319 research outputs found
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
RXTE observations of the dipping low-mass X-ray binary 4U 1624-49
We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer
(RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We
find that outside the dips the tracks in the colour-colour and
hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll
sources in the middle and upper parts of the banana branch. The tracks show
secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a
function of the position in the CD. This is the first time power spectra of
this source are presented. No quasi-periodic oscillations (QPOs) are found. The
power spectra are dominated by very low frequency noise (VLFN), characteristic
for atoll sources in the banana state, and band limited noise (BLN) which is
not reliably detected but may, uncharacteristically, strengthen and increase in
frequency with spectral hardness. The VLFN fits to a power law, which becomes
steeper when the source moves to the harder part of the CD. We conclude that 4U
1624-49 is an atoll source which in our observations is in the upper banana
branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term
flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and
with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is
most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in A&A. This
version: a few typos correcte
X-ray spectral and timing properties of the 2001 superburst of 4U 1636-536
Preliminary results are reported on the spectral and timing properties of the
spectacular 2001 superburst of 4U 1636-536 as seen by the RXTE/PCA. The
(broad-band) power-spectral and hardness properties during the superburst are
compared to those just before and after the superburst. Not all of the
superburst emission can be fitted by pure black-body radiation. We also
gathered BeppoSAX/WFC and RXTE/ASM data, as well as other RXTE/PCA data,
obtained days to months before and after the superburst to investigate the
normal X-ray burst behavior around the time of the superburst. The first normal
X-ray burst after the 2001 superburst was detected 23 days later. During
inspection of all the RXTE/ASM data we found a third superburst. This
superburst took place on June 26, 1999, which is ~2.9 yrs after the 1996
superburst and ~1.75 yrs before the 2001 superburst. The above findings are the
strongest constraints observed so far on the duration of the cessation of
normal X-ray bursts after a superburst and the superburst recurrence times.Comment: 4 pages, 4 figures,to appear in the proceedings of "X-Ray Timing
2003: Rossi and Beyond", eds. P. Kaaret, F.K. Lamb, & J.H. Swank (Melville,
NY: AIP
The atoll source states of 4U 1608-52
We have studied the atoll source 4U 1608-52 using a large data set obtained
with the Rossi X-ray Timing Explorer. We find that the timing properties of 4U
1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09
and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a
transient covering two orders of magnitude in luminosity. The frequencies of
the variability components of these three sources follow a universal scheme
when plotted versus the frequency of the upper kilohertz QPO, suggesting a very
similar accretion flow configuration. If we plot the Z sources on this scheme
only the lower kilohertz QPO and HBO follow identical relations. Using the
mutual relations between the frequencies of the variability components we
tested several models; the transition layer model, the sonic point beat
frequency model, and the relativistic precession model. None of these models
described the data satisfactory. Recently, it has been suggested that the atoll
sources (among them 4U 1608-52) trace out similar three-branch patterns as the
Z sources in the color-color diagram. We have studied the relation between the
power spectral properties and the position of 4U 1608-52 in the color-color
diagram and conclude that the timing behavior is not consistent with the idea
that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram
along which the timing properties vary gradually, as Z sources do.Comment: 43 pages, 16 figures, ApJ accepte
Timing properties and spectral states in Aquila X-1
We have analyzed five X-ray outbursts of the neutron-star soft X-ray
transient Aql X-1 and investigated the timing properties of the source in
correlation with its spectral states as defined by different positions in the
color-color and hardness-intensity diagrams. The hard color and the source
count rate serve as the distinguishing parameters giving rise to three spectral
states: a low-intensity hard state, an intermediate state and a high-intensity
soft state. These states are respectively identified with the extreme island,
island and banana states that characterize the atoll sources. The large amount
of data analyzed allowed us to perform for the first time a detailed timing
analysis of the extreme island state. Differences in the aperiodic variability
between the rise and the decay of the X-ray outbursts are found in this state:
at the same place in the color-color diagram, during the rise the source
exhibits more power at low frequencies (< 1 Hz), whereas during the decay the
source is more variable at high frequencies (> 100 Hz). The very-low frequency
noise that characterizes the banana-state power spectra below 1 Hz cannot be
described in terms of a single power law but a two-component model is required.
