561 research outputs found
The evolution of the X-ray phase lags during the outbursts of the black hole candidate GX 339-4
Owing to the frequency and reproducibility of its outbursts, the black-hole
candidate GX 339-4 has become the standard against which the outbursts of other
black-hole candidate are matched up. Here we present the first systematic study
of the evolution of the X-ray lags of the broad-band variability component
(0.008-5 Hz) in GX 339-4 as a function of the position of the source in the
hardness-intensity diagram. The hard photons always lag the soft ones,
consistent with previous results. In the low-hard state the lags correlate with
X-ray intensity, and as the source starts the transition to the
intermediate/soft states, the lags first increase faster, and then appear to
reach a maximum, although the exact evolution depends on the outburst and the
energy band used to calculate the lags. The time of the maximum of the lags
appears to coincide with a sudden drop of the Optical/NIR flux, the fractional
RMS amplitude of the broadband component in the power spectrum, and the
appearance of a thermal component in the X-ray spectra, strongly suggesting
that the lags can be very useful to understand the physical changes that GX
339-4 undergoes during an outburst. We find strong evidence for a connection
between the evolution of the cut-off energy of the hard component in the energy
spectrum and the phase lags, suggesting that the average magnitude of the lags
is correlated with the properties of the corona/jet rather than those of the
disc. Finally, we show that the lags in GX 339-4 evolve in a similar manner to
those of the black-hole candidate Cygnus X-1, suggesting similar phenomena
could be observable in other black-hole systems.Comment: 13 pages, 8 figures, Accepted for publication in MNRA
The XMM Newton spectra of the 2012 outburst of the black-hole candidate 4U 1630-47 revisited
Recent XMM-Newton observations of the black-hole candidate 4U 1630-47 during
the 2012 outburst revealed three relativistically Doppler-shifted emission
lines that were interpreted as arising from baryonic matter in the jet of this
source. Here we reanalyse those data and find an alternative model that, with
less free parameters than the model with Doppler-shifted emission lines, fits
the data well. In our model we allow the abundance of S and Fe in the
interstellar material along the line of sight to the source to be non solar.
Among other things, this significantly impacts the emission predicted by the
model at around 7.1 keV, where the edge of neutral Fe appears, and renders the
lines unnecessary. The fits to all the 2012 XMM-Newton observations of this
source require a moderately broad emission line at around 7 keV plus several
absorption lines and edges due to highly ionised Fe and Ni, which reveal the
presence of a highly-ionised absorber close to the source. Finally, our model
also fits well the observations in which the lines were detected when we apply
the most recent calibration files, whereas the model with the three
Doppler-shifted emission lines does not.Comment: 8 pages, 6 figure
Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636-53
We detected millihertz quasi-periodic oscillations (QPOs) in an XMM-Newton
observation of the neutron-star low-mass X-ray binary 4U 1636-53. These QPOs
have been interpreted as marginally-stable burning on the neutron-star surface.
At the beginning of the observation the QPO was at around 8 mHz, together with
a possible second harmonic. About 12 ks into the observation a type I X-ray
burst occurred and the QPO disappeared; the QPO reappeared ~25 ks after the
burst and it was present until the end of the observation. We divided the
observation into four segments to study the evolution of the spectral
properties of the source during intervals with and without mHz QPO. We find
that the temperature of the neutron-star surface increases from the QPO segment
to the non-QPO segment, and vice versa. We also find a strong correlation
between the frequency of the mHz QPO and the temperature of a black-body
component in the energy spectrum representing the temperature of neutron-star
surface. Our results are consistent with previous results that the frequency of
the mHz QPO depends on the variation of the heat flux from the neutron star
crust, and therefore supports the suggestion that the observed QPO frequency
drifts could be caused by the cooling of deeper layers.Comment: Accepted for publication in the MNRA
Iron Emission Lines from Extended X-ray Jets in SS 433: Reheating of Atomic Nuclei
Powerful relativistic jets are among the most ubiquitous and energetic
observational consequences of accretion around supermassive black holes in
active galactic nuclei and neutron stars and stellar-mass black holes in x-ray
binary (XRB) systems. But despite more than three decades of study, the
structure and composition of these jets remain unknown. Here we present
spatially resolved x-ray spectroscopy of arc second-scale x-ray jets from the
XRB SS 433 analized with the Chandra advanced charge-coupled device imaging
spectrometer. These observations reveal evidence for a hot continuum and
Doppler-shifted iron emission lines from spatially resolved regions.
Apparently, in situ reheating of the baryonic component of the jets takes place
in a flow that moves with relativistic bulk velocity even more than 100 days
after launch from the binary core
The distribution of kHz QPO frequencies in bright LMXBs
We analyzed all published frequencies, nu_1 and nu_2, of the twin kilohertz
quasi-periodic oscillations (kHz QPOs) in bright neutron star low-mass X-ray
binaries. The two frequencies are well correlated but, contrary to recent
suggestions, the frequency-frequency correlation is significantly different
from a nu_2 = (3/2) nu_1 relation. To check whether, although not following the
the 3/2 relation, the QPO frequencies cluster around a region where nu_2/nu_1 ~
3/2, we re-analyzed the Sco X-1 data that were used to report that ratio and
show that, because the distribution of ratios of linearly correlated
measurements is intrinsically biased, although the significance of the
clustering around nu_2/nu_1 ~ 3/2 previously reported in the case of Sco X-1 is
formally correct, it does not provide any useful information about a possible
underlying resonance mechanism in this source. Using the same data, we then
show that the (unbiased) distribution of QPO frequencies is consistent with a
uniform distribution at a 2.4 sigma level. To investigate this further, we
analyzed a larger data set of Sco X-1 and four other sources, 4U 1608-52, 4U
1636-53, 4U 1728-34 and 4U 1820-30. We find that for all five sources the
distribution of the kHz QPO frequencies is not uniform and has multiple peaks,
which have no analogy in the distribution of points in the spectral color-color
diagrams of these sources. Finally, we demonstrate that a simple random walk of
the QPO frequencies can reproduce qualitatively the observed distributions in
frequency and frequency ratio. This result weakens the support for resonance
models of kHz QPOs in neutron stars.Comment: 8 pages, 7 figures. Accepted for publication on Astronomy and
Astrophyics (2005 January 8); references updated, minor changes to text.
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