1,054 research outputs found
Eclipse Timings of the Transient Low Mass X-ray Binary EXO0748-676. IV. The Rossi X-Ray Timing Explorer Eclipses
We report our complete database of X-ray eclipse timings of the low mass
X-ray binary EXO0748-676 observed by the Rossi X-Ray Timing Explorer (RXTE)
satellite. As of this writing we have accumulated 443 full X-ray eclipses, 392
of which have been observed with the Proportional Counter Array on RXTE. These
include both observations where an eclipse was specifically targeted and those
eclipses found in the RXTE data archive. Eclipse cycle count has been
maintained since the discovery of the EXO0748-676 system in February 1985. We
describe our observing and analysis techniques for each eclipse and describe
improvements we have made since the last compilation by Wolff et al. (2002).
The principal result of this paper is the database containing the timing
results from a seven-parameter fit to the X-ray light curve for each observed
eclipse along with the associated errors in the fitted parameters. Based on the
standard O-C analysis, EXO0748-676 has undergone four distinct orbital period
epochs since its discovery. In addition, EXO0748-676 shows small-scale events
in the O-C curve that are likely due to short-lived changes in the secondary
star.Comment: Accepted for publication in The Astrophysical Journal Supplement
Series, 5 figures. Analysis revised. Tables 1 & 3 update
Dynamics of the Flow Pattern in a Baffled Mixing Vessel with an Axial Impeller
This paper deals with the primary circulation of an agitated liquid in a flat-bottomed cylindrical stirred tank. The study is based on experiments, and the results of the experiments are followed by a theoretical evaluation. The vessel was equipped with four radial baffles and was stirred with a six pitched blade impeller pumping downwards. The experiments were concentrated on the lower part of the vessel, where the space pulsations of the primary loop, originated due to the pumping action of the impeller. This area is considered to be the birthplace of the flow macroinstabilities in the system – a phenomenon which has been studied and described by several authors. The flow was observed in a vertical plane passing through the axis of the vessel. The flow patterns of the agitated liquid were visualized by means of Al micro particles illuminated by a vertical light knife and scanned by a digital camera. The experimental conditions corresponded to the turbulent regime of agitated liquid flow.It was found that the primary circulation loop is elliptical in shape. The main diameter of the primary loop is not constant. It increases in time and after reaching a certain value the loop disintegrates and collapses. This process is characterized by a certain periodicity and its period proved to be correlated to the occurrence of flow macroinstability. The instability of the loop can be explained by a dissipated energy balance. When the primary loop reaches the level of disintegration, the whole impeller power output is dissipated and under this condition any flow alteration requiring additional energy, even a very small vortex separation, causes the loop to collapse.
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
Discovery of a Peculiar Dip from GX 301-2
We present temporal and spectral properties of a unique X-ray dip in GX 301-2
as seen with Rossi X-ray Timing Explorer in May 2010. The X-ray pulsation from
the source gradually declined prior to the dip, disappears for one spin cycle
during the dip and is abruptly restored in the spin cycle immediately after the
dip. Moreover, the phase-integrated spectrum of the source becomes softer
before and during the dip and it quickly hardens again following the dip. Our
findings indicate the fact that the mechanism for pulsations gradually turned
off briefly and underlying dim and softer emission likely from the accretion
column became observable in the brief absence of high level emission due to
wind accretion.Comment: Accepted for publication in A&A Letter
The cooling rate of neutron stars after thermonuclear shell flashes
Thermonuclear shell flashes on neutron stars are detected as bright X-ray
bursts. Traditionally, their decay is modeled with an exponential function.
However, this is not what theory predicts. The expected functional form for
luminosities below the Eddington limit, at times when there is no significant
nuclear burning, is a power law. We tested the exponential and power-law
functional forms against the best data available: bursts measured with the
high-throughput Proportional Counter Array (PCA) on board the Rossi X-ray
Timing Explorer. We selected a sample of 35 'clean' and ordinary (i.e., shorter
than a few minutes) bursts from 14 different neutron stars that 1) show a large
dynamic range in luminosity, 2) are the least affected by disturbances by the
accretion disk and 3) lack prolonged nuclear burning through the rp-process. We
find indeed that for every burst a power law is a better description than an
exponential function. We also find that the decay index is steep, 1.8 on
average, and different for every burst. This may be explained by contributions
from degenerate electrons and photons to the specific heat capacity of the
ignited layer and by deviations from the Stefan-Boltzmann law due to changes in
the opacity with density and temperature. Detailed verification of this
explanation yields inconclusive results. While the values for the decay index
are consistent, changes of it with the burst time scale, as a proxy of ignition
depth, and with time are not supported by model calculations.Comment: 10 pages, 7 figures, recommended for publication in A&
Energy-Dependent Harmonic Ratios of the Cyclotron Features of X0331+53 in the 2004-2005 Outburst
We report on changes of the cyclotron resonance energies of the recurrent
transient pulsar, X0331+53 (V0332+53). The whole RXTE data acquired in the
2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between
1.7x10^36 and 3.5x10^38 ers/s, assuming a distance of 7 kpc. We confirmed that
the fundamental cyclotron resonance energy changed from ~22 to ~27 keV in a
clear anti-correlation to the source luminosity, and without any hysteresis
effects between the rising and declining phases of the outburst. In contrast,
the second harmonic energy changed from ~49 to ~54 keV, implying a weaker
fractional change as a function of the luminosity. As a result, the observed
resonance energy ratio between the second harmonic and the fundamental was ~2.2
when the source was most luminous, whereas the ratio decreased to the nominal
value of 2.0 at the least luminous state. Although the significance of this
effect is model dependent, these results suggest that the fundamental and
second harmonic resonances represent different heights in the accretion column,
depending on the mass accretion rate.Comment: 39 pages, 15 figures, 4 tables. Accepted for publication in
Astrophysical Journa
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