13,463 research outputs found
Weak lensing evidence for a filament between A222/A223
We present a weak lensing analysis and comparison to optical and X-ray maps
of the close pair of massive clusters A222/223. Indications for a filamentary
connection between the clusters are found and discussed.Comment: 6 pages, 1 figure. To appear in Proc. IAU Colloquium 195: Outskirts
of Galaxy Clusters - Intense Life in the Suburbs. Version with higher
resolution available at
http://www.astro.uni-bonn.de/~dietrich/torino_proc.ps.g
A Suzaku X-ray observation of one orbit of the supergiant fast X-ray transient IGR J16479-4514
We report on a 250 ks long X-ray observation of the supergiant fast X-ray
transient (SFXT) IGR J16479-4514 performed with Suzaku in 2012 February. About
80% of the short orbital period (Porb=3.32 days) was covered as continuously as
possible for the first time. The source light curve displays variability of
more than two orders of magnitude, starting with a very low emission state
lasting the first 46 ks (1E-13 erg/cm2/s, 1-10 keV), consistent with being due
to the X-ray eclipse by the supergiant companion. The transition to the
uneclipsed X-ray emission is energy dependent. Outside the eclipse, the source
spends most of the time at a level of (6-7)x10^-12 erg/cm2/s punctuated by two
structured faint flares with a duration of about 10 and 15 ks. Remarkably, the
first faint flare occurs at a similar orbital phase of the bright flares
previously observed in the system. This indicates the presence of a
phase-locked large scale structure in the supergiant wind, driving a higher
accretion rate onto the compact object. The scattered component visible during
the X-ray eclipse allowed us to directly probe the wind density at the orbital
separation, resulting in rho=7E-14 g/cm3. Assuming a spherical geometry for the
supergiant wind, the derived wind density translates into a ratio
Mdot_w/v_terminal = 7E-17 solar masses/km which, assuming terminal velocities
in a large range 500-3000 km/s, implies an accretion luminosity two orders of
magnitude higher than that observed. As a consequence, a mechanism is at work
reducing the mass accretion rate. Different possibilities are discussed.Comment: Accepted for publication in MNRAS. 10 pages, 5 figure
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
Swift/XRT orbital monitoring of the candidate supergiant fast X-ray transient IGR J17354-3255
We report on the Swift/X-ray Telescope (XRT) monitoring of the field of view
around the candidate supergiant fast X-ray transient (SFXT) IGR J17354-3255,
which is positionally associated with the AGILE/GRID gamma-ray transient AGL
J1734-3310. Our observations, which cover 11 days for a total on-source
exposure of about 24 ks, span 1.2 orbital periods (P_orb=8.4474 d) and are the
first sensitive monitoring of this source in the soft X-rays. These new data
allow us to exploit the timing variability properties of the sources in the
field to unambiguously identify the soft X-ray counterpart of IGR J17354-3255.
The soft X-ray light curve shows a moderate orbital modulation and a dip. We
investigated the nature of the dip by comparing the X-ray light curve with the
prediction of the Bondi-Hoyle-Lyttleton accretion theory, assuming both
spherical and nonspherical symmetry of the outflow from the donor star. We
found that the dip cannot be explained with the X-ray orbital modulation. We
propose that an eclipse or the onset of a gated mechanism is the most likely
explanation for the observed light curve.Comment: Accepted for publication in Astronomy and Astrophysics. 9 page
Resampling-based confidence regions and multiple tests for a correlated random vector
We derive non-asymptotic confidence regions for the mean of a random vector
whose coordinates have an unknown dependence structure. The random vector is
supposed to be either Gaussian or to have a symmetric bounded distribution, and
we observe i.i.d copies of it. The confidence regions are built using a
data-dependent threshold based on a weighted bootstrap procedure. We consider
two approaches, the first based on a concentration approach and the second on a
direct boostrapped quantile approach. The first one allows to deal with a very
large class of resampling weights while our results for the second are
restricted to Rademacher weights. However, the second method seems more
accurate in practice. Our results are motivated by multiple testing problems,
and we show on simulations that our procedures are better than the Bonferroni
procedure (union bound) as soon as the observed vector has sufficiently
correlated coordinates.Comment: submitted to COL
A spherical model with directional interactions: I. Static properties
We introduce a simple spherical model whose structural properties are similar
to the ones generated by models with directional interactions, by employing a
binary mixture of large and small hard spheres, with a square-well attraction
acting only between particles of different size. The small particles provide
the bonds between the large ones. With a proper choice of the interaction
parameters, as well as of the relative concentration of the two species, it is
possible to control the effective valence. Here we focus on a specific choice
of the parameters which favors tetrahedral ordering and study the equilibrium
static properties of the system in a large window of densities and
temperatures. Upon lowering the temperature we observe a progressive increase
in local order, accompanied by the formation of a four-coordinated network of
bonds. Three different density regions are observed: at low density the system
phase separates into a gas and a liquid phase; at intermediate densities a
network of fully bonded particles develops; at high densities -- due to the
competition between excluded volume and attractive interactions -- the system
forms a defective network. The very same behavior has been previously observed
in numerical studies of non-spherical models for molecular liquids, such as
water, and in models of patchy colloidal particles. Differently from these
models, theoretical treatments devised for spherical potentials, e.g. integral
equations and ideal mode coupling theory for the glass transition can be
applied in the present case, opening the way for a deeper understanding of the
thermodynamic and dynamic behavior of low valence molecules and particles.Comment: 11 pages, 11 figure
Approximate analytical description of the nonaffine response of amorphous solids
An approximation scheme for model disordered solids is proposed that leads to
the fully analytical evaluation of the elastic constants under explicit account
of the inhomogeneity (nonaffinity) of the atomic displacements. The theory is
in quantitative agreement with simulations for central-force systems and
predicts the vanishing of the shear modulus at the isostatic point with the
linear law {\mu} ~ (z - 2d), where z is the coordination number. The vanishing
of rigidity at the isostatic point is shown to be a consequence of the
canceling out of positive affine and negative nonaffine terms
Giant outburst from the supergiant fast X-ray transient IGR J17544-2619: accretion from a transient disc?
