337 research outputs found
Correlations in Quasi-Periodic Oscillation and Noise Frequencies Among Neutron-Star and Black-Hole X-ray Binaries
We study systematically the ~0.1-1200 Hz quasi-periodic oscillations (QPOs)
and broad noise components observed in the power spectra of non-pulsing
neutron-star and black-hole low-mass X-ray binaries. We show that among these
components we can identify two, occurring over a wide range of source types and
luminosities, whose frequencies follow a tight correlation. The variability
components involved in this correlation include neutron-star kilohertz QPOs and
horizontal-branch oscillations, as well as black-hole QPOs and noise
components. Our results suggest that the same types of variability may occur in
both neutron-star and black-hole systems over three orders of magnitude in
frequency and with coherences that vary widely but systematically. Confirmation
of this hypothesis will strongly constrain theoretical models of these
phenomena and provide additional clues to understanding their nature.Comment: 15 pages, 2 figures (one color figure), to appear in the
Astrophysical Journa
X-Ray and UV Orbital Phase Dependence in LMC X-3
The black-hole binary LMC X-3 is known to be variable on time scales of days
to years. We investigate X-ray and ultraviolet variability in the system as a
function of the 1.7 day binary phase using a 6.4 day observation with the Rossi
X-ray Timing Explorer (RXTE) from December 1998. An abrupt 14% flux decrease,
lasting nearly an entire orbit, is followed by a return to previous flux
levels. This behavior occurs twice, at nearly the same binary phase, but it is
not present in consecutive orbits. When the X-ray flux is at lower intensity, a
periodic amplitude modulation of 7% is evident in data folded modulo the
orbital period. The higher intensity data show weaker correlation with phase.
This is the first report of X-ray variability at the orbital period of LMC X-3.
Archival RXTE observations of LMC X--3 during a high flux state in December
1996 show similar phase dependence. An ultraviolet light curve obtained with
the High Speed Photometer aboard the Hubble Space Telescope shows orbital
modulation consistent with that in the optical, caused by the ellipsoidal
variation of the spatially deformed companion.
The X-ray spectrum of LMC X-3 can be acceptably represented by a
phenomenological disk-black-body plus a power law. Changes in the spectrum of
LMC X-3 during our observations are compatible with earlier observations during
which variations in the 2-10 keV flux are tracked closely by the disk geometry
spectral model parameter.Comment: 11 pages, 7 figures, ApJ in pres
``Canonical'' Black Hole States in the Superluminal Source GRO J1655-40
We analyze RXTE/PCA observations of the black hole candidate and galactic
superluminal source GRO J1655-40 during its recent outburst. We show that
during its decay to quiescence, GRO J1655-40 goes through the high,
intermediate, and low state (and that at the beginning of its decay it might
have even shown signatures of a very high state), just like other black hole
candidates. This is the first time that such a transition is observed in a
galactic superluminal source. We discuss what are the implications of these
results on the hypothesis that the spin of the black hole in superluminal
sources is much higher than in other black hole candidates.Comment: To appear in ApJL. AAS LaTex v4.0 (9 pages, 2 ps-figures
Testing the transition layer model of quasi-periodic oscillations in neutron star X-ray binarie
We compare the theoretical predictions of the transition layer model with
some observational features of quasi-periodic oscillations (QPOs) in neutron
star X-ray binaries. We found that the correlation between horizontal branch
oscillation (HBO) frequencies and kilohertz (kHz) QPO frequencies, the
difference between the low-frequency QPOs in atoll sources and HBOs in Z
sources, and the correlation between the frequencies of low-frequency QPOs and
break frequencies can be well explained by the transition layer model, provided
the neutron star mass is around 1.4 solar mass and the angle between
magnetosphere equator and accretion disk plane is around 6 degree. The observed
