318 research outputs found
Macrophage Phenotype Modulation by CXCL4 in Atherosclerosis
During atherogenesis, blood monocytes transmigrate into the subendothelial space and differentiate toward macrophages and foam cells. The major driver of monocyte–macrophage differentiation is macrophage colony-stimulating factor (M-CSF). M-CSF-induced macrophages are important promoters of atherogenesis as demonstrated in M-CSF and M-CSF receptor knock out mice. However, M-CSF is not the only relevant promoter of macrophage differentiation. The platelet chemokine CXCL4 also prevents monocyte apoptosis and promotes macrophage differentiation in vitro. It is secreted from activated platelets and has effects on various cell types relevant in atherogenesis. Knocking out the Pf4 gene coding for CXCL4 in Apoe−/− mice leads to reduced atherogenesis. Thus, it seems likely that CXC4-induced macrophages may have specific pro-atherogenic capacities. We have studied CXC4-induced differentiation of human macrophages using gene chips, systems biology, and functional in vitro and ex vivo experiments. Our data indicate that CXCL4-induced macrophages are distinct from both their M-CSF-induced counterparts and other known macrophage polarizations like M1 macrophages (induced by lipopolysaccharide and interferon-gamma) or M2 macrophages (induced by interleukin-4). CXCL4-induced macrophages have distinct phenotypic and functional characteristics, e.g., the complete loss of the hemoglobin–haptoglobin (Hb–Hp) scavenger receptor CD163 which is necessary for effective hemoglobin clearance after plaque hemorrhage. Lack of CD163 is accompanied by the inability to upregulate the atheroprotective enzyme heme oxygenase-1 in response to Hb–Hp complexes. This review covers the current knowledge about CXCL4-induced macrophages. Based on their unique properties, we have suggested to call these macrophages “M4.” CXCL4 may represent an important orchestrator of macrophage heterogeneity within atherosclerotic lesions. Further dissecting its effects on macrophage differentiation may help to identify novel therapeutic targets in atherogenesis
Short term X-ray rms variability of Cyg X-1
A linear dependence of the amplitude of broadband noise variability on flux
for GBHC and AGN has been recently shown by Uttley & McHardy (2001). We present
the long term evolution of this rms-flux-relation for Cyg X-1 as monitored from
1998-2002 with RXTE. We confirm the linear relationship in the hard state and
analyze the evolution of the correlation for the period of 1996-2002. In the
intermediate and the soft state, we find considerable deviations from the
otherwise linear relationship. A possible explanation for the rms-flux-relation
is a superposition of local mass accretion rate variations.Comment: 3 pages, 3 figures, Proceedings of the 4th Microquasar Workshop, eds.
Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space
Physics: Kolkat
Different kinds of long-term variability from Cygnus X-1
We present a study of the long-term variability of Cyg X-1 using data from
the RXTE/ASM and the RXTE/PCA during the time between the two soft states of
1996 and 2001/2002. This period has been characterized by many short ASM
flaring episodes which we have identified as "failed state transitions". The
150 d period which has been seen before and shortly after the 1996 soft state
is not obviously present in the ASM rate during most of this time. Applying
selection criteria from our pointed RXTE/PCA observations to exclude the
flaring episodes we show that the 150 d period can indeed still be
significantly detected in the hard state. Furthermore, while the ~420 d
timescale associated with the flaring is reduced in the selected hard state
count rate, it is still pronounced in the temporal evolution of the
corresponding hardness ratios. The Ryle radio flux is also consistent with the
150 d period being present but distorted during this time.Comment: 4 pages, 6 figures, to appear in Proceedings of "X-ray Timing 2003:
Rossi and Beyond", ed. P. Kaaret, F.K. Lamb, & J.H. Swan
INTEGRAL-RXTE observations of Cygnus X-1
We present first results from contemporaneous observations of Cygnus X-1 with
INTEGRAL and RXTE, made during INTEGRAL's performance verification phase in
2002 November and December. Consistent with earlier results, the 3-250 keV data
are well described by Comptonization spectra from a Compton corona with a
temperature of kT~50-90 keV and an optical depth of tau~1.0-1.3 plus reflection
from a cold or mildly ionized slab with a covering factor of Omega/2pi~0.2-0.3.
