the data archive from the Chandra and XMM-Newton satellites to find
the observed limitations on the wind physical properties, such as its velocity
and ionization state. We also investigated the long-term evolution of this
source, which lasted over about 600 days of observations, using the data
collected by the Swift and RXTE satellites. During this long period, the
oscillations pattern and the observable wind properties changed systematically.
We found that this source probably exhibits observable outbursts of appropriate
timescales and amplitudes as a result of the disk instability. Our model
requires a substantial wind component to explain the proper variability
pattern, and even complete suppression of flares in some states. The wind
mass-loss rate extracted from the data agrees quantitatively well with our
scenario.Comment: 12 pages, 8 figures. Published in Astronomy and Astrophysic
Solar wind He pickup ions as source of tens-of-keV/n neutral He atoms observed by the HSTOF/SOHO detector
Context. Since 1996, during periods of low solar activity, the HSTOF
instrument onboard the SOHO satellite has been measuring weak fluxes of He
atoms of 28-58 keV/n (helium energetic neutral atoms - He ENA). The probable
source region is the inner heliosheath.
Aims. We aim to understand the emission mechanism of He ENA based on
knowledge of heliosheath spatial extent and plasma content resulting from
Voyager 1 & 2 measurements in the period after termination shock crossings.
Methods. He ENA are generated by charge-exchange neutralization of energetic
helium ions on interstellar neutral H and He. Energy spectra of helium ions in
the heliosheath are calculated by following the evolution of their velocity
distribution functions when carried by and undergoing binary interactions with
plasma constituents of a background flow whose particle populations are modeled
to approximately render post-termination-shock Voyager data.
Results. The observed HSTOF He ENA form a higher energy part of general
heliospheric He ENA fluxes and can be explained by the proposed mechanism to
within 2{\sigma} error. The main factor determining the level of emission (and
its uncertainty) is the energy spectrum of He^+ pickup ions in post-termination
shock plasmas.Comment: 6 pages, 2 figures, v2: version accepted for publication in Astronomy
and Astrophysic
Solar ions in the heliosheath: a possible new source of heavy neutral atoms
We show that multiply ionized coronal C, N, O, Mg, Si, S ions carried by the
solar wind and neutralized by consecutive electron captures from neutral
interstellar atoms constitute an important new source of neutral atoms in the
inner heliosheath, with energies up to ~ 1 keV/n. In the model we developed,
the heavy ions are treated as test particles carried by hydrodynamic plasma
flow (with a Monte-Carlo description of interstellar neutrals) and undergoing
all relevant atomic processes determining the evolution of all charge-states of
considered species (radiative and dielectronic recombination, charge exchange,
photo-, and electron impact ionization). The total strength of the source is
from ~10^6 g/s for S to ~10^8 g/s for O, deposited as neutrals below the
heliopause. These atoms should provide, as they drift to supersonic wind
region, important sources of PUIs and eventually ACRs, especially for species
that are excluded from entering the heliosphere because of their ionization in
the LISM. The expected corresponding ENA fluxes at 1 AU are in the range 10^-4
- 10^0 at./(cm^2 s sr), depending on the species and direction (Table 2).Comment: Submitted for IGGP Astrophysics Conference, March 2006; 6 page
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