12,963 research outputs found
A multi-objective DIRECT algorithm for ship hull optimization
The paper is concerned with black-box nonlinear constrained multi-objective optimization problems. Our interest is the definition of a multi-objective deterministic partition-based algorithm. The main target of the proposed algorithm is the solution of a real ship hull optimization problem. To this purpose and in pursuit of an efficient method, we develop an hybrid algorithm by coupling a multi-objective DIRECT-type algorithm with an efficient derivative-free local algorithm. The results obtained on a set of “hard” nonlinear constrained multi-objective test problems show viability of the proposed approach. Results on a hull-form optimization of a high-speed catamaran (sailing in head waves in the North Pacific Ocean) are also presented. In order to consider a real ocean environment, stochastic sea state and speed are taken into account. The problem is formulated as a multi-objective optimization aimed at (i) the reduction of the expected value of the mean total resistance in irregular head waves, at variable speed and (ii) the increase of the ship operability, with respect to a set of motion-related constraints. We show that the hybrid method performs well also on this industrial problem
Quiescent Thermal Emission from the Neutron Star in Aql X-1
We report on the quiescent spectrum measured with Chandra/ACIS-S of the
transient, type-I X-ray bursting neutron star Aql X-1, immediately following an
accretion outburst. The neutron star radius, assuming a pure hydrogen
atmosphere and hard power-law spectrum, is =13.4{+5}{-4} (d/5 \kpc)
km. Based on the historical outburst record of RXTE/ASM, the quiescent
luminosity is consistent with that predicted by Brown, Bildsten and Rutledge
from deep crustal heating, lending support to this theory for providing a
minimum quiescent luminosity of transient neutron stars. While not required by
the data, the hard power-law component can account for 18+/-8% of the 0.5-10
keV thermal flux. Short-timescale intensity variability during this observation
is less than 15% rms (3 sigma; 0.0001-1 Hz, 0.2-8 keV). Comparison between the
Chandra spectrum and three X-ray spectral observations made between Oct 1992
and Oct 1996 find all spectra consistent with a pure H atmosphere, but with
temperatures ranging from 145--168 eV, spanning a factor of 1.87+/-0.21 in
observed flux. The source of variability in the quiescent luminosity on long
timescales (greater than years) remains a puzzle. If from accretion, then it
remains to be explained why the quiescent accretion rate provides a luminosity
so nearly equal to that from deep crustal heating.Comment: 15 pages, 1 figure, 2 tables; ApJ, accepte
On the bolometric quiescent luminosity and luminosity swing of black hole candidate and neutron star low mass X-ray transients
Low mass X-ray transients (LMXRTs) hosting black hole candidates (BHCs)
display on average a factor of ~100 larger swing in the minimum (quiescent) to
maximum (outburst) X-ray luminosity than neutron stars (NSs), despite the fact
that the swing in the mass inflow rate is likely in the same range. Advection
dominated accretion flows (ADAFs) were proposed to interpret such a difference.
The residual optical/UV emission of quiescent LMXRTs, after subtraction of the
companion star spectrum, is produced by synchrotron radiation in the (latest
version) of ADAF and therefore is part of the ADAF's luminosity budget. We
demonstrate that, once the residual optical/UV emission is taken into account,
the bolometric luminosity swing of BHCs is consistent with that of NSs. We
explore here an alternative scenario to ADAFs in which very little mass
accretion onto the collapsed star takes place in the quiescence intervals. The
residual optical/UV emission of BHCs are expected to derive from the energy
released by the matter transferred from the companion star at radii comparable
to the circularisation radius. The quiescent X-ray luminosity originates either
from accretion onto the BH at very low rates and/or from coronal activity in
the companion star or in the outer disk. For comparably small mass inflow
rates, the NSs in these systems are likely in the radio pulsar regime. In the
interaction of the radio pulsar relativistic wind with matter transferred from
the companion star, a shock forms, the power law-like emission of which powers
both the harder X-ray emission and most of the residual optical/UV. The soft,
thermal-like X-ray component may arise from the cooling of the NS surface. This
scenario matches well both the X-ray and bolometric luminosity swing of LMXRTs.
(ABRIDGED).Comment: 13 pages (including 2 postscript figures - use emulateapj macro).