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries
associated with OB supergiant companions and characterised by an X-ray flaring
behaviour whose dynamical range reaches 5 orders of magnitude on timescales of
a few hundred to thousands of seconds. Current investigations concentrate on
finding possible mechanisms to inhibit accretion in SFXTs and explain their
unusually low average X-ray luminosity. We present the Swift observations of an
exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014
October 10 when the source achieved a peak luminosity of erg
s. This extends the total source dynamic range to 10, the
largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence
for pulsations at a period of 11.6 s is also reported. We show that these
observations challenge, for the first time, the maximum theoretical luminosity
achievable by an SFXT and propose that this giant outburst was due to the
formation of a transient accretion disc around the compact object.Comment: Accepted for publication in Astronomy and Astrophysics Letters. 5
pages, 5 figures, 2 table
Improving performances of biomimetic wings with leading-edge tubercles
The present study aims investigating experimentally wing/blade geometries in which the leading edge is modified by the presence of artificial bumps, following examples in nature (“biomimetics”). Specifically, the tubercles observed in humpback whales are considered with a special focus on easy manufacturing and performance improvements, trying to overcome the observed lift coefficient reduction before stall in comparison with a standard wing. To this end, different tubercle geometries are tested, by measuring overall forces acting on the wings and by deriving detailed velocity fields using particle image
velocimetry. Measurements indicate performance improvements for all trailing edge tubercle geometries here tested. In addition, the detailed analysis of mechanisms underlying the improvement of performances suggests that a triangular shape of the leading edge combines the advantages of easy manufacturing and improvements of pre-stall behaviour. So far, a simple mathematical model, describing tubercles as delta wings, is presented and verified by experimental data. The objective of the present work is focusing on the basic fluid-mechanics phenomena involved, to show that beneficial effects of tubercles are present even when tubercle details are simplified, in order to couple performance improvement and ease of assembly
The 2015 outburst of the accreting millisecond pulsar IGR J17511-3057 as seen by INTEGRAL, Swift and XMM-Newton
We report on INTEGRAL, Swift and XMM-Newton observations of IGR J17511-3057
performed during the outburst that occurred between March 23 and April 25,
2015. The source reached a peak flux of 0.7(2)E-9 erg/cm/s and decayed to
quiescence in approximately a month. The X-ray spectrum was dominated by a
power-law with photon index between 1.6 and 1.8, which we interpreted as
thermal Comptonization in an electron cloud with temperature > 20 keV . A broad
({\sigma} ~ 1 keV) emission line was detected at an energy (E =
6.9 keV) compatible with the K{\alpha} transition of ionized
Fe, suggesting an origin in the inner regions of the accretion disk. The
outburst flux and spectral properties shown during this outburst were
remarkably similar to those observed during the previous accretion event
detected from the source in 2009. Coherent pulsations at the pulsar spin period
were detected in the XMM-Newton and INTEGRAL data, at a frequency compatible
with the value observed in 2009. Assuming that the source spun up during the
2015 outburst at the same rate observed during the previous outburst, we derive
a conservative upper limit on the spin down rate during quiescence of 3.5E-15
Hz/s. Interpreting this value in terms of electromagnetic spin down yields an
upper limit of 3.6E26 G/cm to the pulsar magnetic dipole (assuming a
magnetic inclination angle of 30{\deg}). We also report on the detection of
five type-I X-ray bursts (three in the XMM-Newton data, two in the INTEGRAL
data), none of which indicated photospheric radius expansion.Comment: 10 pages, 7 figures, accepted for publication in A&
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