230 research outputs found
A possible signature of Lense-Thirring precession in dipping and eclipsing neutron-star low-mass X-ray binaries
Relativistic Lense-Thirring precession of a tilted inner accretion disk
around a compact object has been proposed as a mechanism for low-frequency
(~0.01-70 Hz) quasi-periodic oscillations (QPOs) in the light curves of X-ray
binaries. A substantial misalignment angle (~15-20 degrees) between the
inner-disk rotation axis and the compact-object spin axis is required for the
effects of this precession to produce observable modulations in the X-ray light
curve. A consequence of this misalignment is that in high-inclination X-ray
binaries the precessing inner disk will quasi-periodically intercept our line
of sight to the compact object. In the case of neutron-star systems this should
have a significant observational effect, since a large fraction of the
accretion energy is released on or near the neutron-star surface. In this
Letter I suggest that this specific effect of Lense-Thirring precession may
already have been observed as ~1 Hz QPOs in several dipping/eclipsing
neutron-star X-ray binaries.Comment: Typo correcte
The distribution of kHz QPO frequencies in bright LMXBs
We analyzed all published frequencies, nu_1 and nu_2, of the twin kilohertz
quasi-periodic oscillations (kHz QPOs) in bright neutron star low-mass X-ray
binaries. The two frequencies are well correlated but, contrary to recent
suggestions, the frequency-frequency correlation is significantly different
from a nu_2 = (3/2) nu_1 relation. To check whether, although not following the
the 3/2 relation, the QPO frequencies cluster around a region where nu_2/nu_1 ~
3/2, we re-analyzed the Sco X-1 data that were used to report that ratio and
show that, because the distribution of ratios of linearly correlated
measurements is intrinsically biased, although the significance of the
clustering around nu_2/nu_1 ~ 3/2 previously reported in the case of Sco X-1 is
formally correct, it does not provide any useful information about a possible
underlying resonance mechanism in this source. Using the same data, we then
show that the (unbiased) distribution of QPO frequencies is consistent with a
uniform distribution at a 2.4 sigma level. To investigate this further, we
analyzed a larger data set of Sco X-1 and four other sources, 4U 1608-52, 4U
1636-53, 4U 1728-34 and 4U 1820-30. We find that for all five sources the
distribution of the kHz QPO frequencies is not uniform and has multiple peaks,
which have no analogy in the distribution of points in the spectral color-color
diagrams of these sources. Finally, we demonstrate that a simple random walk of
the QPO frequencies can reproduce qualitatively the observed distributions in
frequency and frequency ratio. This result weakens the support for resonance
models of kHz QPOs in neutron stars.Comment: 8 pages, 7 figures. Accepted for publication on Astronomy and
Astrophyics (2005 January 8); references updated, minor changes to text.
Matches journal versio
Common Patterns in the Evolution between the Luminous Neutron Star Low-Mass X-ray Binary Subclasses
The X-ray transient XTE J1701-462 was the first source observed to evolve
through all known subclasses of low-magnetic-field neutron star low-mass X-ray
binaries (NS-LMXBs), as a result of large changes in its mass accretion rate.
To investigate to what extent similar evolution is seen in other NS-LMXBs we
have performed a detailed study of the color-color and hardness-intensity
diagrams (CDs and HIDs) of Cyg X-2, Cir X-1, and GX 13+1 -- three luminous
X-ray binaries, containing weakly magnetized neutron stars, known to exhibit
strong secular changes in their CD/HID tracks. Using the full set of Rossi
X-ray Timing Explorer Proportional Counter Array data collected for the sources
over the 16 year duration of the mission, we show that Cyg X-2 and Cir X-1
display CD/HID evolution with close similarities to XTE J1701-462. Although GX
13+1 shows behavior that is in some ways unique, it also exhibits similarities
to XTE J1701-462, and we conclude that its overall CD/HID properties strongly
indicate that it should be classified as a Z source, rather than as an atoll
source. We conjecture that the secular evolution of Cyg X-2, Cir X-1, and GX
13+1 -- illustrated by sequences of CD/HID tracks we construct -- arises from
changes in the mass accretion rate. Our results strengthen previous suggestions
that within single sources Cyg-like Z source behavior takes place at higher
luminosities and mass accretion rates than Sco-like Z behavior, and lend
support to the notion that the mass accretion rate is the primary physical
parameter distinguishing the various NS-LMXB subclasses.Comment: 20 pages, 14 figures, 5 tables -- matches published version in Ap
On the geometric nature of low-frequency quasi-periodic oscillations in neutron-star low-mass X-ray binaries
We report on a detailed analysis of the so-called ~1 Hz quasi-periodic
oscillation (QPO) in the eclipsing and dipping neutron-star low-mass X-ray
binary EXO 0748-676. This type of QPO has previously been shown to have a
geometric origin. Our study focuses on the evolution of the QPO as the source
moves through the color-color diagram, in which it traces out an
atoll-source-like track. The QPO frequency increases from ~0.4 Hz in the hard
state to ~25 Hz as the source approaches the soft state. Combining power
spectra based on QPO frequency reveals additional features that strongly
resemble those seen in non-dipping/eclipsing atoll sources. We show that the
low-frequency QPOs in atoll sources and the ~1 Hz QPO in EXO 0748-676 follow
similar relations with respect to the noise components in their power spectra.
