34 research outputs found
Self-similar solutions of viscous and resistive ADAFs with thermal conduction
We have studied the effects of thermal conduction on the structure of viscous
and resistive advection-dominated accretion flows (ADAFs). The importance of
thermal conduction on hot accretion flow is confirmed by observations of hot
gas that surrounds Sgr A and a few other nearby galactic nuclei. In this
research, thermal conduction is studied by a saturated form of it, as is
appropriated for weakly-collisional systems. It is assumed the viscosity and
the magnetic diffusivity are due to turbulence and dissipation in the flow. The
viscosity also is due to angular momentum transport. Here, the magnetic
diffusivity and the kinematic viscosity are not constant and vary by position
and -prescription is used for them. The govern equations on system have
been solved by the steady self-similar method. The solutions show the radial
velocity is highly subsonic and the rotational velocity behaves sub-Keplerian.
The rotational velocity for a specific value of the thermal conduction
coefficient becomes zero. This amount of conductivity strongly depends on
magnetic pressure fraction, magnetic Prandtl number, and viscosity parameter.
Comparison of energy transport by thermal conduction with the other energy
mechanisms implies that thermal conduction can be a significant energy
mechanism in resistive and magnetized ADAFs. This property is confirmed by
non-ideal magnetohydrodynamics (MHD) simulations.Comment: 8 pages, 5 figures, accepted by Ap&S
Modelling spectral and timing properties of accreting black holes: the hybrid hot flow paradigm
The general picture that emerged by the end of 1990s from a large set of
optical and X-ray, spectral and timing data was that the X-rays are produced in
the innermost hot part of the accretion flow, while the optical/infrared (OIR)
emission is mainly produced by the irradiated outer thin accretion disc. Recent
multiwavelength observations of Galactic black hole transients show that the
situation is not so simple. Fast variability in the OIR band, OIR excesses
above the thermal emission and a complicated interplay between the X-ray and
the OIR light curves imply that the OIR emitting region is much more compact.
One of the popular hypotheses is that the jet contributes to the OIR emission
and even is responsible for the bulk of the X-rays. However, this scenario is
largely ad hoc and is in contradiction with many previously established facts.
Alternatively, the hot accretion flow, known to be consistent with the X-ray
spectral and timing data, is also a viable candidate to produce the OIR
radiation. The hot-flow scenario naturally explains the power-law like OIR
spectra, fast OIR variability and its complex relation to the X-rays if the hot
flow contains non-thermal electrons (even in energetically negligible
quantities), which are required by the presence of the MeV tail in Cyg X-1. The
presence of non-thermal electrons also lowers the equilibrium electron
temperature in the hot flow model to <100 keV, making it more consistent with
observations. Here we argue that any viable model should simultaneously explain
a large set of spectral and timing data and show that the hybrid
(thermal/non-thermal) hot flow model satisfies most of the constraints.Comment: 26 pages, 13 figures. To be published in the Space Science Reviews
and as hard cover in the Space Sciences Series of ISSI - The Physics of
Accretion on to Black Holes (Springer Publisher
Vertically Self-Gravitating ADAFs in the Presence of Toroidal Magnetic Field
Force due to the self-gravity of the disc in the vertical direction is
considered to study its possible effects on the structure of a magnetized
advection-dominated accretion disc. We present steady-sate self similar
solutions for the dynamical structure of such a type of the accretion flows.
Our solutions imply reduced thickness of the disc because of the self-gravity.
It also imply that the thickness of the disc will increase by adding the
magnetic field strength.Comment: Accepted for publication in Astrophysics and Space Science
Electronic structure, charge transfer, and intrinsic luminescence of gadolinium oxide nanoparticles: Experiment and theory
The cubic (c) and monoclinic (m) polymorphs of Gd2O3 were studied using the
combined analysis of several materials science techniques - X-ray diffraction
(XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy
(XPS), and photoluminescence (PL) spectroscopy. Density functional theory (DFT)
based calculations for the samples under study were performed as well. The
cubic phase of gadolinium oxide (c-Gd2O3) synthesized using a precipitation
method exhibits spheroidal-like nanoclusters with well-defined edges assembled
from primary nanoparticles with an average size of 50 nm, whereas the
monoclinic phase of gadolinium oxide (m-Gd2O3) deposited using explosive
pyrolysis has a denser structure compared with natural gadolinia. This phase
also has a structure composed of three-dimensional complex agglomerates without
clear-edged boundaries that are ~21 nm in size plus a cubic phase admixture of
only 2 at. % composed of primary edge-boundary nanoparticles ~15 nm in size.
