437 research outputs found
The Interpretations For the Low and High Frequency QPO Correlations of X-ray Sources Among White Dwarfs, Neutron Stars and Black Holes
It is found that there exists an empirical linear relation between the high
frequency \nhigh and low frequency \nlow of quasi-periodic oscillations
(QPOs) for black hole candidate (BHC), neutron star (NS) and white dwarf (WD)
in the binary systems, which spans five orders of magnitude in frequency.
For the NS Z (Atoll) sources,
and are identified as the lower kHz QPO frequency
and horizontal branch oscillations (HBOs) \nh (broad noise components); for
the black hole candidates and low-luminosity neutron stars, they are the QPOs
and broad noise components at frequencies between 1 and 10 Hz; for WDs, they
are the ``dwarf nova oscillations'' (DNOs) and QPOs of cataclysmic variables
(CVs). To interpret this relation, our model ascribes to the
Alfv\'en wave oscillation frequency at a preferred radius and to
the same mechanism at another radius. Then, we can obtain \nlow = 0.08
\nhigh and the relation between the upper kHz QPO frequency \nt and HBO to
be \nh \simeq 56 ({\rm Hz}) (\nt/{\rm kHz})^{2}, which are in accordance with
the observed empirical relations. Furthermore, some implications of model are
discussed, including why QPO frequencies of white dwarfs and neutron stars span
five orders of magnitude in frequency. \\Comment: 11 pages, 1 figure, accepted by PAS
Precise Measurements of the Kilohertz Quasi-Periodic Oscillations in 4U 1728-34
We have analyzed seventeen observations of the low-mass X-ray binary and
atoll source 4U 1728-34, carried out by the Rossi X-ray Timing Explorer in 1996
and 1997. We obtain precise measurements of the frequencies of the two
simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source.
We show that the frequency separation between the two QPO, , is
always significantly smaller than the frequency of the nearly-coherent
oscillations seen in this source during X-ray bursts, even at the lowest
inferred mass accretion rate, when seems to reach its maximum
value. We also find that decreases significantly, from Hz to Hz, as the frequency of the lower frequency kHz QPO
increases from 615 to 895 Hz. This is the first time that variations of the kHz
QPO peak separation are measured in a source which shows nearly-coherent
oscillations during bursts.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses
AAS LaTex v4.0 (5 pages plus 4 postscript figures
Gravitational Lensing of the X-Ray Background by Clusters of Galaxies
Gravitational lensing by clusters of galaxies affects the cosmic X-ray
background (XRB) by altering the observed density and flux distribution of
background X-ray sources. At faint detection flux thresholds, the resolved
X-ray sources appear brighter and diluted, while the unresolved component of
the XRB appears dimmer and more anisotropic, due to lensing. The diffuse X-ray
intensity in the outer halos of clusters might be lower than the sky-averaged
XRB, after the subtraction of resolved sources. Detection of the lensing signal
with a wide-field X-ray telescope could probe the mass distribution of a
cluster out to its virialization boundary. In particular, we show that the
lensing signature imprinted on the resolved component of the XRB by the cluster
A1689, should be difficult but possible to detect out to 8' at the 2-4 sigma
level, after 10^6 seconds of observation with the forthcoming AXAF satellite.
The lensing signal is fairly insensitive to the lens redshift in the range
0.1<z<0.6. The amplitude of the lensing signal is however sensitive to the
faint end slope of the number-flux relation for unresolved X-ray sources, and
can thus help constrain models of the XRB. A search for X-ray arcs or arclets
could identify the fraction of all faint sources which originate from extended
emission of distant galaxies. The probability for a 3 sigma detection of an
arclet which is stretched by a factor of about 3 after a 10^6 seconds
observation of A1689 with AXAF, is roughly comparable to the fraction of all
background X-ray sources that have an intrinsic size of order 1''.Comment: 41 LaTeX pages, 11 postscript figures, 1 table, in AASTeX v4.0
format. To appear in ApJ, April 1, 1997, Vol. 47
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
Milli-second Oscillations in the Persistent and Bursting Flux of Aql X-1 During an Outburst
The Rossi X-Ray Timing Explorer observed the soft X-Ray transient Aql X-1
during its outburst in February and March 1997. We report the discovery of
quasi-periodic oscillations (QPOs) in its persistent flux with frequencies in
the range of 740 to 830 Hz, Q-value of over 100, and a fractional RMS amplitude
of (6.8 +- 0.6)%, and nearly coherent oscillations (NCOs) during a Type-I burst
with a frequency of 549 Hz. The frequency of the QPOs in the persistent flux is
correlated with the mass accretion rate on time scale of hours, but not on time
scale of days. This is most likely the manifestation in a single source of the
kHz QPO puzzle observed among many sources, i.e., on the one hand, individual
sources show a correlation between the QPO frequency and the inferred mass
accretion rate, on the other hand, the dozen or so sources with luminosities
spanning two decades have essentially the same QPO frequencies. We propose that
this multi-valued QPO frequency and mass accretion rate correlation indicates
the existence of many similar regimes of the accretion disk. These regimes,
with a very similar energy spectrum and QPO frequency, are distinguished from
each other by the mass accretion rate or the total X-ray flux. The NCOs during
the burst can be made almost perfectly coherent by taking into account a large
frequency derivative. This strongly suggests that this frequency is related to
the neutron star spin frequency. The large frequency derivative is attributable
to the expansion or contraction of the neutron star photosphere during the
burst.Comment: 6 pages, LaTex (aas2pp4), Accepted for publication in ApJ Let
The mass of the neutron star in Cyg X-2 (V1341 Cyg)
Cygnus X-2 is one of the brightest and longest known X-ray sources. We
present high resolution optical spectroscopy of Cyg X-2 obtained over 4 years
which gives an improved mass function of 0.69 +/- 0.03 Msun (1 sigma error). In
addition, we resolve the rotationally broadened absorption features of the
secondary star for the first time, deriving a rotation speed of vsin(i) = 34.2
+/- 2.5 km per s (1 sigma error) which leads to a mass ratio of q = M_c/M_x =
0.34 +/- 0.04 (1 sigma error), assuming a tidally-locked and Roche lobe-filling
secondary). Hence with the lack of X-ray eclipses (i.e. i <~ 73 degrees) we can
set firm 95% confidence lower limits to the neutron star mass of M_x > 1.27
Msun and to the companion star mass of M_c > 0.39 Msun. However, by
additionally requiring that the companion must exceed 0.75 Msun (as required
theoretically to produce a steady low-mass X-ray binary), then M_x > 1.88 Msun
and i < 61 degrees (95% confidence lower and upper limit, respectively),
thereby making Cyg X-2 the highest mass neutron star measured to date. If
confirmed this would set significant constraints on the equation of state of
nuclear matter.Comment: 16 pages, 4 figures, ApJ Letters, accepted, LaTeX, aasms4.st
XMM-Newton observation of the Lockman Hole; I. The X-ray Data
We report on the first deep X-ray survey with the XMM-Newton observatory
during the performance verification phase. The field of the Lockman Hole, one
of the best studied sky areas over a very wide range of wavelengths, has been
observed. A total of ~100 ksec good exposure time has been accumulated.