We conclude that the frequencies of both types of QPOs are likely set by (the
same) precession of a misaligned inner accretion disk. For high-inclination
systems, like EXO 0748-676, this results in modulations of the neutron-star
emission due to obscuration or scattering, while for lower-inclination systems
the modulations likely arise from relativistic Doppler boosting and
light-bending effects.Comment: Updated to published version (ApJ, 812, 80
Spectral Softening Between Outburst and Quiescence In The Neutron Star Low-Mass X-Ray Binary SAX J1750.8-2900
Tracking the spectral evolution of transiently accreting neutron stars
between outburst and quiescence probes relatively poorly understood accretion
regimes. Such studies are challenging because they require frequent monitoring
of sources with luminosities below the thresholds of current all-sky X-ray
monitors. We present the analysis of over 30 observations of the neutron star
low-mass X-ray binary SAX J1750.8-2900 taken across four years with the X-ray
telescope aboard Swift. We find spectral softening with decreasing luminosity
both on long (1 year) and short (days to week) timescales. As the
luminosity decreases from erg s to erg s (0.5-10 keV), the power law photon index increases from from 1.4
to 2.9. Although not statistically required, our spectral fits allow an
additional soft component that displays a decreasing temperature as the
luminosity decreases from to erg
s. Spectral softening exhibited by SAX J1750.8-2900 is consistent both
with accretion emission whose spectral shape steepens with decreasing
luminosity and also with being dominated by a changing soft component, possibly
associated with accretion onto the neutron star surface, as the luminosity
declines.Comment: 11 pages, 9 figures, 3 Tables. Resubmitted to The Astrophysical
Journal after incorporating comments from the refere
The Spectral Evolution along the Z track of the Bright Neutron Star X-ray Binary GX 17+2
Z sources are bright neutron-star X-ray binaries, accreting at around the
Eddington limit. We analyze the 68 RXTE observations (270 ks) of Sco-like Z
source GX 17+2 made between 1999 October 3-12, covering a complete Z track. We
create and fit color-resolved spectra with a model consisting of a thermal
multicolor disk, a single-temperature-blackbody boundary layer and a weak
Comptonized component. We find that, similar to what was observed for XTE
J1701-462 in its Sco-like Z phase, the branches of GX 17+2 can be explained by
three processes operating at a constant accretion rate Mdot into the disk:
increase of Comptonization up the horizontal branch, transition from a standard
thin disk to a slim disk up the normal branch, and temporary fast decrease of
the inner disk radius up the flaring branch. We also model the Comptonization
in an empirically self-consistent way, with its seed photons tied to the
thermal disk component and corrected for to recover the pre-Comptonized thermal
disk emission. This allows us to show a constant Mdot along the entire Z track
based on the thermal disk component. We also measure the upper kHz QPO
frequency and find it to depend on the apparent inner disk radius R_in (prior
to Compton scattering) approximately as frequency \propto R_in^(-3/2),
supporting the idenfitication of it as the Keplerian frequency at R_in. The
horizontal branch oscillation is probably related to the dynamics in the inner
disk as well, as both its frequency and R_in vary significantly on the
horizontal branch but become relatively constant on the normal branch.Comment: 17 pages, 14 figures. Accepted for publication in Ap
A detailed study of the 5 Hz quasi-periodic oscillations in the bright X-ray transient and black-hole candidate GRS 1739-278
We present a detailed study of the 5 Hz quasi-periodic oscillation (QPO)
recently discovered in the bright X-ray transient and black-hole candidate GRS
1739-278 (Borozdin & Trudolyubov 2000) during a Rossi X-ray Timing Explorer
observation taken on 1996 March 31. In total 6.6 ksec of on-source data were
obtained, divided in two data sets of 3.4 and 3.2 ksec which were separated by
2.6 ksec. The 5 Hz QPO was only present during the second data set. The QPO
increased in strength from below 2% rms amplitude for photon energies below 4
keV to ~5% rms amplitude for energies above 10 keV. The soft QPO photons (below
5 keV) lagged the hard ones (above 10 keV) by almost 1.5 radian. Besides the
QPO fundamental, its first overtone was detected. The strength of the overtone
increased with photon energy (from <2% rms below 5 keV to ~8% rms above 10
keV). Although the limited statistics did not allow for an accurate
determination of the lags of the first overtone, indications are that also for
this QPO the soft photons lagged the hard ones. When the 5 Hz QPO was not
detected (i.e., during the first part of the observation), a broad noise
component was found for photon energies below 10 keV but it became almost a
true QPO (with a Q value of ~1.9) above that energy, with a frequency of ~3 Hz.
Its hard photons preceded the soft ones in a way reminiscent of the 5 Hz QPO,
strongly suggesting that both features are physically related. We discuss our
finding in the frame work of low-frequency QPOs and their properties in BHCs.Comment: Accepted for publication in MNRAS, 2 August 200
Discovery of the near-infrared counterpart to the luminous neutron-star low-mass X-ray binary GX 3+1
Using the High Resolution Camera onboard the Chandra X-ray Observatory, we
have measured an accurate position for the bright persistent neutron-star X-ray
binary and atoll source GX 3+1. At a location that is consistent with this new
position we have discovered the near-infrared (NIR) counterpart to GX 3+1 in
images taken with the PANIC and FourStar cameras on the Magellan Baade
Telescope. The identification of this K_s=15.8+-0.1 mag star as the counterpart
is based on the presence of a Br-gamma emission line in a NIR spectrum taken
with the FIRE spectrograph on the Baade Telescope. The absolute magnitude
derived from the best available distance estimate to GX 3+1 indicates that the
mass donor in the system is not a late-type giant. We find that the NIR light
in GX 3+1 is likely dominated by the contribution from a heated outer accretion
disk. This is similar to what has been found for the NIR flux from the brighter
class of Z sources, but unlike the behavior of atolls fainter (Lx ~ 1e36 to
1e37 erg/s) than GX 3+1, where optically-thin synchrotron emission from a jet
probably dominates the NIR flux.Comment: Accepted for publication in Ap
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