1,453 research outputs found
Aperiodic variability of low-mass X-ray binaries at very low frequencies
We have obtained discrete Fourier power spectra of a sample of persistent
low-mass neutron-star X-ray binaries using long-term light curves from the All
Sky Monitor on board the Rossi X-ray Timing Explorer. Our aim is to investigate
their aperiodic variability at frequencies in the range 1 x 10^{-7}-5 x 10^{-6}
Hz and compare their properties with those of the black-hole source Cyg X-1. We
find that the classification scheme that divides LMXBs into Z and atoll sources
blurs at very low frequencies. Based on the long-term (~ years) pattern of
variability and the results of power-law fits (P ~ v^{-a}) to the 1 x 10^{-7}-5
x 10^{-6} Hz power density spectra, low-mass neutron-star binaries fall into
three categories. Type I includes all Z sources, except Cyg X-2, and the atoll
sources GX9+1 and GX13+1. They show relatively flat power spectra (a < 0.9) and
low variability (rms < 20%). Type II systems comprise 4U 1636-53, 4U 1735-44
and GX3+1. They are more variable (20% < rms < 30%) and display steeper power
spectra (0.9 < a < 1.2) than Type I sources. Type III systems are the most
variable (rms > 30%) and exhibit the steepest power spectra (a > 1.2). The
sources 4U 1705-44, GX354-0 and 4U 1820-30 belong to this group. GX9+9 and Cyg
X-2 appear as intermediate systems in between Type I and II and Type II and III
sources, respectively. We speculate that the differences in these systems may
be caused by the presence of different types of mass-donor companions. Other
factors, like the size of the accretion disc and/or the presence of weak
magnetic fields, are also expected to affect their low-frequency X-ray
aperiodic varibility.Comment: 9 pages, 6 figures. To be published in A&
Discovery of multiple Lorentzian components in the X-ray timing properties of the Narrow Line Seyfert 1 Ark 564
We present a power spectral analysis of a 100 ksec XMM-Newton observation of
the narrow line Seyfert 1 galaxy Ark~564. When combined with earlier RXTE and
ASCA observations, these data produce a power spectrum covering seven decades
of frequency which is well described by a power law with two very clear breaks.
This shape is unlike the power spectra of almost all other AGN observed so far,
which have only one detected break, and resemble Galactic binary systems in a
soft state. The power spectrum can also be well described by the sum of two
Lorentzian-shaped components, the one at higher frequencies having a hard
spectrum, similar to those seen in Galactic binary systems. Previously we have
demonstrated that the lag of the hard band variations relative to the soft band
in Ark 564 is dependent on variability time-scale, as seen in Galactic binary
sources. Here we show that the time-scale dependence of the lags can be
described well using the same two-Lorentzian model which describes the power
spectrum, assuming that each Lorentzian component has a distinct time lag. Thus
all X-ray timing evidence points strongly to two discrete, localised, regions
as the origin of most of the variability. Similar behaviour is seen in Galactic
X-ray binary systems in most states other than the soft state, i.e. in the
low-hard and intermediate/very high states. Given the very high accretion rate
of Ark 564 the closest analogy is with the very high (intermediate) state
rather than the low-hard state. We therefore strengthen the comparison between
AGN and Galactic binary sources beyond previous studies by extending it to the
previously poorly studied very high accretion rate regime.Comment: 11 pages, 11 figures, accepted for publication in MNRA
In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems
Using a novel technique, we make quantitative measurements of the spin
polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional
holes as a function of an in-plane magnetic field. As the field is increased
the system gradually becomes spin polarized, with the degree of spin
polarization depending on the orientation of the field relative to the crystal
axes. Moreover, the behavior of the system turns from metallic to insulating
\textit{before} it is fully spin polarized. The minority-spin population at the
transition is ~8*10^{9} cm^{-2}, close to the density below which the system
makes a transition to an insulating state in the absence of a magnetic field.Comment: 4 pages with figure
A jet model for black-hole X-ray sources
A jet model for Galactic black-hole X-ray binaries will be presented that appears to explain several observational characteristics. In particular, it explains the energy spectrum from radio to hard X-rays, the time-lags as a function of Fourier frequency, the increase of the variability amplitude (QPO and high frequency) with increasing photon energy, and the narrowing of the autocorrelation function with increasing photon energy. On the other hand, there are additional observational constraints that no model has tried to explain yet. It is important that we all try to address these constraints if we are to make any progress in understanding black-hole X-ray source
Low-field magnetoresistance in GaAs 2D holes
We report low-field magnetotransport data in two-dimensional hole systems in
GaAs/AlGaAs heterostructures and quantum wells, in a large density range, cm, with primary focus on
samples grown on (311)A GaAs substrates. At high densities, cm, we observe a remarkably strong positive magnetoresistance.
