5,787 research outputs found
Around 200 new X-ray binary IDs from 13 years of Chandra observations of the M31 center
We have created 0.3--10 keV, 13 year, unabsorbed luminosity lightcurves for
528 X-ray sources in the central 20' of M31. We have 174 Chandra observations
spaced at ~1 month intervals thanks to our transient monitoring program, deeper
observations of the M31 nucleus, and some public data from other surveys. We
created 0.5--4.5 keV structure functions (SFs) for each source, for comparison
with the ensemble structure function of AGN. We find 220 X-ray sources with
luminosities > ~1E+35 erg/s that have SFs with significantly more variability
than the ensemble AGN SF, and are likely X-ray binaries (XBs). A further 30
X-ray sources were identified as XBs using other methods. We therefore have 250
probable XBs in total, including ~200 new identifications. This result
represents great progress over the ~50 XBs and ~40 XB candidates previously
identified out of the ~2000 X-ray sources within the D_25 region of M31; it
also demonstrates the power of SF analysis for identifying XBs in external
galaxies. We also identify a new transient black hole candidate, associated
with the M31 globular cluster B128.Comment: Accepted for publication in ApJ; 20 pages, 7 figures; Tables 1 and 2
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Predicting space climate change
The recent decline in the open magnetic flux of the Sun heralds the end of the Grand Solar Maximum (GSM) that has persisted throughout the space age, during which the largestâfluence Solar Energetic Particle (SEP) events have been rare and Galactic Cosmic Ray (GCR) fluxes have been relatively low. In the absence of a predictive model of the solar dynamo, we here make analogue forecasts by studying past variations of solar activity in order to evaluate how longâterm change in space climate may influence the hazardous energetic particle environment of the Earth in the future. We predict the probable future variations in GCR flux, nearâEarth interplanetary magnetic field (IMF), sunspot number, and the probability of large SEP events, all deduced from cosmogenic isotope abundance changes following 24 GSMs in a 9300âyear record
The Distribution of X-ray Dips with Orbital Phase in Cygnus X-1
We present results of a comprehensive study of the distribution of absorption
dips with orbital phase in Cygnus X-1. Firstly, the distribution was obtained
using archival data from all major X-ray observatories and corrected for the
selection effect that phase zero (superior conjunction of the black hole) has
been preferentially observed. Dip occurrence was seen to vary strongly with
orbital phase \phi, with a peak at \phi ~ 0.95, i.e. was not symmetric about
phase zero. Secondly, the RXTE ASM has provided continuous coverage of the Low
State of Cygnus X-1 since Sept. 1996, and we have selected dip data based on
increases in hardness ratio. The distribution, with much increased numbers of
dip events, confirms that the peak is at \phi ~ 0.95, and we report the
discovery of a second peak at \phi ~ 0.6. We attribute this peak to absorption
in an accretion stream from the companion star HDE 226868. We have estimated
the ionization parameter at different positions showing that radiative
acceleration of the wind is suppressed by photoionization in particular regions
in the binary system. To obtain the variation of column density with phase, we
make estimates of neutral wind density for the extreme cases that acceleration
of the wind is totally suppressed, or not suppressed at all. An accurate
description will lie between these extremes. In each case, a strong variation
of column density with orbital phase resulted, similar to the variation of dip
occurrence. This provides evidence that formation of the blobs in the wind
which lead to absorption dips depends on the density of the neutral component
in the wind, suggesting possible mechanisms for blob growth.Comment: 9 pages, Latex, 7 ps figures. accepted by MNRA
Identification of diverse database subsets using property-based and fragment-based molecular descriptions
This paper reports a comparison of calculated molecular properties and of 2D fragment bit-strings when used for the selection of structurally diverse subsets of a file of 44295 compounds. MaxMin dissimilarity-based selection and k-means cluster-based selection are used to select subsets containing between 1% and 20% of the file. Investigation of the numbers of bioactive molecules in the selected subsets suggest: that the MaxMin subsets are noticeably superior to the k-means subsets; that the property-based descriptors are marginally superior to the fragment-based descriptors; and that both approaches are noticeably superior to random selection
Oxidation Behavior of a Pd_(43)Cu_(27)Ni_(10)P_(20) Bulk Metallic Glass and Foam in Dry Air
The oxidation behavior of both Pd_(43)Cu_(27)Ni_(10)P_(20) bulk metallic glass (Pd4-BMG) and its amorphous foam containing 45 pct porosity (Pd4-AF) was investigated over the temperature range of 343 K (70 °C) to 623 K (350 °C) in dry air. The results showed that virtually no oxidation occurred in the Pd4-BMG at T < 523 K (250 °C), revealing the alloyâs favorable oxidation resistance in this temperature range. In addition, the oxidation kinetics at T â„ 523 K (250 °C) followed a parabolic-rate law, and the parabolic-rate constants (k_p values) generally increased with temperature. It was found that the oxidation k_p values of the Pd4-AF are slightly lower than those of the Pd4-BMG, indicating that the porous structure contributes to improving the overall oxidation resistance. The scale formed on the alloys was composed exclusively of CuO at T â„ 548 K (275 °C), whose thickness gradually increased with increasing temperature. In addition, the amorphous structure remained unchanged at T †548 K (275 °C), while a triplex-phase structure developed after the oxidation at higher temperatures, consisting of Pd_2Ni_2P, Cu_3P, and Pd_3P
Tomographic reconstruction of treponemal cytoplasmic filaments reveals novel bridging and anchoring components
An understanding of the involvement of bacterial cytoplasmic filaments in cell division requires the elucidation of the structural organization of those filamentous structures. Treponemal cytoplasmic filaments are composed of one protein, CfpA, and have been demonstrated to be involved in cell division. In this study, we used electron tomography to show that the filaments are part of a complex with a novel molecular organization that includes at least two distinct features decorating the filaments. One set of components appears to anchor the filaments to the cytoplasmic membrane. The other set of components appears to bridge the cytoplasmic filaments on the cytoplasmic side, and to be involved in the interfilament spacing within the cell. The filaments occupy between 3 and 18% of the inner surface of the cytoplasmic membrane. These results reveal a novel filamentous molecular organization of independent filaments linked by bridges and continuously anchored to the membrane
Four new black hole candidates identified in M31 globular clusters with Chandra and XMM-Newton
We have identified four new black hole candidates in M31 globular clusters
using 123 Chandra, and 4 XMM-Newton observations of the M31 central region. The
X-ray source associated with Bo 163 (XB163) is a recurrent transient, with the
highest luminosity ~1.4E+38 erg/s, considerably brighter than any outbursts
from the neutron star transients Aql X-1 or 4U 1608-452; the outburst
apparently started ~45 days earlier than the observed peak, hence the
luminosity could have been considerably higher. We identified XB082, XB153 and
XB185 as BHCs by observing "low state" emission spectra at luminosities that
exceed the threshold for neutron star binaries. The probability that these are
neutron star systems with anisotropic emission beamed toward us is <4E-4, and
their variability suggests emission from a single source. We therefore conclude
that these systems likely contain black holes rather than neutron stars. We
have now identified 4 persistently bright BHCs in the region; the probability
that these are all background AGN is <1E-20. According to theory, the donors
could be tidally captured main sequence stars, or white dwarves in
ultra-compact binaries. We find that GCs that are particularly massive (XB082)
or metal rich (XB144) can host bright X-ray sources in addition to those that
are both (XB163). Our method may reveal BHCs in other bright X-ray sources.Comment: Accepted for publication in ApJ. 17 pages, 5 figure
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