190 research outputs found
The superorbital variability and triple nature of the X-ray source 4U 1820-303
We perform a comprehensive analysis of the superorbital modulation in the ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting onto a neutron star. Based on RXTE data, we measure the fractional amplitude of the source superorbital variability (with a 170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of about 2. As proposed before, the superorbital variability can be explained by oscillations of the binary eccentricity. We now present detailed calculations of the eccentricity-dependent flow through the inner Lagrangian point, and find a maximum of the eccentricity of about 0.004 is sufficient to explain the observed fractional amplitude. We then study hierarchical triple models yielding the required quasi-periodic eccentricity oscillations through the Kozai process. We find the resulting theoretical light curves to match well the observed ones. We constrain the ratio of the semimajor axes of the outer and inner systems, the component masses, and the inclination angle between the inner and outer orbits. Last but not least, we discover a remarkable and puzzling synchronization between the observed period of the superorbital variability (equal to the period of the eccentricity oscillations in our model) and the period of the general-relativistic periastron precession of the binary
The superorbital variability and triple nature of the X-ray source 4U 1820-303
We perform a comprehensive analysis of the superorbital modulation in the
ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting
onto a neutron star. Based on RXTE data, we measure the fractional amplitude of
the source superorbital variability (with a 170-d quasi-period) in the folded
and averaged light curves, and find it to be by a factor of about 2. As
proposed before, the superorbital variability can be explained by oscillations
of the binary eccentricity. We now present detailed calculations of the
eccentricity-dependent flow through the inner Lagrangian point, and find a
maximum of the eccentricity of about 0.004 is sufficient to explain the
observed fractional amplitude. We then study hierarchical triple models
yielding the required quasi-periodic eccentricity oscillations through the
Kozai process. We find the resulting theoretical light curves to match well the
observed ones. We constrain the ratio of the semimajor axes of the outer and
inner systems, the component masses, and the inclination angle between the
inner and outer orbits. Last but not least, we discover a remarkable and
puzzling synchronization between the observed period of the superorbital
variability (equal to the period of the eccentricity oscillations in our model)
and the period of the general-relativistic periastron precession of the binary.Comment: MNRAS, in pres
Modeling The X-ray Timing Properties Of Cygnus X-1 As Caused By Waves Propagating In A Transition Disk
We show that waves propagating in a transition disk can explain the short
term temporal behavior of Cygnus X-1. In the transition disk model the spectrum
is produced by saturated Comptonization within the inner region of the
accretion disk where the temperature varies rapidly with radius. Recently, the
spectrum from such a disk has been shown to fit the average broad band spectrum
of this source better than that predicted by the soft-photon Comptonization
model. Here, we consider a simple model where waves are propagating
cylindrically symmetrical ly in the transition disk with a uniform propagation
speed (). We show that this model can qualitatively explain (a) the
variation of the power spectral density (PSD) with energy, (b) the hard lags as
a function of frequency and (c) the hard lags as a function of energy for
various frequencies. Thus the transition disk model can explain the average
spectrum and the short term temporal behavior of Cygnus X-1.Comment: accepted for publication in APJ letter
A comment on the colour-colour diagrams of low-mass X-ray binaries
Disc-accreting neutron stars come in two distinct varieties, atolls and Z sources, named after their differently shaped tracks on a colour–colour diagram as the source luminosity changes. Here we present analysis of three transient atoll sources showing that there is an additional branch in the colour–colour diagram of atoll sources which appears at very low luminosities. This new branch connects to the top of previously known C-shaped (atoll) path, forming a horizontal track where the average source flux decrease from right to left. This turns the C-shape into a Z. Thus both atolls and Z sources share the same topology on the colour–colour diagram and evolve in similar way, as a function of increasing averaged mass accretion rate. This strongly favours models in which the underlying geometry of these sources changes in similar ways. A possible scenario is one where the truncated disc approaches the neutron star when the accretion rate increases, but in the atolls the disc is truncated by evaporation (similarly to black holes), and in the Z sources it is truncated by the magnetic field
Discovery of powerful millisecond flares from Cygnus X-1
We have found a large number of very strong flares in the available XTE PCA
data of Cyg X-1 (also seen in available HEXTE and BATSE data) with 13 flares
satisfying our chosen threshold criterion, occuring both in the hard and the
soft states. We analyze here in detail two of them. The strongest one took
place in the soft state, with the 3-30 keV energy flux increasing 30 times with
respect to the preceding 16-s average. The e-folding time is ~ 7 ms for the
main flare and ~ 1 ms for its precursor. The spectrum strongly hardens during
the flare. On the other hand, flares in the hard state have generally lower
amplitudes and longer e-folding times, and their spectra soften during the
flare, with the hardness of the spectrum at the flare peak similar for both
types of the flares. The presence of the flares shows unusually dramatic events
taking place in the accretion flow of Cyg X-1. On the other hand, the rate of
occurence of hard-state flares shows they may represent a high-flux end of the
distribution of shots present in usual lightcurves of Cyg X-1.Comment: Accepted for publication in MNRA
A fit to the simultaneous broadband spectrum of Cygnus X-1 using the transition disk model
We have used the transition disk model to fit the simultaneous broad band
( keV) spectrum of Cygnus X-1 from OSSE and Ginga observations. In this
model, the spectrum is produced by saturated Comptonization within the inner
region of the accretion disk, where the temperature varies rapidly with radius.
