3,225 research outputs found
Breakdown of self-similar evolution in homogeneous perfect fluid collapse
The stability analysis of self-similar solutions is an important approach to
confirm whether they act as an attractor in general non-self-similar
gravitational collapse. Assuming that the collapsing matter is a perfect fluid
with the equation of state , we study spherically symmetric
non-self-similar perturbations in homogeneous self-similar collapse described
by the flat Friedmann solution. In the low pressure approximation , we analytically derive an infinite set of the normal modes and their growth
(or decay) rate. The existence of one unstable normal mode is found to conclude
that the self-similar behavior in homogeneous collapse of a sufficiently low
pressure perfect fluid must terminate and a certain inhomogeneous density
profile can develop with the lapse of time.Comment: 9 pages, 1 figure, references added, published in Physical Review
Stability analysis of self-similar behaviors in perfect fluid gravitational collapse
Stability of self-similar solutions for gravitational collapse is an
important problem to be investigated from the perspectives of their nature as
an attractor, critical phenomena and instability of a naked singularity. In
this paper we study spherically symmetric non-self-similar perturbations of
matter and metrics in spherically symmetric self-similar backgrounds. The
collapsing matter is assumed to be a perfect fluid with the equation of state
. We construct a single wave equation governing the
perturbations, which makes their time evolution in arbitrary self-similar
backgrounds analytically tractable. Further we propose an analytical
application of this master wave equation to the stability problem by means of
the normal mode analysis for the perturbations having the time dependence given
by , and present some sufficient conditions for the
absence of non-oscillatory unstable normal modes with purely imaginary
.Comment: 17 pages, 3 figures, matched to the published versio
Detection of Iron Emission Line from the Galaxy Cluster Including the Radio Galaxy 3C220.1 at z=0.62
We have detected an emission line feature at 4 keV in the X-ray emission from
a sky region including the distant radio galaxy 3C220.1(z=0.62) obtained with
ASCA. The line energy is 6.1 - 7.0 keV (90% confidence) in the rest frame of
3C220.1. Within the present statistics, the observed spectra are consistent
with two different models: a non-thermal model consisting of a power-law
continuum plus a 6.4 keV iron emission line, and a Raymond-Smith thin-thermal
emission model of kT ~6 keV with a metal abundance of ~0.5 solar. However,
because of the large (~ 500 eV) equivalent width of the line, a significant
fraction of the X-ray emission is likely to arise from the hot intracluster gas
associated with the galaxy cluster that includes 3C220.1. The spectral
parameters of the thermal emission are consistent with the
luminosity-temperature relation of nearby clusters.Comment: 15 pages, 5 ps figures, accepted for publication in the Astrophysical
Journa
No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81
Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds
are likely white dwarfs that steadily or cyclically burn accreted matter on
their surface, which are promising type Ia supernova progenitors. Observations
of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources
that are generally hotter and more luminous, including some ultraluminous
supersoft sources (ULSs) that are possibly intermediate mass black holes of a
few thousand solar masses. In this paper we report our X-ray spectral and
timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby
spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS
observations spanning six years, and its spectrum can be described by either a
eV blackbody for a white dwarf, or a
eV multicolor accretion disk for a
intermediate mass black hole. In two observations, the light curves exhibited
dramatic flux drop/rise on time scales of seconds, reminiscent of
eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive
search for periodicity in the reasonable range of 50 ksec to 50 days failed to
reveal an orbital period. The failure to reveal any periodicity is consistent
with the long period ( yrs) predicted for this system given the optical
identification of the secondary with an asymptotic giant star. Also, the
eclipse-like dramatic flux changes in hours are hard to explain under the white
dwarf model, but can in principle be explained by disk temperature changes
induced by accretion rate variations under the intermediate mass black hole
model.Comment: 19 pages, 7 figures, 1 table, to appear in ApJ
Spectral evolution of the microquasar XTE J1550-564 over its entire 2000 outburst
We report on RXTE observations of the microquasar XTE J1550-564 during a ~70
day outburst in April-June 2000. We study the evolution of the PCA+HEXTE
spectra over the outburst. The source transited from an initial Low Hard State
(LS), to an Intermediate State (IS), and then back to the LS. The source shows
an hysteresis effect similar to what is observed in other sources, favoring a
common origin for the state transitions in soft X-ray transients. The first
transition occurs at a ~ constant 2-200 keV flux, which probably indicates a
change in the relative importance of the emitting media. The second transition
is more likely driven by a drop in the mass accretion rate.
In both LS, the spectra are characterized by the presence of a strong
power-law tail (Compton corona) with a variable high energy cut-off. During the
IS, the spectra show the presence of a ~0.8 keV thermal component (accretion
disk). We discuss the apparently independent evolution of the two media, and
show that right after the X-ray maximum on MJD 51662, the decrease of the
source luminosity is due to a decrease of the power-law luminosity, at a
constant disk luminosity. This, together with the detection of radio emission
(with a spectrum typical of optically thin synchrotron emission), may suggest
that the corona is ejected and further detected as a discrete radio ejection.Comment: Accepted for publication in ApJ. 9 pages, 4 figures, abstract
abridge
A Grid of Relativistic, non-LTE Accretion Disk Models for Spectral Fitting of Black Hole Binaries
Self-consistent vertical structure models together with non-LTE radiative
transfer should produce spectra from accretion disks around black holes which
differ from multitemperature blackbodies at levels which may be observed. High
resolution, high signal-to-noise observations warrant spectral modeling which
both accounts for relativistic effects, and treats the physics of radiative
transfer in detail. In Davis et al. (2005) we presented spectral models which
accounted for non-LTE effects, Compton scattering, and the opacities due to
ions of abundant metals. Using a modification of this method, we have tabulated
spectra for black hole masses typical of Galactic binaries. We make them
publicly available for spectral fitting as an Xspec model. These models
represent the most complete realization of standard accretion disk theory to
date. Thus, they are well suited for both testing the theory's applicability to
observed systems and for constraining properties of the black holes, including
their spins.Comment: 7 pages, emulate ApJ, accepted to Ap
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