11,838 research outputs found
Strong Correlation Between Noise Features at Low Frequency and the Kilohertz QPOs in the X-Ray Binary 4U 1728-34
We study the timing properties of the low mass X-ray binary 4U 1728-34 using
recently released data from the Rossi X-Ray Timing Explorer. This binary, like
many others with accreting neutron stars, is known to exhibit strong
quasi-periodic oscillations (QPOs) of its X-ray flux near 1 kHz. In addition to
the kilohertz QPOs, the Fourier power spectra show a broken power law noise
component, with a break frequency between 1 and 50 Hz, and a Lorentzian between
10 and 50 Hz. We find that the frequencies of the break and the low-frequency
Lorentzian are well correlated with the frequencies of the kilohertz QPOs. The
slope of the correlation is similar to that expected if the oscillations are
due to relativistic frame dragging (Lense-Thirring precession) in the inner
accretion disk (Stella & Vietri 1998). The correlation is also nearly identical
to the one found in Z-sources between the the well known QPOs on the horizontal
branch and the kilohertz QPOs, suggesting that the low frequency oscillations
are a similar phenomenon in these sources. The frequency of the break in the
power spectra is also correlated with the frequencies of the kilohertz QPOs. As
previously noted for the similar binaries 4U 1608-50 and 4U 1705-44, this
broken power law component closely resembles that of black hole candidates in
the low state, where the break frequency is taken as an indicator of mass
accretion rate. The relation between break frequency and kilohertz QPO
frequency thus provides additional proof that the frequency of the kilohertz
QPOs increases with mass accretion rate.Comment: ApJL in press, see the 'QPO page' at
http://www.astro.uva.nl/ecford/qpos.htm
Detection of a 1258 Hz high-amplitude kilohertz quasi-periodic oscillation in the ultra-compact X-ray binary 1A 1246-588
We have observed the ultra-compact low-mass X-ray binary (LMXB) 1A 1246-588
with the Rossi X-ray Timing Explorer (RXTE). In this manuscript we report the
discovery of a kilohertz quasi-periodic oscillation (QPO) in 1A 1246-588. The
kilohertz QPO was only detected when the source was in a soft high-flux state
reminiscent of the lower banana branch in atoll sources. Only one kilohertz QPO
peak is detected at a relatively high frequency of 1258+-2 Hz and at a single
trial significance of more than 7 sigma. Kilohertz QPOs with a higher frequency
have only been found on two occasions in 4U 0614+09. Furthermore, the frequency
is higher than that found for the lower kilohertz QPO in any source, strongly
suggesting that the QPO is the upper of the kilohertz QPO pair often found in
LMXBs. The full-width at half maximum is 25+-4 Hz, making the coherence the
highest found for an upper kilohertz QPO. From a distance estimate of ~6 kpc
from a radius expansion burst we derive that 1A 1246-588 is at a persistent
flux of ~0.2-0.3 per cent of the Eddington flux, hence 1A 1246-588 is one of
the weakest LMXBs for which a kilohertz QPO has been detected. The
root-mean-square (rms) amplitude in the 5-60 keV band is 27+-3 per cent, this
is the highest for any kilohertz QPO source so far, in line with the general
anti-correlation between source luminosity and rms amplitude of the kilohertz
QPO peak identified before. Using the X-ray spectral information we produce a
colour-colour diagram. The source behaviour in this diagram provides further
evidence for the atoll nature of the source.Comment: 4 pages, 3 figures, accepted for publication in MNRA
The aperiodic timing behaviour of the accretion-driven millisecond pulsar SAX J1808.4-3658
We studied the aperiodic X-ray timing behaviour of the accreting millisecond
pulsar SAX J1808.4-3658. The source was recently found to be the first
accreting millisecond pulsar that shows the kilohertz quasi-periodic
oscillations (kilohertz QPOs) that are found in many other X-ray binaries with
accreting neutron stars. The high frequency of these signals reflects the short
dynamical time scales in the region near the compact object where they
originate. We find that in addition to the kilohertz QPOs SAX J1808.4-3658
shows several low frequency timing features, based on which the source can be
classified as a so-called atoll source. The frequencies of the variability
components of the atoll sources follow a universal scheme of correlations. The
correlations in SAX J1808.4-3658 are similar but show a shift in upper
kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or
differently configured magnetic field.Comment: 4 pages, 3 figures. To appear in the proceedings of the "The Restless
High-Energy Universe" (Amsterdam, The Netherlands), 2003, eds. E.P.J. van den
Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijer
Sonic-Point Model of Kilohertz Quasi-Periodic Brightness Oscillations in Low-Mass X-ray Binaries
Strong, coherent, quasi-periodic brightness oscillations (QPOs) with
frequencies ranging from about 300 Hz to 1200 Hz have been discovered with the
Rossi X-ray Timing Explorer in the X-ray emission from some fifteen neutron
stars in low-mass binary systems. Two simultaneous kilohertz QPOs differing in
frequency by 250 to 350 Hertz have been detected in twelve of the fifteen
sources. Here we propose a model for these QPOs. In this model the X-ray source
is a neutron star with a surface magnetic field of 10^7 to 10^10 G and a spin
frequency of a few hundred Hertz, accreting gas via a Keplerian disk. The
frequency of the higher-frequency QPO in a kilohertz QPO pair is the Keplerian
frequency at a radius near the sonic point at the inner edge of the Keplerian
flow whereas the frequency of the lower-frequency QPO is approximately the
difference between the Keplerian frequency at a radius near the sonic point and
