1,689 research outputs found
Non-detection of kHz QPOs in GX 9+1 and GX 9+9
In numerous low-mass X-ray binaries kHz quasi-periodic oscillations (kHz
QPOs) are detected. We observed the atoll sources GX 9+1 and GX 9+9 with the
RXTE satellite. Both sources were on the (upper) banana branch during our
observations. No kHz QPOs were detected, with upper limits of 1.3-1.8 %. It is
known from other atoll sources (e.g. 4U 1636-53 and 4U 1820-30) that when they
are in the upper banana branch the kHz QPOs are not detected. Thus, it remains
possible that when GX 9+1 and GX 9+9 are observed longer on the lower banana,
or even in the island state, kHz QPOs are detected in these sources.Comment: 2 pages, 1 figure. To appear in the proceedings of IAU Symposium 188
"The Hot Universe
Timing Properties of Magnetars
We study the pulse morphologies and pulse amplitudes of thermally emitting
neutron stars with ultrastrong magnetic fields. The beaming of the radiation
emerging from a magnetar was recently shown to be predominantly non-radial,
with a small pencil and a broad fan component. We show that the combination of
this radiation pattern with the effects of strong lensing in the gravitational
field of the neutron star yields pulse profiles that show a qualitatively
different behavior compared to that of the radially-peaked beaming patterns
explored previously. Specifically, we find that: (i) the pulse profiles of
magnetars with a single hot emission region on their surface exhibit 1-2 peaks,
whereas those with an antipodal emission geometry have 1-4 peaks, depending on
the neutron star compactness, the observer's viewing angle, and the size of the
hot regions; (ii) the energy dependence of the beaming pattern may give rise to
weakly or strongly energy-dependent pulse profiles and may introduce phase lags
between different energy bands; (iii) the non-radial beaming pattern can give
rise to high pulsed fractions even for very relativistic neutron stars; (iv)
the pulsed fraction may not vary monotonically with neutron star compactness;
(v) the pulsed fraction does not decrease monotonically with the size of the
emitting region; (vi) the pulsed fraction from a neutron star with a single hot
pole has, in general, a very weak energy dependence, in contrast to the case of
an antipodal geometry. Comparison of these results to the observed properties
of anomalous X-ray pulsars strongly suggests that they are neutron stars with a
single hot region of ultrastrong magnetic field.Comment: 22 pages, 13 color figures, ApJ in pres
Difference Frequency of Kilohertz QPOs Not Equal to Half the Burst Oscillation Frequency in 4U 1636-53
We have analyzed data obtained during two observations with the Rossi X-ray
Timing Explorer on January 5 and 8, 1997, of the low-mass X-ray binary (LMXB)
and atoll source 4U 1636-53. We measure the frequency separation of the two
simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source to
be 253.7 +/- 4.7 and 246.4 +/- 5.4 Hz, respectively. These values are
inconsistent with being equal to 0.5 times the frequency of the 581-Hz
oscillations that have been detected previously in 4U 1636-53 during type I
bursts. The weighted average discrepancy is 39.5 +/- 3.5 Hz. This result shows
that a simple beat-frequency interpretation of the kHz QPOs, in which the
frequency of the oscillations detected during type I bursts equals the
separation between the two kHz QPOs (or twice that value), is incorrect.Comment: Accepted for publication in The Astrophysical Journal Letters. 4
pages plus 1 postscript figur
Discovery of a Second Kilohertz QPO in the X-ray Binary 4U 1735-44
In recent observations with the Rossi X-Ray Timing Explorer we have detected
two simultaneous quasi-periodic oscillation (QPO) peaks in the low mass X-ray
binary and atoll source 4U 1735-44. The lower and higher frequency QPOs have
frequencies varying between 632 and 729 Hz, and 982 and 1026 Hz, respectively.
The fractional rms amplitudes are 3.7 to 8.1% and 5.0 to 5.8%. The frequency
separation between the two QPOs changes from 341+-7 Hz to 296+-12 Hz. The
inferred mass accretion rate during our observations is relatively low compared
to that during the previous observations, where only a single QPO was present.
There is weak evidence that the frequency of the QPOs correlates with the mass
accretion rate, as observed in other binaries. Five X-ray bursts were recorded
with no detectable oscillations with upper limits for the rms fraction of 4% to
13%.Comment: accepted by ApJ Letters, for more information see the QPO page at
http://www.astro.uva.nl/ecford/qpos.htm
On the masses of neutron stars
We analyze the currently available observations of X-ray binaries in a
consistent way, to re-determine the masses of the neutron stars in these
systems. In particular, our attention is focussed on a realistic and consistent
assessment of observational uncertainties and sources of systematic error.