Combining the images of the European Photon Imaging Camera (EPIC) detectors we
reach a flux limit of 0.31, 1.4 and 2.4 X 10^{-15} erg cm^{-2} s^{-1},
respectively in the 0.5-2, 2-10, and 5-10 keV band. Within an off-axis angle of
10 arcmin we detect 148, 112 and 61 sources, respectively. The log(N)-log(S)
relation in the three bands is compared with previous results. In particular in
the 5-10 keV band these observations present the deepest X-ray survey ever,
about a factor 20 more sensitive than the previous BeppoSAX observations. Using
X-ray spectral diagnostics and the set of previously known, spectroscopically
identified ROSAT sources in the field, the new sources can be classified.
XMM-Newton detects a significant number (~40%) of X-ray sources with hard,
probably intrinsically absorbed X-ray spectra, confirming a prediction of the
population synthesis models for the X-ray background.Comment: 6 pages, based on the new A&A style file (included), to be published
in A&A 365 (2001, special XMM issue), one page contains color image
The (un)resolved X-ray background in the Lockman Hole
Most of the soft and a growing fraction of the harder X-ray background has
been resolved into emission from point sources, yet the resolved fraction above
7 keV has only been poorly constrained. We use ~700 ks of XMM-Newton
observations of the Lockman Hole and a photometric approach to estimate the
total flux attributable to resolved sources in a number of different energy
bands. We find the resolved fraction of the X-ray background to be ~90 per cent
below 2 keV but it decreases rapidly at higher energies with the resolved
fraction above ~7 keV being only ~50 per cent. The integrated X-ray spectrum
from detected sources has a slope of Gamma~1.75, much softer than the Gamma=1.4
of the total background spectrum. The unresolved background component has the
spectral signature of highly obscured AGN.Comment: 6 pages, 6 figures, MNRAS Letters, in press, changed to reflect
accepted versio
Discovery of a binary AGN in the ultraluminous infrared galaxy NGC 6240 using Chandra
Ultraluminous infrared galaxies (ULIRGs) are outstanding due to their huge
luminosity output in the infrared, which is predominantly powered by super
starbursts and/or hidden active galactic nuclei (AGN). NGC 6240 is one of the
nearest ULIRGs and is considered a key representative of its class.
Here, we report the first high-resolution imaging spectroscopy of NGC 6240 in
X-rays. The observation, performed with the ACIS-S detector aboard the Chandra
X-ray observatory, led to the discovery of two hard nuclei, coincident with the
optical-IR nuclei of NGC 6240. The AGN character of both nuclei is revealed by
the detection of absorbed hard, luminous X-ray emission and two strong neutral
Fe_K_alpha lines. In addition, extended X-ray emission components are present,
changing their rich structure in dependence of energy. The close correlation of
the extended emission with the optical Halpha emission of NGC 6240, in
combination with the softness of its spectrum, clearly indicates its relation
to starburst-driven superwind activity.Comment: ApJ Letters in press, 7 colour figures included; preprint and related
papers on NGC 6240 also available at http://www.xray.mpe.mpg.de/~skomossa
The Optical Counterpart of the Isolated Neutron Star RX J1605.3+3249
We have detected the optical counterpart to the nearby isolated neutron star
RX J1605.3+3249 using observations from the Space Telescope Imaging
Spectrometer aboard the Hubble Space Telescope. The counterpart, with
m_{50CCD}=26.84+/-0.07 mag and very blue colors, lies close to the ROSAT HRI
error circle and within the Chandra error circle. The spectrum is consistent
with a Rayleigh-Jeans tail whose emission is a factor of ~14 above the
extrapolation of the X-ray blackbody, and the source has an unabsorbed
X-ray-to-optical flux ratio of log(fX/fopt)=4.4, similar to that of other
isolated neutron stars. This confirms the classification of RX J1605.3+3249 as
a neutron star.Comment: 5 pages, 4 figures. Uses emulateapj5.sty, onecolfloat5.sty. Accepted
by ApJL. Small changes from ver. 1 following referee's repor
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