6,708 research outputs found
A theory on power in networks
The eigenvector centrality equation is a successful
compromise between simplicity and expressivity. It claims that central actors
are those connected with central others. For at least 70 years, this equation
has been explored in disparate contexts, including econometrics, sociometry,
bibliometrics, Web information retrieval, and network science. We propose an
equally elegant counterpart: the power equation , where
is the vector whose entries are the reciprocal of those of . It
asserts that power is in the hands of those connected with powerless others. It
is meaningful, for instance, in bargaining situations, where it is advantageous
to be connected to those who have few options. We tell the parallel, mostly
unexplored story of this intriguing equation
INTEGRAL and Swift observations of IGRJ19294+1816 in outburst
IGRJ19294+1816 was discovered by INTEGRAL in 2009 during a bright X-ray
outburst and was classified as a possible Be X-ray binary or supergiant fast
X-ray transient. On 2010 October 28, the source displayed a second X-ray
outburst and a 2 months-long monitoring with Swift was carried out to follow
the evolution of the source X-ray flux during the event. We report on the
INTEGRAL and Swift observations of the second X-ray outburst observed from
IGRJ19294+1816. We detected pulsations in the X-ray emission from the source at
\sim12.5 s up to 50 keV. The source X-ray flux decreased smoothly during the
two months of observation displaying only marginal spectral changes. Due to the
relatively rapid decay of the source X-ray flux, no significant variations of
the source spin period across the event could be measured. This prevented a
firm confirmation of the previously suggested orbital period of the source at
117 d. This periodicity was also searched by using archival Swift /BAT data. We
detected a marginally significant peak in the periodogram and determined the
best period at 116.2\pm0.6 days (estimated chance probability of a spurious
detection 1%). The smooth decline of the source X-ray flux across the two
months of observations after the onset of the second outburst, together with
its relatively low value of the spin period and the absence of remarkable
changes in the spectral parameters (i.e., the absorption column density),
suggests that IGRJ19294+1816 is most likely another member of the Be X-ray
binaries discovered by INTEGRAL and not a supergiant fast X-ray transient.Comment: Accepted for publication in A&A. 7 pages, 10 figure
SFXTs versus classical SgXBs: Does the difference lie in the companion wind?
We present a comparative study of stellar winds in classical supergiant high
mass X-ray binaries (SgXBs) and supergiant fast X-ray transients (SFXTs) based
on the analysis of publicly available out-of-eclipse observations performed
with Suzaku and XMM-Newton. Our data-set includes 55 observations of classical
SgXBs and 21 observations of SFXTs. We found that classical SgXBs are
characterized by a systematically higher absorption and luminosity compared to
the SFXTs, confirming the results of previous works in the literature.
Additionally, we show that the equivalent width of the fluorescence K{\alpha}
iron line in the classical SgXBs is significantly larger than that of the SFXTs
(outside X-ray eclipses). Based on our current understanding of the physics of
accretion in these systems, we conclude that the most likely explanation of
these differences is to be ascribed to the presence of mechanisms inhibiting
accretion for most of the time in the SFXTs and leading to a much less
efficient photoionization of the stellar wind compared to classical SgXBs.We do
not find evidence for the previously reported anti-correlation between the
equivalent width of the fluorescence iron line and the luminosity of SgXBs.Comment: 12 pages, 8 figures, 2 tables, Accepted for publication in A&
INTEGRAL and Swift observations of the hard X-ray transient MAXI J1828-249
In this paper we report on the observations performed with INTEGRAL and Swift
of the first outburst detected from the hard X-ray transient MAXI J1828-249.
During the first about two days of the outburst, the source was observed by
MAXI to undergo a very rapid transition from a hard to a softer spectral state.
While the hard state was not efficiently monitored because the transition
occurred so rapidly, the evolution of the source outburst in the softer state
was covered quasi-simultaneously in a broad energy range (0.6-150 keV) by the
instruments on-board INTEGRAL and Swift. During these observations, the spectra
measured from the source displayed both a prominent thermal emission with
temperature kT 0.7 keV and a power-law hard component with a photon index gamma
2.2 extending to 200 keV. The properties of the source in the X-ray domain are
reminiscent of those displayed by black hole transients during the soft
intermediate state, which supports the association of MAXI J1828-249 with this
class of objects.Comment: 5 pages, 5 figures, 2 tables. Accepter for publication in A&
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