39 research outputs found
Discovery of Two Simultaneous Kilohertz Quasi-Periodic Oscillations in KS 1731-260
We have discovered two simultaneous quasi-periodic oscillations (QPOs) at
898.3+/-3.3 Hz and 1158.6+/-9.0 Hz in the 1996 August 1 observation of the
low-mass X-ray binary KS 1731-260 with the Rossi X-ray Timing Explorer. The rms
amplitude and FWHM of the lower frequency QPO were 5.3+/-0.7 % and 22+/-8 Hz,
whereas those of the higher frequency QPO were 5.2+/-1.0 % and 37+/-21 Hz. At
low inferred mass accretion rate both QPOs are visible, at slightly higher mass
accretion rate the lower frequency QPO disappears and the frequency of the
higher frequency QPO increases to ~1178 Hz. At the highest inferred mass
accretion rate this QPO is only marginally detectable (2.1 sigma) near 1207 Hz,
which is the highest frequency so far observed in an X-ray binary. The
frequency difference (260.3+/-9.6 Hz) between the QPOs is equal to half the
frequency of the oscillations observed in a type I burst in this source (at
523.92+/-0.05 Hz, Smith, Morgan and Bradt 1997). This suggests that the neutron
star spin frequency is 261.96 Hz (3.8 ms), and that the lower frequency QPO is
the beat between the higher frequency QPO, which could be a preferred orbital
frequency around the neutron star, and the neutron star spin. During the 1996
August 31 observation we detected an additional QPO at 26.9+/-2.3 Hz, with a
FWHM and rms amplitude of 11+/-5 Hz and 3.4+/-0.6 %.Comment: 6 pages including 3 figures, Astrophysical Journal Letters, in press
(issue 482
Kilohertz QPO Peak Separation Is Not Constant in Scorpius X-1
We report on a series of twenty ~10^5 c/s, 0.125 msec time-resolution RXTE
observations of the Z source and low-mass X-ray binary Scorpius X-1. Twin
kilohertz quasi-periodic oscillation (QPO) peaks are obvious in nearly all
observations. We find that the peak separation is not constant, as expected in
some beat-frequency models, but instead varies from ~310 to ~230 Hz when the
centroid frequency of the higher-frequency peak varies from ~875 to ~1085 Hz.
We detect none of the additional QPO peaks at higher frequencies predicted in
the photon bubble model (PBM), with best-case upper limits on the peaks' power
ratio of 0.025. We do detect, simultaneously with the kHz QPO, additional QPO
peaks near 45 and 90 Hz whose frequency increases with mass accretion rate. We
interpret these as first and second harmonics of the so-called
horizontal-branch oscillations well known from other Z sources and usually
interpreted in terms of the magnetospheric beat-frequency model (BFM). We
conclude that the magnetospheric BFM and the PBM are now unlikely to explain
the kHz QPO in Sco X-1. In order to succeed in doing so, any BFM involving the
neutron star spin (unseen in Sco X-1) will have to postulate at least one
additional unseen frequency, beating with the spin to produce one of the kHz
peaks.Comment: 6 pages including 3 figure
Discovery of KiloHertz Quasi-Periodic Oscillations in the Z source Cygnus X-2
During observations with RXTE we discovered two simultaneous kHz QPOs near
500 Hz and 860 Hz in the Z source Cyg X-2. In the X-ray color-color diagram and
hardness-intensity diagram (HID) a clear Z track was traced out. A single kHz
QPO peak was detected at the left end of the horizontal branch (HB) of the Z
track, with a frequency of 731+/-20 Hz and an amplitude of 4.7+0.8-0.6 % rms in
the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher
count rates, an additional peak at 532+/-43 Hz was detected with an rms
amplitude of 3.0+1.0-0.7 %. When the source moved down the HB, thus when the
inferred mass accretion rate increased, the frequency of the higher-frequency
QPO increased to 839+/-13 Hz, and its amplitude decreased to 3.5+0.4-0.3 % rms.