Accepted for publication in Ap
Lagrangian fibrations of holomorphic-symplectic varieties of K3^[n]-type
Let X be a compact Kahler holomorphic-symplectic manifold, which is
deformation equivalent to the Hilbert scheme of length n subschemes of a K3
surface. Let L be a nef line-bundle on X, such that the 2n-th power of c_1(L)
vanishes and c_1(L) is primitive. Assume that the two dimensional subspace
H^{2,0}(X) + H^{0,2}(X), of the second cohomology of X with complex
coefficients, intersects trivially the integral cohomology. We prove that the
linear system of L is base point free and it induces a Lagrangian fibration on
X. In particular, the line-bundle L is effective. A determination of the
semi-group of effective divisor classes on X follows, when X is projective. For
a generic such pair (X,L), not necessarily projective, we show that X is
bimeromorphic to a Tate-Shafarevich twist of a moduli space of stable torsion
sheaves, each with pure one dimensional support, on a projective K3 surface.Comment: 34 pages. v3: Reference [Mat5] and Remark 1.8 added. Incorporated
improvement to the exposition and corrected typos according to the referees
suggestions. To appear in the proceedings of the conference Algebraic and
Complex Geometry, Hannover 201
Search for low instability strip variables in the young open cluster NGC 2516
In this paper we revise and complete the photometric survey of the
instability strip of the southern open cluster NGC 2516 published by Antonello
and Mantegazza (1986). No variable stars with amplitudes larger than
were found. However by means of an accurate analysis based on a new statistical
method two groups of small amplitude variables have been disentangled: one with
periods (probably Scuti stars) and one with periods
. The position in the HR diagram and the apparent time-scale may
suggest that the stars of the second group belong to a recently discovered new
class of variables, named Dor variables. They certainly deserve
further study. We also present a comparison between the results of the
photometric survey and the available pointed ROSAT observations of this
cluster.Comment: 7 pages, 2 ps figures. Accepted for P.A.S.
Three Additional Quiescent Low-Mass X-ray Binary Candidates in 47 Tucanae
We identify through their X-ray spectra one certain (W37) and two probable
(W17 and X4) quiescent low-mass X-ray binaries (qLMXBs) containing neutron
stars in a long Chandra X-ray exposure of the globular cluster 47 Tucanae, in
addition to the two previously known qLMXBs. W37's spectrum is dominated by a
blackbody-like component consistent with radiation from the hydrogen atmosphere
of a 10 km neutron star. W37's lightcurve shows strong X-ray variability which
we attribute to variations in its absorbing column depth, and eclipses with a
probable 3.087 hour period. For most of our exposures, W37's blackbody-like
emission (assumed to be from the neutron star surface) is almost completely
obscured, yet some soft X-rays (of uncertain origin) remain. Two additional
candidates, W17 and X4, present X-ray spectra dominated by a harder component,
fit by a power-law of photon index ~1.6-3. An additional soft component is
required for both W17 and X4, which can be fit with a 10 km hydrogen-atmosphere
neutron star model. X4 shows significant variability, which may arise from
either its power-law or hydrogen-atmosphere spectral component. Both W17 and X4
show rather low X-ray luminosities, Lx(0.5-10 keV)~5*10^{31} ergs/s. All three
candidate qLMXBs would be difficult to identify in other globular clusters,
suggesting an additional reservoir of fainter qLMXBs in globular clusters that
may be of similar numbers as the group of previously identified objects. The
number of millisecond pulsars inferred to exist in 47 Tuc is less than 10 times
larger than the number of qLMXBs in 47 Tuc, indicating that for typical
inferred lifetimes of 10 and 1 Gyr respectively, their birthrates are
comparable.Comment: Accepted for publication in ApJ. 13 pages, 7 figures (2 color
Discovery of a candidate quiescent low-mass X-ray binary in the globular cluster NGC 6553
This paper reports the search for quiescent low-mass X-ray binaries (qLMXBs)
in the globular cluster (GC) NGC 6553 using an XMM-Newton observation designed
specifically for that purpose. We spectrally identify one candidate qLMXB in
the core of the cluster, based on the consistency of the spectrum with a
neutron star H-atmosphere model at the distance of NGC 6553. Specifically, the
best-fit radius found using the three XMM European Photon Imaging Camera
spectra is R_NS=6.3(+2.3)(-0.8) km (for M_NS=1.4 Msun) and the best-fit
temperature is kTeff=136 (+21)(-34) eV. Both physical parameters are in
accordance with typical values of previously identified qLMXBs in GC and in the
field, i.e., R_NS~5-20 km and kTeff~50-150 eV. A power-law (PL) component with
a photon index Gamma=2.1(+0.5)(-0.8) is also required for the spectral fit and
contributes to ~33% of the total flux of the X-ray source. A detailed analysis
supports the hypothesis that the PL component originates from nearby sources in
the core, unresolved with XMM. The analysis of an archived Chandra observation
provides marginal additional support to the stated hypothesis. Finally, a
catalog of all the sources detected within the XMM field of view is presented
here.Comment: 10 pages, 5 figures, 3 tables. Accepted to ApJ (to be published in
August 2011
The discovery of the optical/IR counterpart of the 12s transient X-ray pulsar GS 0834-43
We report the discovery of the optical/infra-red counterpart of the 12.3s
transient X-ray pulsar GS0834-43. We re-analysed archival ROSAT PSPC
observations of GS0834-43, obtaining two new refined positions, about 14" and
18" away from the previously published one, and a new spin period measurement.
Within the new error circles we found a relatively faint (V=20.1) early type
reddened star (V-R=2.24). The optical spectrum shows a strong Halpha emission
line. The IR observations of the field confirm the presence of an IR excess for
the Halpha-emitting star (K'=11.4, J-K'=1.94) which is likely surrounded by a
conspicuous circumstellar envelope. Spectroscopic and photometric data indicate
a B0-2 V-IIIe spectral-type star located at a distance of 3-5kpc and confirm
the Be-star/X-ray binary nature of GS0834-43.Comment: 6 pages. Accepted for publication in MNRA
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