14 research outputs found
An analytic relation for the thickness of accretion flows
We take the vertical distribution of the radial and azimuthal velocity into
account in spherical coordinates, and find that the analytic relation
c_{s0}/(v_K \Theta) = [(\gamma -1)/(2\gamma)]^{1/2} is valid for both
geometrically thin and thick accretion flows, where c_{s0} is the sound speed
on the equatorial plane, v_K is the Keplerian velocity, \Theta is the
half-opening angle of the flow, and \gamma is the adiabatic index.Comment: 4 pages, 2 figures, accepted by Science in China Series
Serendipitous Kepler observations of a background dwarf nova of SU UMa type
We have discovered a dwarf nova (DN) of type SU UMa in Kepler data which is
7.0 arcsec from the G-type exoplanet survey target KIC 4378554. The DN appears
as a background source in the pixel aperture of the foreground G star. We
extracted only the pixels where the DN is present and observed the source to
undergo five outbursts -- one a superoutburst -- over a timespan of 22 months.
The superoutburst was triggered by a normal outburst, a feature that has been
seen in all DNe superoutburst observed by Kepler. Superhumps during the super
outburst had a period of 1.842+/-0.004 h and we see a transition from
disc-dominated superhump signal to a mix of disc and accretion stream impact.
Predictions of the number of DNe present in Kepler data based on previously
published space densities vary from 0.3 to 258. An investigation of the
background pixels targets would lead to firmer constraints on the space density
of DN.Comment: Accepted for publication in MNRA
Revisiting vertical structure of neutrino-dominated accretion disks: Bernoulli parameter, neutrino trapping and other distributions
We revisit the vertical structure of neutrino dominated accretion flows
(NDAFs) in spherical coordinates with a new boundary condition based on the
mechanical equilibrium. The solutions show that NDAF is significantly thick.
The Bernoulli parameter and neutrino trapping are determined by the mass
accretion rate and the viscosity parameter. According to the distribution of
the Bernoulli parameter, the possible outflow may appear in the outer region of
the disk. The neutrino trapping can essentially affect the neutrino radiation
luminosity. The vertical structure of NDAF is like a "sandwich", and the
multilayer accretion may account for the flares in gamma-ray bursts.Comment: 7 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Quasi-periodic oscillations in accreting magnetic white dwarfs II. The asset of numerical modelling for interpreting observations
Magnetic cataclysmic variables are close binary systems containing a strongly
magnetized white dwarf that accretes matter coming from an M-dwarf companion.
High-energy radiation coming from those objects is emitted from the accretion
column close to the white dwarf photosphere at the impact region. Its
properties depend on the characteristics of the white dwarf and an accurate
accretion column model allows the properties of the binary system to be
inferred, such as the white dwarf mass, its magnetic field, and the accretion
rate. We study the temporal and spectral behaviour of the accretion region and
use the tools we developed to accurately connect the simulation results to the
X-ray and optical astronomical observations. The radiation hydrodynamics code
Hades was adapted to simulate this specific accretion phenomena. Classical
approaches were used to model the radiative losses of the two main radiative
processes: bremsstrahlung and cyclotron. The oscillation frequencies and
amplitudes in the X-ray and optical domains are studied to compare those
numerical results to observational ones. Different dimensional formulae were
developed to complete the numerical evaluations. The complete characterization
of the emitting region is described for the two main radiative regimes: when
only the bremsstrahlung losses and when both cyclotron and bremsstrahlung
losses are considered. The effect of the non-linear cooling in- stability
regime on the accretion column behaviour is analysed. Variation in luminosity
on short timescales (~ 1 s quasi-periodic oscillations) is an expected
consequence of this specific dynamic. The importance of secondary shock
instability on the quasi-periodic oscillation phenomenon is discussed. The
stabilization effect of the cyclotron process is confirmed by our numerical
simulations, as well as the power distribution in the various modes of
oscillation.Comment: 13 pages, 13 figures, 2 tables. Accepted for publication in A&