In an earlier attempt, we demonstrated the viability of this model by fitting
the data from EXOSAT, XMPC balloon and OSSE observations, though these were not
made simultaneously. Since the source is known to be variable, however, the
results of this fit were not conclusive. In addition, since only once set of
observations was used, the good agreement with the data could have been a
chance occurrence. Here, we improve considerably upon our earlier analysis by
considering four sets of simultaneous observations of Cygnus X-1, using an
empirical model to obtain the disk temperature profile. The vertical structure
is then obtained using this profile and we show that the analysis is self-
consistent. We demonstrate conclusively that the transition disk spectrum is a
better fit to the observations than that predicted by the soft photon
Comptonization model. Since the temperature profile is obtained by fitting the
data, the unknown viscosity mechanism need not be specified. The disk structure
can then be used to infer the viscosity parameter , which appears to
vary with radius and luminosity. This behavior can be understood if
depends intrinsically on the local parameters such as density, height and
temperature. However, due to uncertainties in the radiative transfer,
quantitative statements regarding the variation of cannot yet be made.Comment: 8 figures. uses aasms4.sty, accepted by ApJ (Mar 98
Ultraluminous X-ray Sources Powered by Radiatively Efficient Two-Phased Super-Eddington Accretion onto Stellar Mass Black holes
The radiation spectra of many of the brightest ultraluminous X-ray sources
(ULXs) are dominated by a hard power law component, likely powered by a hot,
optically thin corona that Comptonizes soft seed photons emitted from a cool,
optically thick black hole accretion disk. Before its dissipation and
subsequent conversion into coronal photon power, the randomized gravitational
binding energy responsible for powering ULX phenomena must separate from the
mass of its origin by a means other than, and quicker than, electron
scattering-mediated radiative diffusion. Therefore, the release of accretion
power in ULXs is not necessarily subject to Eddington-limited photon trapping,
as long as it occurs in a corona. Motivated by these basic considerations, we
present a model of ULXs powered by geometrically thin accretion onto stellar
mass black holes. We argue that the radiative efficiency of the flow remains
high if the corona is magnetized or optically thin and the majority of the
accretion power escapes in the form of radiation rather than an outflow. Within
the context of the current black hole X-ray binary paradigm, our ULX model may
be viewed as an extension of the very high state observed in Galactic sources.
(abridged)Comment: 11 page
Radiation mechanisms and geometry of Cygnus X-1 in the soft state
We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition
from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in
1996 May and June. The spectra consist of a dominant soft component below ~2
keV and a power-law-like continuum extending to at least ~800 keV. We interpret
them as emission from an optically-thick, cold accretion disc and from an
optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total
available power is dissipated in the corona. We model the soft component by
multi-colour blackbody disc emission taking into account the torque-free
inner-boundary condition. If the disc extends down to the minimum stable orbit,
the ASCA/RXTE data yield the most probable black hole mass of about 10 solar
masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft
state in the upper part of the stable, gas-pressure dominated, accretion-disc
solution branch. The spectrum of the corona is well modelled by repeated
Compton scattering of seed photons from the disc off electrons with a hybrid,
thermal/non-thermal distribution. The electron distribution can be
characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50
keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The
compactness of the corona is between 2 and 7, and a presence of a significant
population of electron-positron pairs is ruled out. We find strong signatures
of Compton reflection from a cold and ionized medium, presumably an accretion
disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum
is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted
to MNRA
Probing the temporal variability of Cygnus X-1 into the soft state
Building on results from previous studies of Cygnus~X-1, we analyze Rossi
X-ray Timing Explorer (RXTE) data taken when the source was in the soft and
transitional spectral states. We look at the power spectrum in the 0.01 -- 50
Hz range, using a model consisting of a cut-off power-law and two Lorentzian
components. We are able to constrain the relation between the characteristic
frequencies of the Lorentzian components, and show that it is consistent with a
power-law relation having the same index (1.2) as previously reported for the
hard state, but shifted by a factor ~2. Furthermore, it is shown that the
change in the frequency relation seen during the transitions can be explained
by invoking a shift of one Lorentzian component to a higher harmonic, and we
explore the possible support for this interpretation in the other component
parameters. With the improved soft state results we study the evolution of the
fractional variance for each temporal component. This approach indicates that
the two Lorentzian components are connected to each other, and unrelated to the
power-law component in the power spectrum, pointing to at least two separate
emission components.Comment: 11 pages, 10 figures, 1 electronic table. Accepted for publication in
A&
- …