The higher-frequency QPO was also detected on the upper normal branch (NB) with
an rms amplitude of 1.8+0.6-0.4 % and a frequency of 1007+/-15 Hz; its peak
width did not show a clear correlation with inferred mass accretion rate. The
lower-frequency QPO was most of the time undetectable, with typical upper
limits of 2 % rms, no conclusion on how this QPO behaved with mass accretion
rate can be drawn. If the peak separation between the QPOs is the neutron star
spin frequency (as required in some beat-frequency models) then the neutron
star spin period is 2.9+/-0.2 ms (346+/-29 Hz).Comment: 10 pages, including 3 figures. Accepted for publication in ApJ
Letter
Discovery of a ~7 Hz Quasi-Periodic Oscillation in the low-luminosity low-mass X-ray binary 4U 1820-30
We have discovered a 7.06+-0.08 Hz quasi-periodic oscillation (QPO) in the
X-ray flux of the low-luminosity low-mass X-ray binary (LMXB) and atoll source
4U 1820-30. This QPO was only observable at the highest observed mass accretion
rate, when the source was in the uppermost part of the banana branch, at a 2-25
keV luminosity of 5.4x10^37 erg/s (for a distance of 6.4 kpc). The QPO had a
FWHM of only 0.5+-0.2 Hz during small time intervals (32-s of data), and showed
erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms
amplitude over the energy range 2-60 keV was 5.6%+-0.2%. The amplitude
increased with photon energy from 3.7%+-0.5% between 2.8 and 5.3 keV to
7.3%+-0.6% between 6.8 and 9.3 keV, above which it remained approximately
constant at ~7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was
consistent with being zero (-1.2+-3.4 ms).
The properties of the QPO (i.e., its frequency and its presence only at the
highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO
observed in the highest luminosity LMXBs (the Z sources) when they are
accreting near the Eddington mass accretion limit. If this is indeed the same
phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which
require the near-Eddington accretion rates, will not hold. Assuming isotropic
emission, the 2-25 keV luminosity of 4U 1820-30 at the time of the 7 Hz QPOs is
at maximum only 40% (for a companion star with cosmic abundances), but most
likely ~20% (for a helium companion star) of the Eddington accretion limit.Comment: Accepted for publication in ApJ Letters (6 pages, including 3
figures
XMM-Newton observations of the neutron star X-ray transient KS 1731-260 in quiescence
We report on XMM-Newton observations performed on 2001 September 13-14 of the
neutron star X-ray transient KS 1731-260 in quiescence. The source was detected
at an unabsorbed 0.5-10 keV flux of only 4 - 8 x 10^{-14} erg/s, depending on
the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10
keV X-ray luminosity of approximately 2 - 5 x 10^{32} erg/s. The September 2001
quiescent flux of KS 1731-260 is lower than that observed during the Chandra
observation in March 2001. In the cooling neutron star model for the quiescent
X-ray emission of neutron star X-ray transients, this decrease in the quiescent
flux implies that the crust of the neutron star in KS 1731-260 cooled down
rapidly between the two epochs, indicating that the crust has a high
conductivity. Furthermore, enhanced cooling in the neutron star core is also
favored by our results.Comment: Accepter for publication in ApJ Letters, 22 May 200
Chandra Detections of Two Quiescent Black Hole X-Ray Transients
Using the Chandra X-ray Observatory, we have detected the black hole
transients V4641 Sgr and XTE J1859+226 in their low luminosity, quiescent
states. The 0.3-8 keV luminosities are (4.0^(+3.3)_(-2.4))E31 (d/7 kpc)^2 erg/s
and (4.2^(+4.8)_(-2.2))E31 (d/11 kpc)^2 erg/s for V4641 Sgr and XTE J1859+226,
respectively. With the addition of these 2 systems, 14 out of the 15 transients
with confirmed black holes (via compact object mass measurements) now have
measured quiescent luminosities or sensitive upper limits. The only exception
is GRS 1915+105, which has not been in quiescence since its discovery in 1992.
The luminosities for V4641 Sgr and XTE J1859+226 are consistent with the median
luminosity of 2E31 erg/s for the systems with previous detections. Our analysis
suggests that the quiescent X-ray spectrum of V4641 Sgr is harder than for the
other systems in this group, but, due to the low statistical quality of the
spectrum, it is not clear if V4641 Sgr is intrinsically hard or if the column
density is higher than the interstellar value. Focusing on V4641 Sgr, we
compare our results to theoretical models for X-ray emission from black holes
in quiescence. Also, we obtain precise X-ray positions for V4641 Sgr and XTE
J1859+226 via cross-correlation of the X-ray sources detected near our targets
with IR sources in the 2 Micron All-Sky Survey catalog.Comment: 4 pages, Accepted by ApJ Letter
RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs
(Abridged) We report on ten weeks of RXTE observations of the X-ray transient
XTE J1701-462. Comparisons with other sources suggest it had all the
characteristics of the neutron star Z sources (the brightest persistent neutron
star LMXBs). These include Z tracks in X-ray color diagrams and typical
variability components detected in the power spectra. XTE J1701-462 is the
first transient Z source and provides unique insights into mass accretion rate
(Mdot) and luminosity dependencies in neutron star LMXBs. As its overall
luminosity decreased, we observed a switch between two types of Z-source
behavior, with most of the branches of the Z-track changing their shape and/or
orientation. We interpret this switch as an extreme case of the longterm
