20 research outputs found
Self-consistent triaxial de Zeeuw-Carollo Models
We use the usual method of Schwarzschild to construct self-consistent
solutions for the triaxial de Zeeuw & Carollo (1996) models with central
density cusps. ZC96 models are triaxial generalisations of spherical
-models of Dehnen whose densities vary as near the center
and at large radii and hence, possess a central density core for
and cusps for . We consider four triaxial models from
ZC96, two prolate triaxials: with and
1.5, and two oblate triaxials: with and
1.5. We compute 4500 orbits in each model for time periods of .
We find that a large fraction of the orbits in each model are stochastic by
means of their nonzero Liapunov exponents. The stochastic orbits in each model
can sustain regular shapes for or longer, which suggests
that they diffuse slowly through their allowed phase-space. Except for the
oblate triaxial models with , our attempts to construct
self-consistent solutions employing only the regular orbits fail for the
remaining three models. However, the self-consistent solutions are found to
exist for all models when the stochastic and regular orbits are treated in the
same way because the mixing-time, , is shorter than the
integration time, . Moreover, the ``fully-mixed'' solutions can
also be constructed for all models when the stochastic orbits are fully mixed
at 15 lowest energy shells. Thus, we conclude that the self-consistent
solutions exist for our selected prolate and oblate triaxial models with
and 1.5.Comment: 6 Pages, 3 Figures, 2 Tables. Accepted for Publication in A&
A family of triaxial modified Hubble mass models: effects of the additional radial functions
The projected properties of triaxial generalization of the modified Hubble
mass models are studied. These models are constructed by adding the additional
radial functions, each multiplied by a low-order spherical harmonic, to the
models of \citet{ct00}. The projected surface density of mass models can be
calculated analytically which allows us to derive the analytic expressions of
axial ratio and position angles of major axis of constant density elliptical
contours at asymptotic radii. The models are more general than those studied
earlier in the sense that the inclusions of additional terms in density
distribution, allows one to produce varieties of the radial profile of axial
ratio and position angle, in particular, their small scale variations at inner
radii. Strong correlations are found to exist between the observed axial ratio
evaluated at and at which occupy well-separated regions
in the parameter space for different choices of the intrinsic axial ratios.
These correlations can be exploited to predict the intrinsic shape of the mass
model, independent of the viewing angles. Using Bayesian statistics, the result
of a test case launched for an estimation of the shape of a model galaxy is
found to be satisfactory.Comment: 23 pages, 8 figures, accepted for publication in New Astronom
AstroSat view of the newly discovered X-ray transient MAXI~J1803--298 in the Hard-intermediate state
We perform comprehensive temporal and spectral analysis of the newly
discovered X-ray transient MAXI~J1803--298 using an AstroSat target of
opportunity observation on May 11, 2021 during its outburst. The source was
found to be in the hard intermediate state. We detect type C quasi-periodic
oscillations (QPOs) at the frequencies of Hz and Hz along
with a sub-harmonic at Hz in the keV band. The frequency and
fractional rms amplitude of the QPO in the keV band are found to be
higher than those in the keV band. We find soft lags of ms and
ms for the respective QPOs at Hz and Hz, whereas
soft lag of ms is found at the sub-harmonic frequency. The increase
in the soft lags at the QPO frequencies with energy is also observed in other
black hole transients and is attributed to the inclination dependence of the
lags. The rms-energy spectra indicate the power-law component to be more
variable than the disk and the reflection components. We find a broad iron line
with an equivalent width of keV and a reflection hump above
keV in the energy spectrum. Based on the X-ray spectroscopy and
considering the distance to the source as 8 kpc, the estimated mass
( M) and spin () of the black hole suggest
that the source is likely to be a stellar mass Kerr black hole X-ray binary.Comment: Accepted for publication in The Astrophysical Journal (ApJ). 17 pages
and 9 figure
Effect of Central Mass Concentration on the Formation of Nuclear Spirals in Barred Galaxies
We have performed smoothed particle hydrodynamics (SPH) simulations to study
the response of the central kiloparsec region of a gaseous disk to the
imposition of nonaxisymmetric bar potentials. The model galaxies are composed
of the three axisymmetric components (halo, disk, and bulge) and a
non-axisymmetric bar. These components are assumed to be invariant in time in
the frame corotating with the bar. The potential of spherical -models
of Dehnen is adopted for the bulge component whose density varies as
near the center and at larger radiiand hence, possesses
a central density core for and cusps for . Since the
central mass concentration of the model galaxies increases with the cusp
parameter , we have examined here the effect of the central mass
concentration by varying the cusp parameter on the mechanism
responsible for the formation of the symmetric two-armed nuclear spirals in
barred galaxies. Our simulations show that the symmetric two-armed nuclear
spirals are formed by hydrodynamic spiral shocks driven by the gravitational
torque of the bar for the models with and 0.5. On the other hand,
the symmetric two-armed nuclear spirals in the models with and 1.5
are explained by gas density waves. Thus, we conclude that the mechanism
responsible for the formation of the symmetric two-armed nuclear spirals in
barred galaxies changes from the hydrodynamic shocks to the gas density waves
when the central mass concentration increases from to 1.5.Comment: 29 pages, 5 figures (Color Figures 3-5), Accepted for Publication in
Astrophysical Journal (ApJ