Confidence limits for these new mass estimates are generally less optimistic
than previously assumed. The available observations, including data on six
radio pulsars, do not firmly constrain the equation of state of neutron star
matter. In particular, a firm upper mass limit cannot yet be established. An
improvement of the accuracy of optical data holds the key to further progress.Comment: 5 pages of compressed, uuencoded postscript (text+figure). Accepted
for publication in A&A. In case of problems, contact [email protected]
Optical Counterpart of the Ultraluminous X-ray Source IC 342 X-1
We present Chandra and HST observations of the ultraluminous X-ray source
(ULX) IC 342 X-1. The Chandra and HST images are aligned using two X-ray
emitting foreground stars. The astrometry corrected position for X-1 is R.A. =
03h45m55.61s, Decl. = +68d04m55.3s (J2000) with an error circle of 0.2". One
extended optical source is found in the error circle, which could be the
optical counterpart of X-1. The source shows an extended feature in HST images
at long wavelengths, which is likely to be a superposition of two point
sources, although it is possible that the dimmer one could be a jet. Both
sources are much redder than typical for ULX optical counterparts. The brighter
one has an absolute magnitude M_V = -5.2 +/- 0.2 and (B-V)_0 = 0.66 +/- 0.13
and the dimmer star is not detected in B and has (B-V)_0 > 2.1. Their colors
are consistent with an F8 to G0 Ib supergiant or a carbon star, respectively.
However, it is likely that part or most of the optical emission may be due to
X-rays reprocessed by the companion star or the accretion disk. The stellar
neighborhood of IC 342 X-1 lacks O stars and has a minimum age of ~10 Myr. This
excludes the possibility that the surrounding nebula is powered by an energetic
explosion of a single massive star that formed a black hole. We suggest that
the nebula is most likely powered by an outflow from the X-ray source.Comment: 10 pages, 8 figures, accepted for publication in Ap
Simultaneous Measurements of X-Ray Luminosity and Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries
We measure simultaneously the properties of the energy spectra and the
frequencies of the kilohertz quasi-periodic oscillations (QPOs) in fifteen low
mass X-ray binaries covering a wide range of X-ray luminosities. In each source
the QPO frequencies cover the same range of approximately 300 Hz to 1300 Hz,
though the sources differ by two orders of magnitude in their X-ray
luminosities (as measured from the unabsorbed 2-50 keV flux). So the X-ray
luminosity does not uniquely determine the QPO frequency. This is difficult to
understand since the evidence from individual sources indicates that the
frequency and luminosity are very well correlated at least over short
timescales. Perhaps beaming effects or bolometric corrections change the
observed luminosities, or perhaps part of the energy in mass accretion is used
to power outflows reducing the energy emitted in X-rays. It is also possible
that the parameters of a QPO model are tuned in such a way that the same range
of frequencies appears in all sources. Different modes of accretion may be
involved for example (disk and radial) or multiple parameters may conspire to
yield the same frequencies.Comment: 14 pages, 2 figures (1 in color), accepted by ApJ, see the 'QPO
page': http://www.astro.uva.nl/~ecford/qpos.htm
The eclipsing Cataclysmic Variable GS Pavonis: Evidence for disk radius changes
We have obtained differential time series photometry of the cataclysmic
variable GS Pavonis over a timespan of 2 years. These show that this system is
deeply eclipsing (~2-3.5 mag) with an orbital period of 3.72 hr. The eclipse
depth and out-of-eclipse light levels are correlated. From this correlation we
deduce that the disk radius is changing and that the eclipses in the low state
are total. The derived distance to GS Pav is 790+/-90 pc, with a height above
the galactic plane of 420+/-60 pc. We classify GS Pav as a novalike system.Comment: 4 pages, including 2 figures, accepted for publication in A&A Letter
A model-independent analysis of the variability of GRS 1915+105
We analyzed 163 observations of the microquasar GRS 1915+105 made with the
Rossi X-ray Timing Explorer (RXTE) in the period 1996-1997. For each
observation, we produced light curves and color-color diagrams. We classified
the observations in 12 separate classes, based on their count rate and color
characteristics. From the analysis of these classes, we reduced the variability
of the source to transitions between three basic states: a hard state
corresponding to the non-observability of the innermost parts of the accretion
disk, and two softer states with a fully observable disk. These two soft states
represent different temperatures of the accretion disk, related to different
local values of the accretion rate. The transitions between these states can be
extremely fast. The source moves between these three states following certain
patterns and avoiding others, giving rise to a relatively large but limited
number of variability classes. These results are the first step towards a
linking of the properties of this exceptional source with standard black-hole
systems and with accretion disk models.Comment: Accepted for publication in Astronomy & Astrophysics, 2000 January
6t
RXTE Observations of the Anomalous Pulsar 4U 0142+61
We observed the anomalous X-ray pulsar 4U 0142+61 using the Proportional
Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE) in March
1996. The pulse frequency was measured as f = 0.11510039(3) Hz with an upper
limit of df/dt < 4 * 10^(-13) Hz/s upon the short term change in frequency over
the 4.6 day span of the observations. A compilation of all historical
measurements showed an overall spin-down trend with slope df/dt = (-3.0 +/-
0.1) * 10^(-14) Hz/s. Searches for orbital modulations in pulse arrival times
yielded an upper limit of a_x sin i < 0.26 lt-s (99% confidence) for the period
range 70 s to 2.5 days. These limits combined with previous optical limits and
evolutionary arguments suggest that 4U 0142+61 is probably not a member of a
binary system.Comment: 20 pages (LaTeX) including 7 figures. Accepted for publication in the
Astrophysical Journa
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