changes seen in the persistent Z sources and suggest that these result from
changes in Mdot. We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1,
and GX 340+0) might be more luminous (> 50%) than the Sco-like Z sources (Sco
X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior
of kHz QPOs, which involves a prompt as well as a filtered response to changes
in Mdot, we propose that changes in Mdot can explain both movement along the Z
track and changes in the shape of the Z track. We discuss some consequences of
this and consider the possibility that the branches of the Z will evolve into
the branches observed in the X-ray color diagrams of the less luminous atoll
sources, but not in a way that was previously suggested.Comment: Accepted for publication in ApJ, 11 page
Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550-564: A New Interpretation of Black Hole States
We present an analysis of RXTE data of the X-ray transient XTE J1550-564. The
source went through several states, which were divided into spectrally soft and
hard states. These states showed up as distinct branches in the color-color
diagram, forming a structure with a comb-like topology; the soft state branch
forming the spine and the hard state branches forming the teeth. Variability
was strongly correlated with the position on the branches. The broad band noise
became stronger, and changed from power law like to band limited, as the
spectrum became harder. Three types of QPOs were found: 1-18 Hz and 102-284 Hz
QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The
frequencies of the high and low frequency QPOs on the hard branches were
correlated with each other, and anti-correlated with spectral hardness. The
changes in QPO frequency suggest that the inner disc radius only increases by a
factor of 3-4 as the source changes from a soft to a hard state. Our results on
XTE J1550-564 strongly favor a 2-dimensional description of black hole
behavior, where the regions near the spine of the comb in the color-color
diagram can be identified with the high state, and the teeth with transitions
from the high state, via the intermediate state (which includes the very high
state) to the low state, and back. The two physical parameters underlying this
behavior vary to a large extent independently and could for example be the mass
accretion rate through the disk and the size of a Comptonizing region.Comment: 49 pages (inlcuding 26 figures and 4 tables), accepted for
publication in ApJ Supplement
Discovery of kilohertz quasi-periodic oscillations in the Z source GX 340+0
We have discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in
the Z source GX 340+0 with the Rossi X-ray Timing Explorer. The X-ray
hardness-intensity and color-color diagram each show a full Z-track, with an
extra limb branching off the flaring branch of the Z. Both peaks moved to
higher frequencies when the mass accretion rate increased. The two peaks moved
from 247 +/- 6 and 567 +/- 39 Hz at the left end of the horizontal branch to
625 +/- 18 and 820 +/- 19 Hz at its right end. The higher frequency peak's rms
amplitude (5-60 keV) and FWHM decreased from ~5% and 383 +/- 135 Hz to ~2%, and
145 +/- 62 Hz, respectively. The rms amplitude and FWHM of the lower peak were
consistent with being constant near 2.5 % and 100 Hz. The kHz QPO separation
was consistent with being constant at 325 +/- 10 Hz. Simultaneous with the kHz
QPOs we detected the horizontal branch oscillations (HBO) and its second
harmonic, at frequencies between 20 and 50 Hz, and 38 and 69 Hz, respectively.
The normal branch oscillations were only detected on the upper and middle
normal branch, and became undetectable on the lower normal branch. The HBO
frequencies do not fall within the range predicted for Lense-Thirring (LT)
precession, unless either the ratio of the neutron star moment of inertia to
neutron star mass is at least 4, 10^45 gcm^2/M_sun, the frequencies of the HBO
are in fact the sub-harmonic oscillations, or the observed kHz peak difference
is half the spin frequency and not the spin frequency. During a 1.2 day gap
between two observations, the Z-track in the hardness-intensity diagram moved
to higher count rates by about 3.5%. Comparing data before and after this
shift, we find that the HBO properties are determined by position on the
Z-track and not directly by count rate or X-ray colors.Comment: 12 pages including 4 figures. Accepted for publication in ApJ Letter
The magnetic nature of disk accretion onto black holes
Although disk accretion onto compact objects - white dwarfs, neutron stars,
and black holes - is central to much of high energy astrophysics, the
mechanisms which enable this process have remained observationally elusive.
Accretion disks must transfer angular momentum for matter to travel radially
inward onto the compact object. Internal viscosity from magnetic processes and
disk winds can in principle both transfer angular momentum, but hitherto we
lacked evidence that either occurs. Here we report that an X-ray-absorbing wind
discovered in an observation of the stellar-mass black hole binary GRO J1655-40
must be powered by a magnetic process that can also drive accretion through the
disk. Detailed spectral analysis and modeling of the wind shows that it can
only be powered by pressure generated by magnetic viscosity internal to the
disk or magnetocentrifugal forces. This result demonstrates that disk accretion
onto black holes is a fundamentally magnetic process.Comment: 15 pages, 2 color figures, accepted for publication in Nature.
Supplemental materials may be obtained by clicking
http://www.astro.lsa.umich.edu/~jonmm/nature1655.p