In two outbursts a new 6-10 Hz QPO has been discovered and tentatively
identified with the normal/flaring branch-like oscillation observed only at the
highest inferred mass accretion rates. We have compared the spectral and timing
properties of Aql X-1 with those of other atoll and Z sources. Our results
argue against a unification scheme for these two types of neutron-star X-ray
binaries.Comment: 24 pages, 4 tables, 9 figures, accepted for publication in Ap
Peculiar spectral and power spectral behaviour of the LMXB GX 13+1
We present results of an analysis of all 480 ks of Rossi X-ray Timing
Explorer Proportional Counter Array data obtained from 17 May 1998 to 11
October 1998 on the luminous low mass X-ray binary GX 13+1. We analysed the
spectral properties in colour-colour diagrams (CDs) and hardness-intensity
diagrams (HIDs) and fitted the power spectra with a multi-Lorentzian model. GX
13+1 traces out a curved track in the CDs on a time scale of hours, which is
very reminiscent of a standard atoll track containing an island, and lower and
upper banana branch. However, both count rate and power spectral properties
vary along this track in a very unusual way, not seen in any other atoll or Z
source. The count rate, which varied by a factor of ~1.6, along a given track
first decreases and then increases, causing the motion through the HIDs to be
in the opposite sense to that in the CD, contrary to all other Z and atoll
sources. Along a CD track, the very low frequency noise uniquely decreases in
amplitude from ~5 to ~2% (rms). The high frequency noise amplitude decreases
from ~4% to less than 1% and its characteristic frequency decreases from ~10 to
\~5 Hz. The 57-69 Hz quasi-periodic oscillation (QPO) found earlier is also
detected, and no kHz QPOs are found. In addition the entire track shows secular
motion on a time scale of about a week. The average count rate as well as the
amplitude of the very low frequency noise correlate with this secular motion.
We discuss a possible explanation for the peculiar properties of GX 13+1 in
terms of an unusual orientation or strength of a relativistic jet.Comment: 15 pages, 13 figures. Accepted for publication in A&
Evidence of a decrease of kHz QPO peak separation towards low frequencies in 4U 1728-34 (GX 354-0)
We have produced the colour-colour diagram of all the observations of 4U
1728-34 available in the Rossi X-ray Timing Explorer public archive (from 1996
to 2002) and found observations filling in a previously reported 'gap' between
the island and the banana X-ray states. We have made timing analysis of these
gap observations and found, in one observation, two simultaneous kHz
quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit
in the overall trend of the source. The 'lower' kHz QPO has a centroid
frequency of ~308 Hz. This is the lowest 'lower' kHz QPO frequency ever
observed in 4U 1728-34. The peak frequency separation between the 'upper' and
the 'lower' kHz QPO is 274+/-11 Hz, significantly smaller than the constant
value of ~350 Hz found when the 'lower' kHz QPO frequency is between ~500 and
800 Hz. This is the first indication in this source for a significant decrease
of kHz QPO peak separation towards low frequencies. We briefly compare the
result to theoretical models for kHz QPO production.Comment: accepted for publication in MNRAS Letter
A multi-Lorentzian timing study of the atoll sources 4U 0614+09 and 4U 1728-34
We present the results of a multi-Lorentzian fit to the power spectra of two
kilohertz QPO sources; 4U 0614+09 and 4U 1728-34. This work was triggered by
recent results of a similar fit to the black-hole candidates (BHCs) GX 339-4
and Cyg X-1 by Nowak in 2000. We find that one to six Lorentzians are needed to
fit the power spectra of our two sources. The use of exactly the same fit
function reveals that the timing behaviour of 4U 0614+09 and 4U 1728-34 is
almost identical at luminosities which are about a factor 5 different. As the
characteristic frequency of the Lorentzians we use the frequency, nu_max, at
which each component contributes most of its variance per log frequency as
proposed by Belloni, Psaltis & van der Klis in 2001. When using nu_max instead
of the centroid frequency of the Lorentzian, the recently discovered hectohertz
Lorentzian is practically constant in frequency. We use our results to test the
suggestions by, respectively, Psaltis Belloni and van der Klis in 1999 and
Nowak in 2000 that the two Lorentzians describing the high-frequency end of the
broad-band noise in BHCs in the low state can be identified with the kilohertz
QPOs in the neutron star low mass X-ray binaries. We find, that when the two
kilohertz QPOs are clearly present, the low-frequency part of the power
spectrum is too complicated to draw immediate conclusions from the nature of
the components detected in any one power spectrum. However, the relations we
observe between the characteristic frequencies of the kilohertz QPOs and the
band-limited noise, when compared to the corresponding relations in BHCs, hint
towards the identification of the second-highest frequency Lorentzian in the
BHCs with the lower kilohertz QPO. They do not confirm the identification of
the highest-frequency Lorentzian with the upper kilohertz QPO.Comment: 30 pages, 35 figures, ApJ accepted; changed name of BLN QPO into very
low-frequency Lorentzian, removed table 4 and figure 8 from previous versio
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
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