decrease of peak separation between two kHz QPO frequencies with the increase
of kHz QPO frequencies and the increase of QPO frequencies with the increase of
inferred mass accretion rate are also consistent with the theoretical
predictions of transition layer model. In addition, we derive a simple equation
that can be adopted to estimate the angle () between magnetosphere
equator and accretion disk plane by use of the simultaneously observed QPO
frequency data. We estimate these angles, in the range of 4 to 8 degrees, for
five Z sources and two atoll sources. The nearly constant value for
each source, derived from the different sets of simultaneously observed QPO
frequency data, provides a strong test of the theoretical model. Finally, we
suggest that the similar transition layer oscillations may be also responsible
for the observed QPOs in accretion-powered millisecond X-ray pulsar and
Galactic black hole candidates.Comment: 10 pages, 5 figures, to appear in ApJ, Vol. 55
Reduced magnetic braking and the magnetic capture model for the formation of ultra-compact binaries
A binary in which a slightly evolved star starts mass transfer to a neutron
star can evolve towards ultra-short orbital periods under the influence of
magnetic braking. This is called magnetic capture. In a previous paper we
showed that ultra-short periods are only reached for an extremely small range
of initial binary parameters, in particular orbital period and donor mass. Our
conclusion was based on one specific choice for the law of magnetic braking,
and for the loss of mass and angular momentum during mass transfer. In this
paper we show that for less efficient magnetic braking it is impossible to
evolve to ultra-short periods, independent of the amount of mass and associated
angular momentum lost from the binary.Comment: 7 pages, 7 figures, accepted for publication in Astronomy and
Astrophysics. See http://www.astro.uu.nl/~sluys/PhD
A Unified Description of the Timing Features of Accreting X-ray Binaries
We study an empirical model for a unified description of the power spectra of
accreting neutron stars and black holes. This description is based on a
superposition of multiple Lorentzians and offers the advantage that all QPO and
noise components are dealt with in the same way, without the need of deciding
in advance the nature of each component. This approach also allows us to
compare frequencies of features with high and low coherences in a consistent
manner and greatly facilitates comparison of power spectra across a wide range
of source types and states. We apply the model to six sources, the
low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269 and GS 1826-24, the
high-latitude transient XTE J1118+480, the bright system Cir X-1, and the Z
source GX 17+2. We find that it provides a good description of the observed
spectra, without the need for a scale-free (1/f) component. We update
previously reported correlations between characteristic frequencies of timing
features in the light of this new approach and discuss similarities between
different types of systems which may point towards similar underlying physics.Comment: 13 pages, to appear in The Astrophysical Journa
Discovery of coherent millisecond X-ray pulsations in Aql X-1
We report the discovery of an episode of coherent millisecond X-ray pulsation
in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for
slightly more than 150 seconds, during which the pulse frequency is consistent
with being constant. No X-ray burst or other evidence of thermonuclear burning
activity is seen in correspondence with the pulsation, which can thus be
identified as occurring in the persistent emission. The pulsation frequency is
550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from
burst oscillations in this source. Hence we identify this frequency with the
neutron star spin frequency. The pulsed fraction is strongly energy dependent,
ranging from 10% (16-30 keV). We discuss possible physical
interpretations and their consequences for our understanding of the lack of
pulsation in most neutron star low-mass X-ray binaries. If interpreted as
accretion-powered pulsation, Aql X-1 might play a key role in understanding the
differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions.
