10,085 research outputs found
X-ray Spectroscopy of Candidate Ultracompact X-ray Binaries
We present high-resolution spectroscopy of the neutron star/low-mass X-ray
binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of our continuing study of
known and candidate ultracompact binaries. The LMXB 4U 1850-087 is one of four
systems in which we had previously inferred an unusual Ne/O ratio in the
absorption along the line of sight, most likely from material local to the
binaries. However, our recent Chandra X-ray Observatory LETGS spectrum of 4U
1850-087 finds a Ne/O ratio by number of 0.22+/-0.05, smaller than previously
measured and consistent with the expected interstellar value. We propose that
variations in the Ne/O ratio due to source variability, as previously observed
in these sources, can explain the difference between the low- and
high-resolution spectral results for 4U 1850-087. Our XMM-Newton RGS
observation of 4U 0513-40 also shows no unusual abundance ratios in the
absorption along the line of sight. We also present spectral results from a
third candidate ultracompact binary, 4U 1822-000, whose spectrum is well fit by
an absorbed power-law + blackbody model with absorption consistent with the
expected interstellar value. Finally, we present the non-detection of a fourth
candidate ultracompact binary, 4U 1905+000, with an upper limit on the source
luminosity of < 1 x 10^{32} erg s^{-1}. Using archival data, we show that the
source has entered an extended quiescent state.Comment: 8 pages, 3 figures, accepted for publication to the Astrophysical
Journa
Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification
Based on Rossi X-ray Timing Explorer data, we discuss the classification of
the soft X-ray transient Aquila X-1 in the Z/atoll scheme, and the relation of
its kilohertz quasi-periodic oscillations (kHz QPO) properties to the X-ray
colors. The color-color diagram shows one elongated ("banana") structure and
several "islands" of data points. The power spectra of the island are best
represented by a broken power-law, whereas those of the banana by a power-law
below ~ 1 Hz plus an exponentially cut-off component at intermediate
frequencies (30-60 Hz). The parameters of these two components change in
correlation with the position of the source in the color-color diagram. Based
on the pattern that the source shows in the color-color diagram and its
aperiodic variability we conclude that Aquila X-1 is an atoll source. We have
also investigated the possible correlation between the frequency of the kHz QPO
and the position of the source in the color-color diagram. The complexity seen
in the frequency versus count rate diagram is reduced to a single track when
the frequency is plotted against hard or soft color.Comment: 9 pages, 5 figures. Accepted for publication in Ap
Facilitated diffusion of proteins on chromatin
We present a theoretical model of facilitated diffusion of proteins in the
cell nucleus. This model, which takes into account the successive
binding/unbinding events of proteins to DNA, relies on a fractal description of
the chromatin which has been recently evidenced experimentally. Facilitated
diffusion is shown quantitatively to be favorable for a fast localization of a
target locus by a transcription factor, and even to enable the minimization of
the search time by tuning the affinity of the transcription factor with DNA.
This study shows the robustness of the facilitated diffusion mechanism, invoked
so far only for linear conformations of DNA.Comment: 4 pages, 4 figures, accepted versio
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