These atomic features appear in the electronic structure as different defects
([Gd...O-OH] and [Gd...O-O]) and have dissimilar contributions to the
charge-transfer processes among the appropriate electronic states with
ambiguous contributions in the Gd 5p - O 2s core-like levels in the valence
band structures. The origin of [Gd...O-OH] defects found by XPS was
well-supported by PL analysis. The electronic and atomic structures of the
synthesized gadolinias calculated using DFT were compared and discussed on the
basis of the well-known joint OKT-van der Laan model, and good agreement was
established.Comment: 27 pages, 10 figures, accepted in Appl. Surf. Sc
The balance of power: accretion and feedback in stellar mass black holes
In this review we discuss the population of stellar-mass black holes in our
galaxy and beyond, which are the extreme endpoints of massive star evolution.
In particular we focus on how we can attempt to balance the available accretion
energy with feedback to the environment via radiation, jets and winds,
considering also possible contributions to the energy balance from black hole
spin and advection. We review quantitatively the methods which are used to
estimate these quantities, regardless of the details of the astrophysics close
to the black hole. Once these methods have been outlined, we work through an
outburst of a black hole X-ray binary system, estimating the flow of mass and
energy through the different accretion rates and states. While we focus on
feedback from stellar mass black holes in X-ray binary systems, we also
consider the applicability of what we have learned to supermassive black holes
in active galactic nuclei. As an important control sample we also review the
coupling between accretion and feedback in neutron stars, and show that it is
very similar to that observed in black holes, which strongly constrains how
much of the astrophysics of feedback can be unique to black holes.Comment: To be published in Haardt et al. Astrophysical Black Holes. Lecture
Notes in Physics. Springer 201
Theoretical overview on high-energy emission in microquasars
Microquasar (MQ) jets are sites of particle acceleration and synchrotron
emission. Such synchrotron radiation has been detected coming from jet regions
of different spatial scales, which for the instruments at work nowadays appear
as compact radio cores, slightly resolved radio jets, or (very) extended
structures. Because of the presence of relativistic particles and dense photon,
magnetic and matter fields, these outflows are also the best candidates to
generate the very high-energy (VHE) gamma-rays detected coming from two of
these objects, LS 5039 and LS I +61 303, and may be contributing significantly
to the X-rays emitted from the MQ core. In addition, beside electromagnetic
radiation, jets at different scales are producing some amount of leptonic and
hadronic cosmic rays (CR), and evidences of neutrino production in these
objects may be eventually found. In this work, we review on the different
physical processes that may be at work in or related to MQ jets. The jet
regions capable to produce significant amounts of emission at different
wavelengths have been reduced to the jet base, the jet at scales of the order
of the size of the system orbital semi-major axis, the jet middle scales (the
resolved radio jets), and the jet termination point. The surroundings of the
jet could be sites of multiwavelegnth emission as well, deserving also an
insight. We focus on those scenarios, either hadronic or leptonic, in which it
seems more plausible to generate both photons from radio to VHE and high-energy
neutrinos. We briefly comment as well on the relevance of MQ as possible
contributors to the galactic CR in the GeV-PeV range.Comment: Astrophysics & Space Science, in press (invited talk in the
conference: The multimessenger approach to the high-energy gamma-ray
sources", Barcelona/Catalonia, in July 4-7); 10 pages, 6 figures, 2 tables
(one reference corrected
Unidentified gamma-ray sources off the Galactic plane as low-mass microquasars?
A subset of the unidentified EGRET gamma-ray sources with no active galactic
nucleus or other conspicuous counterpart appears to be concentrated at medium
latitudes. Their long-term variability and their spatial distribution indicate
that they are distinct from the more persistent sources associated with the
nearby Gould Belt. They exhibit a large scale height of 1.3 +/- 0.6 kpc above
the Galactic plane. Potential counterparts for these sources include
microquasars accreting from a low-mass star and spewing a continuous jet.