the stellar spin frequency. This model explains naturally many properties of
the kilohertz QPOs, including their frequencies, amplitudes, and coherence. We
show that if the frequency of the higher-frequency QPO in a pair is an orbital
frequency, as in the sonic-point model, the frequencies of these QPOs place
interesting upper bounds on the masses and radii of the neutron stars in the
kilohertz QPO sources and provide new constraints on the equation of state of
matter at high densities. Further observations of these QPOs may provide
compelling evidence for the existence of a marginally stable orbit, confirming
a key prediction of general relativity in the strong-field regime.Comment: 67 pages, including 15 figures and 5 tables; uses aas2pp4; final
version to appear in the Astrophysical Journal on 1 December 199
Beat-Frequency Models of Kilohertz QPOs
Kilohertz QPO sources are reasonably well-characterized observationally, but
many questions remain about the theoretical framework for these sources and the
consequent implications of the observations for disk physics, strong gravity,
and dense matter. We contrast the predictions and implications of the most
extensively studied class of kilohertz QPO models, the beat-frequency models,
with those of alternative classes of models. We also discuss the expected
impact of new observations of these sources with satellites such as Chandra,
XMM, and Constellation-X.Comment: 10 pages, invited paper at Bologna X-ray Astronomy 1999. To appear in
Astrophysical Letters and Communication
The Beat-Frequency Interpretation of Kilohertz QPOs in Neutron Star Low-Mass X-ray Binaries
Pairs of quasi-periodic oscillations (QPOs) at kilohertz frequencies are a
common phenomenon in several neutron-star low-mass X-ray binaries. The
frequency separation of the QPO peaks in the pair appears to be constant in
many sources and directly related to the neutron star spin frequency. However,
in Sco X-1 and possibly in 4U 1608-52, the frequency separation of the QPOs
decreases with increasing inferred mass accretion rate. We show that the
currently available Rossi X-ray Timing Explorer data are consistent with the
hypothesis that the frequency separations in all sources vary by amounts
similar to the variation in Sco X-1. We discuss the implications for models of
the kilohertz QPOs.Comment: 8 pages, 3 b&w figures and 1 color figure; to appear in the
Astrophysical Journal Letter
Rapid X-Ray Variability of Neutron Stars in Low-Mass Binary Systems
The dramatic discovery with the Rossi X-Ray Timing Explorer satellite of
remarkably coherent 300--1200 Hz oscillations in the X-ray brightness of
some sixteen neutron stars in low-mass binary systems has spurred theoretical
modeling of these oscillations and investigation of their implications for the
neutron stars and accretion flows in these systems. High-frequency oscillations
are observed both during thermonuclear X-ray bursts and during intervals of
accretion-powered emission and appear to be a characteristic feature of
disk-accreting neutron stars with weak magnetic fields. In this review we focus
on the high-frequency quasi-periodic oscillations (QPOs) seen in the
accretion-powered emission. We first summarize the key properties of these
kilohertz QPOs and then describe briefly the models that have been proposed to
explain them. The existing evidence strongly favors beat-frequency models. We
mention several of the difficulties encountered in applying the magnetospheric
beat-frequency model to the kilohertz QPOs. The most fully developed and
successful model is the sonic-point beat-frequency model. We describe the work
on this model in some detail. We then discuss observations that could help to
distinguish between models. We conclude by noting some of the ways in which
study of the kilohertz QPOs may advance our understanding of dense matter and
strong gravitational fields.Comment: 10 pages LaTeX including six figures, uses espcrc2.sty (included),
invited talk at "The Active X-Ray Sky", eds. L. Scarsi, H. Bradt, P. Giommi,
and F. Fior
Sidebands to the lower kilohertz QPO in 4U1636-53
In this Paper we report on further observations of the third and fourth
kilohertz quasi-periodic oscillations (QPOs) in the power spectrum of the
low-mass X-ray binary (LMXB) 4U1636-53. These kilohertz QPOs are sidebands to
the lower kilohertz QPO. The upper sideband has a frequency 55.5+-1.7 Hz larger
than that of the contemporaneously measured lower kilohertz QPO. Such a
sideband has now been measured at a significance >6 sigma in the power spectra
of three neutron star LMXBs (4U1636-53, 4U1728-34, and 4U1608-52). We also
confirm the presence of a sideband at a frequency ~55 Hz less than the
frequency of the lower kilohertz QPO. The lower sideband is detected at a 3.5
sigma level, only when the lower kilohertz QPO frequency is between 800 and 850
Hz. In that frequency interval the sidebands are consistent with being
symmetric around the lower kHz QPO frequency. The upper limit to the rms
amplitude of the lower sideband is significant lower than that of the upper
sideband for lower kilohertz QPO frequencies >850 Hz. Symmetric sidebands are
unique to 4U1636-53. This might be explained by the fact that lower kilohertz
QPO frequencies as high as 800-850 Hz are rare for 4U1728-34 and 4U1608-52.
Finally, we also measured a low frequency QPO at a frequency of ~43 Hz when the
lower kilohertz QPO frequency is between 700-850 Hz. A similar low-frequency
QPO is present in the power spectra of the other two systems for which a
sideband has been observed. We briefly discuss the possibility that the
sideband is caused by Lense-Thirring precession.Comment: 6 pages, 5 Figures, accepted for publication in MNRA
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
- …