439 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
Discovery of high-frequency quasi-periodic oscillations in the black-hole candidate IGR J17091-3624
We report the discovery of 8.5 sigma high-frequency quasi-periodic
oscillations (HFQPOs) at 66 Hz in the RXTE data of the black hole candidate IGR
J17091-3624, a system whose X-ray properties are very similar to those of
microquasar GRS 1915+105. The centroid frequency of the strongest peak is ~66
Hz, its quality factor above 5 and its rms is between 4 and 10%. We found a
possible additional peak at 164 Hz when selecting a subset of data; however, at
4.5 sigma level we consider this detection marginal. These QPOs have hard
spectrum and are stronger in observations performed between September and
October 2011, during which IGR J17091-3624 displayed for the first time light
curves which resemble those of the gamma variability class in GRS 1915+105. We
find that the 66 Hz QPO is also present in previous observations (4.5 sigma),
but only when averaging ~235 ksec of relatively high count rate data. The fact
that the HFQPOs frequency in IGR J17091-3624 matches surprisingly well that
seen in GRS 1915+105 raises questions on the mass scaling of QPOs frequency in
these two systems. We discuss some possible interpretations, however, they all
strongly depend on the distance and mass of IGR J17091-3624, both completely
unconstrained today.Comment: 6 pages, 4 figures, Accepted for publication in ApJ
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
The reflection spectrum of the low-mass X-ray binary 4U 1636-53
We present 3-79 keV NuSTAR observations of the neutron star low-mass X-ray
binary 4U 1636-53 in the soft, transitional and hard state. The spectra display
a broad emission line at 5-10 keV. We applied several models to fit this line:
A GAUSSIAN line, a relativistically broadened emission line model, KYRLINE, and
two models including relativistically smeared and ionized reflection off the
accretion disc with different coronal heights, RELXILL and RELXILLLP. All
models fit the spectra well, however, the KYRLINE and RELXILL models yield an
inclination of the accretion disc of with respect to the line
of sight, which is at odds with the fact that this source shows no dips or
eclipses. The RELXILLLP model, on the other hand, gives a reasonable
inclination of . We discuss our results for these models in this
source and the possible primary source of the hard X-rays.Comment: 9 pages, 8 figure
Intermittent accreting millisecond pulsars: light houses with broken lamps?
Intermittent accreting millisecond X-ray pulsars are an exciting new type of
sources. Their pulsations appear and disappear either on timescales of hundreds
of seconds or on timescales of days. The study of these sources add new
observational constraints to present models that explain the presence or not of
pulsations in neutron star LMXBs. In this paper we present preliminary results
on spectral and aperiodic variability studies of all intermittent AMSPs, with a
particular focus on the comparison between pulsating and non pulsating periods.Comment: 4 pages, 2 figures; to appear in the proceedings of the workshop "A
Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds.
R. Wijnands et al. (AIP Conf. Proc.
Broad iron emission line and kilohertz quasi-periodic oscillations in the neutron star system 4U 1636-53
Both the broad iron (Fe) line and the frequency of the kilohertz
quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries
(LMXBs) can potentially provide independent measures of the inner radius of the
accretion disc. We use XMM-Newton and simultaneous Rossi X-ray Timing Explorer
observations of the LMXB 4U 1636-53 to test this hypothesis. We study the
properties of the Fe-K emission line as a function of the spectral state of the
source and the frequency of the kHz QPOs. We find that the inner radius of the
accretion disc deduced from the frequency of the upper kHz QPO varies as a
function of the position of the source in the colour-colour diagram, in
accordance with previous work and with the standard scenario of accretion disc
geometry. On the contrary, the inner disc radius deduced from the profile of
the Fe line is not correlated with the spectral state of the source. The values
of the inner radius inferred from kHz QPOs and Fe lines, in four observations,
do not lead to a consistent value of the neutron star mass, regardless of the
model used to fit the Fe line. Our results suggest that either the kHz QPO or
the standard relativistic Fe line interpretation does not apply for this
system. Furthermore, the simultaneous detection of kHz QPOs and broad Fe lines
is difficult to reconcile with models in which the broadening of the Fe line is
due to the reprocessing of photons in an outflowing wind.Comment: 12 pages, 6 figures, Accepted for publication in MNRA
The Facilitation of the Brady Plan: Emerging Markets Debt Trading From 1989 to 1993
This Article completes the analysis of the first decade of the modern emerging market\u27s operation by considering two further periods: (iv) Adolescence: March 1989 to October 1991; and (v) Young Adulthood: October 1991 to December 1993. The primary importance of the market in these periods, as will be discovered, lies in its promotion and facilitation of the Brady Plan
Phase lags of quasi-periodic oscillations across source states in the low-mass X-ray binary 4U 1636-53
While there are many dynamical mechanisms and models that try to explain the
origin and phenomenology of the quasi-periodic oscillations (QPOs) seen in the
X-ray light curves of low-mass X-ray binaries, few of them address how the
radiative processes occurring in these extreme environments give rise to the
rich set of variability features actually observed in these light curves. A
step towards this end comes from the study of the energy and frequency
dependence of the phase lags of these QPOs. Here we used a methodology that
allowed us to study, for the first time, the dependence of the phase lags of
all QPOs in the range of 1 Hz to 1300 Hz detected in the low-mass X-ray binary
4U 1636-53 upon energy and frequency as the source changes its states as it
moves through the colour-colour diagram. Our results suggest that within the
context of models of up-scattering Comptonization, the phase lags dependencies
upon frequency and energy can be used to extract size scales and physical
conditions of the medium that produces the lags
The Evolution of the Phase Lags Associated with the Type-C Quasi-periodic Oscillation in GX 339--4 during the 2006/2007 Outburst
We present the evolution of the phase lags associated with the type-C QPO in
GX 339--4 during the rising phase of the 2006/2007 outburst. We find that the
phase lags at the QPO frequency are always positive (hard), and show very
different behavior between QPOs with frequencies below and above Hz:
when the QPO frequency is below Hz, the phase lags increase both with
QPO frequency and energy, while when the QPO frequency is above Hz,
the phase lags remain more or less constant. When the QPO frequency is higher
than Hz, a broad feature is always present in the lag-energy spectra
at around 6.5 keV, suggesting that the reflection component may have a
significant contribution to the phase lags. Below Hz, the QPO rms
first decreases with energy and then turns to almost flat, while above
Hz, the QPO rms increases with energy. During the transition from the
low-hard state to the hard-intermediate state, the second harmonic and
subharmonic of this QPO appear in the power density spectra. The
second-harmonic and subharmonic phase lags show very similar evolution with
their centroid frequencies. However, the energy dependence of the
second-harmonic and subharmonic phase lags are quite different. Our results
suggest that, at different phases of the outburst, different mechanisms may be
responsible for the phase lags of the QPO. We briefly discuss the possible
scenarios for producing the lags.Comment: 15 pages, 12 figures, accepted for publication in Ap
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