A soft excess below 10 keV, interpreted as emission from the accretion disk, is
seen to decrease during the 1.5 months spanned by our observations. Our results
indicate a remarkable consistency among the independently calibrated detectors,
with the remaining issues being mainly related to the flux calibration of
INTEGRAL.Comment: 6 pages, 3 figures. Figs. 2 and 3 are best viewed in color. Accepted
for publication in the INTEGRAL special edition of A&A
Spectro-timing analysis of Cygnus X-1 during a fast state transition
We present the analysis of two long, quasi-uninterrupted RXTE observations of
Cygnus X-1 that span several days within a 10 d interval. The spectral
characteristics during this observation cover the region where previous
observations have shown the source to be most dynamic. Despite that the source
behavior on time scales of hours and days is remarkably similar to that on year
time scales. This includes a variety of spectral/temporal correlations that
previously had only been observed over Cyg X-1's long-term evolution.
Furthermore, we observe a full transition from a hard to a soft spectral state
that occurs within less than 2.5 hours - shorter than previously reported for
any other similar Cyg X-1 transition. We describe the spectra with a
phenomenological model dominated by a broken power law, and we fit the X-ray
variability power spectra with a combination of a cutoff power law and
Lorentzian components. The spectral and timing properties are correlated: the
power spectrum Lorentzian components have an energy-dependent amplitude, and
their peak frequencies increase with photon spectral index. Averaged over
3.2-10 Hz, the time lag between the variability in the 4.5-5.7 keV and 9.5-15
keV bands increases with decreasing hardness when the variability is dominated
by the Lorentzian components during the hard state. The lag is small when there
is a large power law noise contribution, shortly after the transition to the
soft state. Interestingly, the soft state not only shows the shortest lags, but
also the longest lags when the spectrum is at its softest and faintest. We
discuss our results in terms of emission models for black hole binaries.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
Monazite-type SrCrO4 under compression
We report a high-pressure study of monoclinic monazite-type SrCrO4 up to 26 GPa. Therein we combined x-ray diffraction, Raman, and optical-absorption measurements with ab initio calculations, to find a pressure-induced structural phase transition of SrCrO4 near 8–9 GPa. Evidence of a second phase transition was observed at 10–13 GPa. The crystal structures of the high-pressure phases were assigned to the tetragonal scheelite-type and monoclinic AgMnO4-type structures. Both transitions produce drastic changes in the electronic band gap and phonon spectrum of SrCrO4. We determined the pressure evolution of the band gap for the low- and high-pressure phases as well as the frequencies and pressure dependencies of the Raman-active modes. In all three phases most Raman modes harden under compression, however the presence of low-frequency modes which gradually soften is also detected. In monazite-type SrCrO4, the band gap blueshifts under compression, but the transition to the scheelite phase causes an abrupt decrease of the band gap in SrCrO4. Calculations showed good agreement with experiments and were used to better understand the experimental results. From x-ray-diffraction studies and calculations we determined the pressure dependence of the unit-cell parameters of the different phases and their ambient-temperature equations of state. The results are compared with the high-pressure behavior of other monazites, in particular PbCrO4. A comparison of the high-pressure behavior of the electronic properties of SrCrO4 (SrWO4) and PbCrO4 (PbWO4) will also be made. Finally, the possible occurrence of a third structural phase transition is discussed
Long term variability of Cygnus X-1. IV, Spectral evolution 1999–2004
Continuing the observational campaign initiated by our group, we present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15 GHz. We present ∼200 pointed observations taken between early 1999 and late 2004 with the Rossi X-ray
Timing Explorer and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., by Titarchuk (1994, ApJ, 434, 570, compTT) and by Coppi (1999, in High Energy Processes in Accreting Black Holes, ed. J. Poutanen, & R. Svensson (San Francisco: ASP), ASP Conf. Ser., 161, 375, eqpair), can reproduce this simplicity; however, the success of the phenomenological broken power law models cautions against “overparameterizing” the more physical models. Broken power law models reveal a significant linear correlation between the photon index of the lower energy
power law and the hardening of the power law at ∼10 keV. This phenomenological soft/hard power law correlation is partly attributable to correlations of broad band continuum components, rather than being dominated by the weak hardness/reflection fraction correlation present in the Comptonization model. Specifically, the Comptonization models show that the bolometric flux of a soft excess (e.g., disk component) is strongly correlated with the compactness ratio of the Comptonizing medium, with L disk
∝( h / s ) −0.19 . Over the course of our campaign, Cyg X-1 transited several times into the soft state, and exhibited a large number of “failed state transitions”. The fraction of the time spent in such low radio emission/soft X-ray spectral states has increased from ∼10% in 1996–2000 to ∼34% since early 2000. We find that radio flares
typically occur during state transitions and failed state transitions (at h / s ∼ 3), and that there is a strong correlation between the 10–50 keV X-ray flux and the radio luminosity of the source. We demonstrate that rather than there being distinctly separated states, in contrast to the timing properties the spectrum of Cyg X-1 shows variations between extremes of properties, with clear cut examples of spectra at every intermediate point in the observed spectral correlations
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