4,288 research outputs found
Probing X-ray burst -- accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations
The intense radiation flux of Type I X-ray bursts is expected to interact
with the accretion flow around neutron stars. High frequency quasiperiodic
oscillations (kHz QPOs), observed at frequencies matching orbital frequencies
at tens of gravitational radii, offer a unique probe of the innermost disk
regions. In this paper, we follow the lower kHz QPOs, in response to Type I
X-ray bursts, in two prototypical QPO sources, namely 4U 1636-536 and 4U
1608-522, as observed by the Proportional Counter Array of the Rossi X-ray
Timing Explorer. We have selected a sample of 15 bursts for which the kHz QPO
frequency can be tracked on timescales commensurable with the burst durations
(tens of seconds). We find evidence that the QPOs are affected for over ~200 s
during one exceptionally long burst and ~100 s during two others (although at a
less significant level), while the burst emission has already decayed to a
level that would enable the pre-burst QPO to be detected. On the other hand,
for most of our burst-kHz QPO sample, we show that the QPO is detected as soon
as the statistics allow and in the best cases, we are able to set an upper
limit of ~20 s on the recovery time of the QPO. This diversity of behavior
cannot be related to differences in burst peak luminosity. We discuss these
results in the framework of recent findings that accretion onto the neutron
star may be enhanced during Type I X-ray bursts. The subsequent disk depletion
could explain the disappearance of the QPO for ~100 s, as possibly observed in
two events. However, alternative scenarios would have to be invoked for
explaining the short recovery timescales inferred from most bursts. Clearly the
combination of fast timing and spectral information of Type I X-ray bursts
holds great potential in the study of the dynamics of the inner accretion flow
around neutron stars.Comment: 8 pages, 9 figures, appears in Astronomy & Astrophysics, Volume 567,
id.A80, published 07/201
X-ray sources and their optical counterparts in the globular cluster M 22
Using XMM-Newton EPIC imaging data, we have detected 50 low-luminosity X-ray
sources in the field of view of M 22, where 5 +/- 3 of these sources are likely
to be related to the cluster. Using differential optical photometry, we have
identified probable counterparts to those sources belonging to the cluster.
Using X-ray spectroscopic and timing studies, supported by the optical colours,
we propose that the most central X-ray sources in the cluster are cataclysmic
variables, millisecond pulsars, active binaries and a blue straggler. We also
identify a cluster of galaxies behind this globular cluster.Comment: 11 pages, 7 figures, accepted for publication in A&
On the high coherence of kilo-Hz Quasi-Periodic Oscillations
We have carried out a systematic study of the properties of the kilo-Hertz
quasi-periodic oscillations (QPO) observed in the X-ray emission of the neutron
star low-mass X-ray binary 4U1608-52, using archival data obtained with the
Rossi X-ray Timing Explorer. We have investigated the quality factor, Q, of the
oscillations (defined as the ratio of the frequency of the QPO peak to its full
width at half maximum). In order to minimise the effect of long-term frequency
drifts, power spectra were computed over the shortest times permitted by the
data statistics. We show that the high Q of ~200 reported by Berger et al.
(1996) for the lower frequency kilo-Hz QPO in one of their observations is by
no means exceptional, as we observe a mean Q value in excess of 150 in 14 out
of the 21 observations analysed and Q can remain above 200 for thousands of
seconds. The frequency of the QPO varies over the wide range 560--890 Hz and we
find a systematic trend for the coherence time of the QPO, estimated as tau=Q
/(pi nu), to increase with the frequency, up to a maximum level at ~ 800 Hz,
beyond which it appears to decrease, at frequencies where the QPO weakens.
There is a more complex relationship between tau and the QPO root mean squared
amplitude (RMS), in which positive and negative correlations can be found. A
higher-frequency QPO, revealed by correcting for the frequency drift of the
560-890 Hz one, has a much lower Q (~10) which does not follow the same
pattern. We discuss these results in the framework of competing QPO models and
show that those involving clumps orbiting within or above the accretion disk
are ruled out.Comment: Accepted for publication in MNRAS, 8 pages, 6 figures, 3 Table
RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05
We present the results of detailed spectral studies of the ultra-compact low
mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing
Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to
exhibit a ~199-day modulation in its 2--12 keV flux. Observations were
performed with the PCA and HEXTE instruments on RXTE at roughly one-month
intervals to sample this long-term period and study accretion rate-related
spectral changes. We obtain good fits with a model consisting of a blackbody
and an exponentially cut-off power law. The spectral parameters are strongly
correlated with both the broad-band (2--50 keV) luminosity and the position in
the color-color diagram, with the source moving from a low hard state to a high
soft state as the accretion rate increases. The blackbody component appears to
drive the spectral evolution. Our results are consistent with a geometry in
which the soft component arises from an optically thick boundary layer and the
hard component from an extended Comptonizing corona. Comparing our results with
those of a similar study of the brighter source 4U 1820-30 (Bloser et al.
2000), we find that the two ultra-compact LMXBs occupy similar spectral states
even though the transitions occur at very different total luminosities.Comment: 27 pages LaTeX, 8 figures, accepted to the Astrophysical Journa
Simultaneous BeppoSAX and Rossi X-ray Timing Explorer observations of 4U1812-12
4U1812-12 is a faint persistent and weakly variable neutron star X-ray
binary. It was observed by BeppoSAX between April 20th and 21st, 2000 in a hard
spectral state with a bolometric luminosity of ~2x10^36 ergs/s. Its broad band
energy spectrum is characterized by the presence of a hard X-ray tail extending
above ~100 keV. It can be represented as the sum of a dominant hard Comptonized
component (electron temperature of ~36 keV and optical depth ~3) and a weak
soft component. The latter component which can be fitted with a blackbody of
about 0.6 keV and equivalent radius of ~2 km is likely to originate from the
neutron star surface. We also report on the first measurement of the power
density spectrum of the source rapid X-ray variability, as recorded during a
simultaneous snapshot observation performed by the Rossi X-ray Timing Explorer.
As expected for a neutron star system in such hard spectral state, its power
density spectrum is characterized by the presence of a ~0.7 Hz low frequency
quasi-periodic oscillation together with three broad noise components, one of
which extends above ~200 Hz.Comment: 6 pages, 3 figures, accepted for publication in A&
Morphological analysis on the coherence of kHz QPOs
We take the recently published data of twin kHz quasi-period oscillations
(QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and
investigate the morphology of the samples, which focuses on the quality factor,
peak frequency of kHz QPOs, and try to infer their physical mechanism. We
notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in
general) and increase with the kHz QPO peak frequencies for both Z and Atoll
sources. (2) The distribution of quality factor versus frequency for the lower
kHz QPOs are quite different between Z and Atoll sources. For most Z source
samples, the quality factors of lower kHz QPOs are low (usually lower than 15)
and rise steadily with the peak frequencies except for Sco X-1, which drop
abruptly at the frequency of about 750 Hz. While for most Atoll sources, the
quality factors of lower kHz QPOs are very high (from 2 to 200) and usually
have a rising part, a maximum and an abrupt drop. (3) There are three Atoll
sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality
factors for lower kHz QPOs. These three sources have been detected with the
spin frequencies and sidebands, in which the source with higher spin frequency
presents higher quality factor of lower kHz QPOs and lower difference between
sideband frequency and lower kHz QPO frequency.Comment: 8 pages, 8 figures, publishe
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