It appears in samples with an anisotropic in-plane mobility and predominantly
along the low-mobility direction, and is strongly dependent on the
perpendicular electric field and the resulting spin-orbit interaction induced
spin-subband population difference. A careful examination of the data reveals
that the magnetoresistance must result from a combination of factors including
the presence of two spin-subbands, a corrugated quantum well interface which
leads to the mobility anisotropy, and possibly weak anti-localization. None of
these factors can alone account for the observed positive magnetoresistance. We
also present the evolution of the data with density: the magnitude of the
positive magnetoresistance decreases with decreasing density until, at the
lowest density studied ( cm), it vanishes and is
replaced by a weak negative magnetoresistance.Comment: 8 pages, 8 figure
B and I-band optical micro-variability observations of the BL Lac objects S5 2007+777 and 3C371
We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8
nights, respectively, during various observing runs in 2001, 2002 and 2004. The
observations resulted in almost evenly sampled light curves, 6-9 hours long. We
do not detect any flares within the observed light curves, but we do observe
small amplitude, significant variations, in both bands, on time scales of hours
and days. The average variability amplitude on time scales of minutes/hours is
2.5% and 1-1.5% in the case of S5 2007+777 and 3C371, respectively. The average
amplitudes increase to 5-12% and 4-6%, respectively, on time scales of days. We
find that the B and I band variations are highly correlated, on both short and
long time scales. During the 2004 observations, which resulted in the longest
light curves, we observe two well defined flux-decay and rising trends in the
light curves of both objects. When the flux decays, we observe significant
delays, with the B band flux decaying faster than the flux in the I band. As a
result, we also observe significant, flux related spectral variations as well.
The flux-spectral relation is rather complicated, with loop-like structures
forming during the flux evolution. The presence of spectral variations imply
that the observed variability is not caused by geometric effects. On the other
hand, our results are fully consistent with the hypothesis that the observed
variations are caused by perturbations which affect different regions in the
jet of the sources.Comment: Accepted for publication in Astronomy and Astrophysic
Fourier resolved spectroscopy of 4U 1543-47 during the 2002 outburst
We have obtained Fourier-resolved spectra of the black-hole binary 4U 1543-47
in the canonical states (high/soft, very high, intermediate and low/hard)
observed in this source during the decay of an outburst that took place in
2002. Our objective is to investigate the variability of the spectral
components generally used to describe the energy spectra of black-hole systems,
namely a disk component, a power-law component attributed to Comptonization by
a hot corona and the contribution of the iron line due to reprocessing of the
high energy (E > 7 keV) radiation. We find that i) the disk component is not
variable on time scales shorter than ~100 seconds, ii) the reprocessing
emission as manifest by the variability of the Fe Kalpha line responds to the
primary radiation variations down to time scales of ~70 ms in the high and
very-high states, but longer than 2 s in the low state, iii) the low-frequency
QPOs are associated with variations of the X-ray power law spectral component
and not to the disk component and iv) the spectra corresponding to the highest
Fourier frequency are the hardest
(show the flatter spectra) at a given spectral state. These results question
models that explain the observed power spectra as due to modulations of the
accretion rate alone, as such models do not provide any apparent reason for a
Fourier frequency dependence of the power law spectral indices.Comment: 21 pages, 2 tables, 5 figures. Accepted for publication in Ap
A jet model for Galactic black-hole X-ray sources: Some constraining correlations
Some recent observational results impose significant constraints on all the
models that have been proposed to explain the Galactic black-hole X-ray sources
in the hard state. In particular, it has been found that during the hard state
of Cyg X-1 the power-law photon number spectral index is correlated with the
average time lag between hard and soft X-rays. Furthermore, the peak
frequencies of the four Lorentzians that fit the observed power spectra are
correlated with both the photon index and the time lag. We performed Monte
Carlo simulations of Compton upscattering of soft, accretion-disk photons in
the jet and computed the time lag between hard and soft photons and the
power-law index of the resulting photon number spectra. We demonstrate that our
jet model naturally explains the above correlations, with no additional
requirements and no additional parameters
- …