Slightly extended discussion. One author added. Uses emulateapj.cl
Relations Between Timing Features and Colors in the X-Ray Binary 4U 0614+09
We study the correlations between timing and X-ray spectral properties in the
low mass X-ray binary 4U 0614+09 using a large (265-ks) data set obtained with
the Rossi X-ray Timing Explorer. We find strong quasi-periodic oscillations
(QPOs) of the X-ray flux, like the kilohertz QPOs in many other X-ray binaries
with accreting neutron stars, with frequencies ranging from 1329 Hz down to 418
Hz and, perhaps, as low as 153 Hz. We report the highest frequency QPO yet from
any low mass X-ray binary at 1329+-4 Hz, which has implications for neutron
star structure. This QPO has a 3.5-sigma single-trial significance, for an
estimated 40 trials the significance is 2.4-sigma. Besides the kilohertz QPOs,
the Fourier power spectra show four additional components: high frequency noise
(HFN), described by a broken power-law with a break frequency between 0.7 and
45 Hz, very low frequency noise (VLFN), which is fitted as a power-law below 1
Hz, and two broad Lorentzians with centroid frequencies varying from 6 to 38 Hz
and 97 to 158 Hz, respectively. We find strong correlations between the
frequencies of the kilohertz QPOs, the frequency of the 6 to 38 Hz broad
Lorentzian, the break frequency of the HFN, the strength of both the HFN and
the VLFN and the position of the source in the hard X-ray color vs. intensity
diagram. The frequency of the 97 to 158 Hz Lorentzian does not correlate with
these parameters. We also find that the relation between power density and
break frequency of the HFN is similar to that established for black hole
candidates in the low state. We suggest that the changing mass accretion rate
is responsible for the correlated changes in all these parameters.Comment: ApJ, referee
Study of the Temporal Behavior of 4U 1728-34 as a Function of its Position in the Color-Color Diagram
We study the timing properties of the bursting atoll source 4U 1728-34 as a
function of its position in the X-ray color-color diagram. In the island part
of the color-color diagram (corresponding to the hardest energy spectra) the
power spectrum of 4U 1728-34 shows several features such as a band-limited
noise component present up to a few tens of Hz, a low frequency quasi-periodic
oscillation (LFQPO) at frequencies between 20 and 40 Hz, a peaked noise
component around 100 Hz, and one or two QPOs at kHz frequencies. In addition to
these, in the lower banana (corresponding to softer energy spectra) we also
find a very low frequency noise (VLFN) component below ~1 Hz. In the upper
banana (corresponding to the softest energy spectra) the power spectra are
dominated by the VLFN, with a peaked noise component around 20 Hz. We find that
the frequencies of the kHz QPOs are well correlated with the position in the
X-ray color-color diagram. For the frequency of the LFQPO and the break
frequency of the broad-band noise component the relation appears more complex.
These frequencies both increase when the frequency of the upper kHz QPO
increases from 400 to 900 Hz, but at this frequency a jump in the values of the
parameters occurs. We interpret this jump in terms of the gradual appearance of
a QPO at the position of the break at high inferred mass accretion rate, while
the previous LFQPO disappears. Simultaneously, another kind of noise appears
with a break frequency of ~7 Hz, similar to the NBO of Z sources. The 100 Hz
peaked noise does not seem to correlate with the position of the source in the
color-color diagram, but remains relatively constant in frequency. This
component may be similar to several 100 Hz QPOs observed in black hole
binaries.Comment: 27 pages, 9 figures, accepted by Ap
kHz Quasi Periodic Oscillations in Low Mass X-ray Binaries as Probes of General Relativity in the Strong Field Regime
We consider the interpretation of a pair of kHz Quasi Periodic Oscillations
(QPOs) in the Fourier spectra of two Low Mass X-Ray Binaries, Sco X-1 and
4U1608-52, hosting an old accreting neutron star. The observed frequency
difference of these QPOs decreaseas as their frequency increases, contrary to
simple beat frequency models, which predict a constant frequency difference. We
show that the behaviour of these QPOs is instead well matched in terms of the
fundamental frequencies (in the radial and azimuthal directions) for test
particle motion in the gravitational field of the neutron star, for reasonable
star masses, and nearly independent of the star spin. The radial frequency must
be much smaller than the azimuthal one, testifying that kHz QPOs are produced
close to the innermost stable orbit. These results are not reproduced through
the post--Newtonian (PN) approximation of General Relativity (GR). kHz QPOs
from X-ray binaries likely provide an accurate laboratory for strong field GR.Comment: to appear in Physical Review Letters, PRL Latex plus 2 figures in
standard PostScript forma
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