1,098 research outputs found
Normal Branch Quasi-Periodic Oscillations in Sco X-1: Viscous Oscillations of a Spherical Shell Near the Neutron Star
We present a comprehensive classification of all observed QPOs within the
framework of the transition layer model using a large set of Rossi X-ray Timing
Explorer (RXTE) data for Sco X-1. The model assumes an optically thin material
along the observer's line of sight in the horizontal branch and an increasingly
optically thick material while in the other two branches that is consistent
with X-ray and radio observations and the disk transition layer model of QPOs.
We identify the ~ 6 Hz frequencies in the normal branch as acoustic
oscillations of a spherical shell around the neutron star (NS) that is formed
after radiation pressure near the Eddington accretion rate destroys the disk.
The size of the shell is on the order of one NS radii from the NS. We also
estimate the upper limit of Sco X-1's magnetic field to be 0.7 x 10^6 G at
about one NS radii above the NS surface while in the horizontal X-ray branch.Comment: 12 pages and 2 figures, to be published in the Astrophysical Journal
Letter
RXTE observations of the neutron star low-mass X-ray binary GX 17+2: correlated X-ray spectral and timing behavior
We have analyzed ~600 ks of Rossi X-ray Timing Explorer data of the neutron
star low-mass X-ray binary and Z source GX 17+2. A study was performed of the
properties of the noise components and quasi-periodic oscillations (QPOs) as a
function of the spectral properties, with the main goal to study the relation
between the frequencies of the horizontal branch and upper kHz QPOs. It was
found that when the upper kHz QPO frequency is below 1030 Hz these frequencies
correlate, whereas above 1030 Hz they anti-correlate. GX 17+2 is the first
source in which this is observed. We also found that the frequency difference
of the high frequency QPOs was not constant and that the quality factors (Q
values) of the HBO, its second harmonic, and the kHz QPOs are similar, and vary
almost hand in hand by a factor of more than three. Observations of the normal
branch oscillations during two type I X-ray bursts showed that their absolute
amplitude decreased as the flux from the neutron star became stronger. We
discuss these and other findings in terms of models that have been proposed for
these phenomena. We also compare the behavior of GX 17+2 and other Z sources
with that of black hole sources and consider the possibility that the mass
accretion rate might not be driving force behind all spectral and variability
changes.Comment: 35 pages, including 14 figures. Accepted for publication in ApJ.
Revised discussion, one new figure, and some minor figure changes with
respect to old versio
Correlations between X-ray Spectral and Timing Characteristics in Cyg X-2
Correlations between the quasi-periodic oscillations (QPOs) and the spectral
power-law index have been reported for a number of black hole candidate sources
and for four neutron star (NS) sources, 4U 0614+09, 4U 1608-52, 4U 1728-34 and
Sco X-1. An examination of QPO frequencies and index relationship in Cyg X-2 is
reported herein. The RXTE spectrum of Cyg X-2 can be adequately represented by
a simple two-component model of Compton up-scattering with a soft photon
electron temperature of about 0.7 keV and an iron K-line. Inferred spectral
power-law index shows correlation with the low QPO frequencies. We find that
the Thomson optical depth of the Compton cloud (CC) tau, in framework of
spherical geometry, is in the range of ~4-6, which is consistent with the
neutron star's surface being obscured. The NS high frequency pulsations are
presumably suppressed as a result of photon scattering off CC electrons because
of such high values of tau. We also point out a number of similarities in terms
timing (presence of low and high frequency QPOs) and spectral (high CC optical
depth and low CC plasma temperature) appearances between Cyg X-2 and Sco X-1.Comment: 7 pages, 4 figures, accepted for publication in ApJ (October 1, 2007,
v667n2 issue
The IR counterpart of the black-hole candidate 4U 1630-47
We present K band photometry of the region including the radio error box of
the soft X-ray transient 4U 1630-47 during its 1998 outburst. We detect a
variable source at K=16.1 mag located inside the radio error circle which we
identify as the counterpart to the X-ray source. We discuss the properties of
the source, and conclude that it is most likely a black-hole X-ray binary
similar to 4U 1543-47, GRO J1655-40 or SAX J1819.3-2525, containing a
relatively early-type secondary.Comment: 7 pages, 3 figures, A&A, in pres
Recommended from our members
X-rays from cataclysmic variables
Cataclysmic Variables are a distinct class of interacting binaries, transferring mass from a donor star to a degenerate accretor, a white dwarf. We here review X-ray observations of these systems, with some emphasis on what has been achieved in the last decade
Discovery of Two Simultaneous Kilohertz QPOs in the Persistent Flux of GX 349+2
We report the discovery of two simultaneous quasi-periodic oscillations in
the persistent flux of GX 349+2 at frequencies 712 +/- 9 and 978 +/- 9 Hz, with
rms amplitudes 1.25% +/- 0.34% and 1.34 +/- 0.32%, respectively. During our 152
ks observation with the Rossi X-ray Timing Explorer, GX 342+2 was in either the
normal branch or the flaring branch with count rates in the nominal 2-60 keV
RXTE-PCA band ranging from a low of 8,000 cps to a high of 15,000 cps. The kHz
QPOs were observed only when the source was at the top of the normal branch
when the count rate was about 8,200 cps corresponding to a flux of 1.4E-8
ergs/cm**2/s in the 2-10 keV band. With this report, now kHz QPOs have been
observed in all the 6 Z sources.Comment: 5 pages, LaTex (aas2pp4), Accepted for publication in ApJ Let
X-ray burst induced spectral variability in 4U 1728-34
Aims. INTEGRAL has been monitoring the Galactic center region for more than a
decade. Over this time INTEGRAL has detected hundreds of type-I X-ray bursts
from the neutron star low-mass X-ray binary 4U 1728-34, a.k.a. "the slow
burster". Our aim is to study the connection between the persistent X-ray
spectra and the X-ray burst spectra in a broad spectral range. Methods. We
performed spectral modeling of the persistent emission and the X-ray burst
emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI
instruments. Results. We constructed a hardness intensity diagram to track
spectral state variations. In the soft state the energy spectra are
characterized by two thermal components - likely from the accretion disc and
the boundary/spreading layer - together with a weak hard X-ray tail that we
detect in 4U 1728-34 for the first time in the 40 to 80 keV range. In the hard
state the source is detected up to 200 keV and the spectrum can be described by
a thermal Comptonization model plus an additional component: either a powerlaw
tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect
emission up to 80 keV during the X-ray bursts. We find that during the bursts
the emission above 40 keV decreases by a factor of about three with respect to
the persistent emission level. Conclusions. Our results suggest that the
enhanced X-ray burst emission changes the spectral properties of the accretion
disc in the hard state. The likely cause is an X-ray burst induced cooling of
the electrons in the inner hot flow near the neutron star.Comment: 7 pages, 5 figures, Accepted for publication in A&
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