1,454 research outputs found
Towards 4U 1630-47: a black-hole soft X-ray transient odyssey
4U 1630-47 is a black-hole X-ray transient with one of the shortest
recurrence times. Despite its regular outburst behaviour little is known about
this source. Only recently has attention to this system increased. I discuss
there the basic known (X-ray) properties of 4U 1630-47 and report on X-ray and
radio observations obtained during its recent outburst, starting in 1998
February. These observations strengthen some of the similarities seen between
4U 1630-47 and the Galactic superluminal sources GRO J1655-40 and GRS 1915+105,
and provide the first detection of 4U 1630-47 in the radio. Using an updated
outburst ephemeris I predict the next outburst to occur about a week before
Christmas 1999.Comment: 4 pages, 2 figures, updated how to refer to this paper: to appear in:
R.N.Ogley, J. Bell Burnell (eds.), 2nd Workshop on Galactic Sources with
Relativistic Jets, NewAR, 42, in pres
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
The Optical Light Curves of Cygnus X-2 (V1341 Cyg) and the Mass of its Neutron Star
We present U, B and V light curves (taken from the literature) of the low
mass X-ray binary Cygnus X-2. The ``lower envelope'' of the light curves folded
on the orbital period are ellipsoidal. We fit an ellipsoidal model to the lower
envelopes of the B and V light curves to derive inclination constraints. If we
assume the accretion disc is steady-state where its radial temperature profile
goes as T(r) \propto r^{-3/4}, we find an inclination of i = 62.5 +/- 4 deg.
However, the predicted ratio of the disc flux to the total flux in B (the
``disc fraction'') is larger than what is observed (about 0.55 compared to <
0.3). If we use a flatter radial temperature profile of the disc expected for
strongly irradiated discs (T(r) \propto r^{-3/7}), then we find an inclination
of i = 54.6 deg and a disc fraction in B of approximately 0.30. However, in
this case the value of chi^2 is much larger (48.4 with 36 degrees of freedom
compared to 40.9 for the steady-state case). Adopting i = 62.5 +/- 4 deg and
using a previous determination of the mass ratio (q = M_c/M_x = 0.34 +/- 0.04)
and the optical mass function (f(M) = 0.69 +/- 0.03 solar masses), we find that
the mass of the neutron star is M_x = 1.78 +/- 0.23 solar masses and the mass
of the secondary star is M_c = 0.60 +/- 0.13 solar masses. We derive a distance
of d = 7.2 +/- 1.1 kpc, which is significantly smaller than a recent distance
measurement of d = 11.6 +/- 0.3 kpc derived from an observation of a type I
radius-expansion X-ray burst, but consistent with earlier distance estimates.
(abridged)Comment: 12 pages, 9 figures, to appear in MNRAS, LaTeX, uses mn.st
The first radius-expansion X-ray burst from GX 3+1
During several observations in 1999 August with RXTE of the low-mass X-ray
binary GX 3+1, we found a single short and strong X-ray burst. This is the
first burst from GX 3+1 which clearly shows evidence for radius expansion of
the neutron-star photosphere during the thermo-nuclear runaway. We show that
the cooling phase of the neutron star photosphere starts already just before
the end of the contraction phase. Considering the fact that the radius
expansion is due to the burst luminosity being at the Eddington luminosity,
assuming standard burst parameters and accounting for gravitational redshift
effects we derive a distance to the source of ~4.5 kpc, although relaxing these
assumptions may lead to uncertainties up to ~30%. By comparing the persistent
flux with that observed at the peak of the burst we infer that near the time of
the X-ray burst the persistent luminosity of GX 3+1 is ~0.17*L_edd, confirming
predictions from theoretical modeling of X-ray spectra of bright sources like
GX 3+1.Comment: 5 pages, 4 figures, accepted for publication in A&A 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
On the X-ray fast-time variability of Sco X-2 (GX 349+2)
We have analysed archived Ginga data on the Z source Sco X-2 (GX349+2). We
present the first detailed investigation of its X-ray fast-time variability, as
a function of position in the Z track. During the two-day observation over the
period 5-7 March 1989, the source was in the so-called flaring branch, and the
lower part of the so-called normal branch. We found broad peaked noise with a
centroid frequency and width of ~4-7 Hz and ~6-12 Hz respectively. The peaked
noise was strongest in the lower flaring branch, with a maximum fractional rms
amplitude of ~3 %. We conclude that it is not a manifestation of atoll source
high frequency noise, as had been suggested, and compare it with the power
spectral features seen in other Z sources. We find that the peaked noise is
markedly different to the quasi-periodic oscillations found in the normal and
flaring branches of Sco X-1; however it bears some resemblance to that seen in
the flaring branch of Cyg X-2 at low overall intensities.Comment: 9 pages, 7 figues, accepted for publication in A&
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&
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