6,108 research outputs found
AstroSat observation of GX 5-1: Spectral and timing evolution
We report on the first analysis of AstroSat observation of the Z-source GX 5-
1 on February 26-27, 2017. The hardness-intensity plot reveals that the source
traced out the horizontal and normal branches. The 0.8-20 keV spectra from
simultaneous SXT and LAXPC data at different locations of the
hardness-intensity plot can be well described by a disk emission and a thermal
Comptonized component. The ratio of the disk flux to the total i.e. the disk
flux ratio increases monotonically along the horizontal to the normal one.
Thus, the difference between the normal and horizontal branches is that in the
normal branch, the disk dominates the flux while in the horizontal one it is
the Comptonized component which dominates. The disk flux scales with the inner
disk temperature as T_{in}^{5.5} and not as T_{in}{4} suggesting that either
the inner radii changes dramatically or that the disk is irradiated by the
thermal component changing its hardness factor. The power spectra reveal a
Quasi Periodic Oscillation whose frequency changes from \sim 30 Hz to 50 Hz.
The frequency is found to correlate well with the disk flux ratio. In the 3-20
keV LAXPC band the r.m.s of the QPO increases with energy (r.m.s \prop E0.8),
while the harder X-ray seems to lag the soft ones with a time-delay of a
milliseconds. The results suggest that the spectral properties of the source
are characterized by the disk flux ratio and that the QPO has its origin in the
corona producing the thermal Comptonized component
Comprehensive Spectral Analysis of Cyg X-1 using RXTE Data
We analyse a large number () pointed RXTE observations of Cyg X-1 and
model the spectrum of each one. A subset of the observations for which there is
simultaneous reliable measure of the hardness ratio by the All Sky Monitor,
shows that the sample covers nearly all the spectral shapes of Cyg X-1. The
relative strength, width of the Iron line and the reflection parameter are in
general correlated with the high energy photon spectral index . This is
broadly consistent with a geometry where for the hard state (low ) there is a hot inner Comptonizing region surrounded by a truncated cold
disk. The inner edge of the disk moves inwards as the source becomes softer
till finally in the soft state (high ) the disk fills the inner
region and active regions above the disk produce the Comptonized component.
However, the reflection parameter shows non-monotonic behaviour near the
transition region (), suggestive of a more complex geometry or
physical state of the reflector. Additionally, the inner disk temperature,
during the hard state, is on the average higher than in the soft one, albeit
with large scatter. These inconsistencies could be due to limitations in the
data and the empirical model used to fit them. The flux of each spectral
component is well correlated with which shows that unlike some other
black hole systems, Cyg X-1 does not show any hysteresis behaviour. In the soft
state, the flux of the Comptonized component is always similar to the disk one,
which confirms that the ultra-soft state (seen in other brighter black hole
systems) is not exhibited by Cyg X-1. The rapid variation of the Compton
Amplification factor with , naturally explains the absence of spectra
with , despite a large number having .Comment: 12 pages, 8 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
Modulational instability of ion-acoustic wave packets in quantum pair-ion plasmas
Amplitude modulation of quantum ion-acoustic waves (QIAWs) in a quantum
electron-pair-ion plasma is studied. It is shown that the quantum coupling
parameter (being the ratio of the plasmonic energy density to the Fermi
energy) is ultimate responsible for the modulational stability of QIAW packets,
without which the wave becomes modulational unstable. New regimes for the
modulational stability (MS) and instability (MI) are obtained in terms of
and the positive to negative ion density ratio . The growth rate of MI
is obtained, the maximum value of which increases with and decreases
with . The results could be important for understanding the origin of
modulated QIAW packets in the environments of dense astrophysical objects,
laboratory negative ion plasmas as well as for the next generation laser solid
density plasma experiments.Comment: 4 pages, 2 figures (to appear in Astrophysics and Space Science
Dynamic Magnetization-Reversal Transition in the Ising Model
We report the results of mean field and the Monte Carlo study of the dynamic
magnetization-reversal transition in the Ising model, brought about by the
application of an external field pulse applied in opposition to the existing
order before the application of the pulse. The transition occurs at a
temperature T below the static critical temperature T_c without any external
field. The transition occurs when the system, perturbed by the external field
pulse competing with the existing order, jumps from one minimum of free energy
to the other after the withdrawal of the pulse. The parameters controlling the
transition are the strength h_p and the duration Delta t of the pulse. In the
mean field case, approximate analytical expression is obtained for the phase
boundary which agrees well with that obtained numerically in the small Delta t
and large T limit. The order parameter of the transition has been identified
and is observed to vary continuously near the transition. The order parameter
exponent beta was estimated both for the mean field (beta =1) and the Monte
Carlo beta = 0.90 \pm 0.02 in two dimension) cases. The transition shows a
"critical slowing-down" type behaviour near the phase boundary with diverging
relaxation time. The divergence was found to be logarithmic in the mean field
case and exponential in the Monte Carlo case. The finite size scaling technique
was employed to estimate the correlation length exponent nu (= 1.5 \pm 0.3 in
two dimension) in the Monte Carlo case.Comment: 13 pages, latex, 8 figure
Evidence For Advective Flow From Multi-Wavelength Observations Of Nova Muscae
We model the UV/optical spectrum of the black hole binary Nova Muscae as a
sum of black body emissions from the outer region of an accretion disk. We show
for self-consistency that scattering effects in this region are not important.
The black hole mass (), the inclination angle () and the distance to the source ( kpc) have been
constrained by optical observations during quiescence (Orosz et al. 1996).
Using these values we find that the accretion rate during the peak was g sec and subsequently decayed
exponentially. We define a radiative fraction () to be the ratio of the
X-ray energy luminosity to the total gravitational power dissipated for a
keplerian accretion disk. We find that and remains nearly
constant during the Ultra-soft and Soft spectral states. Thus for these states,
the inner region of the accretion disk is advection dominated. probably
increased to during the Hard state and finally decreased to
as the source returned to quiescence.Comment: 5 figures. uses aasms4.sty, accepted by Ap
Gluon Condensates, Chiral Symmetry Breaking and Pion Wave Function
We consider here chiral symmetry breaking in quantum chromodynamics arising
from gluon condensates in vacuum. Through coherent states of gluons simulating
a mean field type of approximation, we show that the off-shell gluon
condensates of vacuum generate a mass-like contribution for the quarks, giving
rise to chiral symmetry breaking. We next note that spontaneous breaking of
global chiral symmetry links the four component quark field operator to the
pion wave function. This in turn yields many hadronic properties in the light
quark sector in agreement with experiments, leading to the conclusion that low
energy hadron properties are primarily driven by the vacuum structure of
quantum chromodynamics.Comment: 25 pages, IP/BBSR/92-76, revte
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