Detailed calculations have been performed of the jet inverse Compton emission
in the radiation fields from the star, the accretion disc, and a hot corona.
Different jet Lorentz factors, powers, and aspect angles have been explored.
The up-scattered emission from the corona predominates below 100 MeV whereas
the disc and stellar contributions are preponderant at higher energies for
moderate (~15 deg) and small (~1 deg) aspect angles, respectively. Yet, unlike
in the high-mass, brighter versions of these systems, the external Compton
emission largely fails to produce the luminosities required for 5 to 10 kpc
distant EGRET sources. Synchrotron-self-Compton emission appears as a promising
alternative.Comment: 11 pages, 5 figures. Contributed paper to the "Multiwavelength
Approach to Unidentified Gamma-Ray Sources", Eds. K.S. Cheng & G.E. Romero,
to appear in Astrophysics and Space Science journa
General Overview of Black Hole Accretion Theory
I provide a broad overview of the basic theoretical paradigms of black hole
accretion flows. Models that make contact with observations continue to be
mostly based on the four decade old alpha stress prescription of Shakura &
Sunyaev (1973), and I discuss the properties of both radiatively efficient and
inefficient models, including their local properties, their expected stability
to secular perturbations, and how they might be tied together in global flow
geometries. The alpha stress is a prescription for turbulence, for which the
only existing plausible candidate is that which develops from the
magnetorotational instability (MRI). I therefore also review what is currently
known about the local properties of such turbulence, and the physical issues
that have been elucidated and that remain uncertain that are relevant for the
various alpha-based black hole accretion flow models.Comment: To be published in Space Science Reviews and as hard cover in the
Space Sciences Series of ISSI: The Physics of Accretion on to Black Holes
(Springer Publisher
Active scars in clinical presentation of postoperative persistent syndrome (Failed back surgery syndrome)
Objective. To assess the role of postoperative scar trigger zones in patients with postoperative persistent syndrome (POPS), effec-tiveness of lidocaine/prilocaine cream and manual treatment methods. Material and methods. Three groups of patients were examined and treated: 1) «early POPS» (e-POPS, n=23), pain after surgery decreased, but continued to significantly disturb the patient; 2) «middle POPS» (m-POPS, n=42), complete regression of pain af-ter surgery with subsequent recurrence within 6—12 months; 3) «late POPS» (l-POPS, n=31) — pain relapse occurred later than 12 months after surgery. Examination included manual diagnosis of skin and soft tissue thresholds within the scar area, tensoalgom-etry with a 1 mm2 nozzle. Treatment included 2 stages: 1) lidocaine/prilocaine cream application on postoperative scar for 2 hours daily for 5 days; 2) manual therapy with soft tissue stretching and pressure. Results. All patients showed a decrease in physiological barrier of skin and soft tissues around the scar by 5—10 mm. Tensoal-gometry revealed scar trigger zones (STZ) in all patients of the e-POPS group with reproduction (complete or partial) of typical pain pattern. The same STZs were found in all patients of other subgroups, but pain pattern reproduction was found in a smaller number of patients (m-POPS subgroup — 13 out of 42, l-POPS subgroup — 2 out of 31). Tensoalgometry data (kg/cm2): e-POPS before treatment — 21.2±12.5, after the first stage of treatment — 64.3±19.5 (p=0.00045), after the second stage of treatment — 87.6±13.5 (p=0.0054); m-POPS before treatment — 51.5±23.2, after the first stage of treatment — 71.5±31.7 (p=0.0054), after the second stage of treatment — 91.4±34.9 (p=0.0043); l-POPS before treatment — 61.3±33.6, after the first stage of treatment — 81.7±41.7 (p=0.035), after the second stage of treatment — 88.7±42.5 (p>0.05). Conclusion. Lidocaine/prilocaine cream showed a good analgesic effect for 4 hours in all patients. Manual therapy also significantly increased pain thresholds, except for the l-POPS group. These patients had a significant effect after lidocaine/prilocaine cream application. The authors recommend lidocaine/prilocaine cream in addition to conventional manual therapy for the treatment of active scars. This cream is characterized by high efficacy, safety and good tolerance