The evolution of the high energy tail in the quiescent spectrum of the soft X-ray transient Aql X-1

A moderate level of variability has been detected in the quiescent luminosity of several neutron star soft X-ray transients. Spectral variability was first revealed by Chandra observations of Aql X-1 in the four months that followed the 2000 X-ray outburst. By adopting the canonical model for quiescent spectrum of soft X-ray transients, i.e. an absorbed neutron star atmosphere model plus a power law tail, Rutledge et al. (2002a) concluded that the observed spectral variations can be ascribed to temperature variations of the neutron star atmosphere. These results can hardly be reconciled with the neutron star cooling that is expected to take place in between outbursts (after deep crustal heating in the accretion phase). Here we reanalyse the Chandra spectra of Aql X-1, together with a long BeppoSAX observation in the same period, and propose a different interpretation of the spectral variability: that this is due to correlated variations of the power law component and the column density (>5, a part of which might be intrinsic to the source), while the temperature and flux of the neutron star atmospheric component remained unchanged. This lends support to the idea that the power law component arises from emission at the shock between a radio pulsar wind and inflowing matter from the companion star.Comment: 6 pages, 2 figures. Accepted for publication on Ap

Structural Studies of Nonionic Dodecanol Ethoxylates at the Oil–Water Interface: Effect of Increasing Head Group Size

The conformation of charged surfactants at the oil-water interface was recently reported. With the aim to assess the role of the head group size on the conformation of the adsorbed layer, we have extended these studies to a series of non-ionic dodecanol ethoxylate surfactants (C12En, ethylene oxide units n from 6 to 12). The study was performed using neutron reflectometry to enable maximum sensitivity to buried interfaces. Similarly to charged surfactants, the interface was found to be broader and rougher compared to the air-water interface. Irrespectively of the head group size, the tail group region was found to assume a staggered conformation. The conformations of the head group were found to be significantly different compared to the air-water interface, moving from a globular to an almost fully extended conformation at the oil-water interface. The stretching of the head groups is attributed to the presence of some hexadecane oil molecules, which may penetrate all the way to this region. It is proposed here that the presence of the oil, which can efficiently solvate the surfactant tail groups, plays a key role in the conformation of the adsorbed layer and is responsible for the broadening of the interface

Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field

Starting from the observation that kilohertz Quasi Period Oscillations (kHz QPO) occur in a very narrow range of X-ray luminosities in neutron star low mass X-ray binaries, we try to link the kHz QPO observability to variations of the neutron star magnetospheric radius, in response to changing mass inflow rate. At low luminosities, the drop off of kHz QPOs activity may be explained by the onset of the centrifugal barrier, when the magnetospheric radius reaches the corotation radius. At the opposite side, at higher luminosities, the magnetospheric radius may reach the neutron star and the vanishing of the magnetosphere may led to the stopping of the kHz QPOs activity. If we apply these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1) and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits naturally apply in the framework of beat frequency models but can also work in the case of general relativistic models.Comment: 4 pages (emulateapj macro), 1 figure. Accepted for publication in ApJ Letter

Crystal structure of poly[[[μ4-5-(9H-carbazol-9-yl)isophthalato][μ3-5-(9H-carbazol-9-yl)isophthalato]bis-(di-methyl-formamide)(methanol)dizinc] di-methyl-formamide monosolvate].

The structure of the polymeric title compound, {[Zn2(C20H11NO4)2(C3H7NO)2(CH3OH)]·C3H7NO} n , comprises carbazolylisophthalate moieties connecting dimetallic tetra-carboxyl-ate zinc secondary building units (SBUs) parallel to [100] and [010], leading to a layer-like arrangement parallel to (001). Each SBU consists of two Zn atoms in slightly distorted tetra-hedral and octa-hedral coordination environments [Zn⋯Zn = 3.5953 (6) Å]. Three carboxyl-ate groups bridge the two Zn atoms in a μ2-O:O' mode, whereas the fourth coordinates through a single carboxyl-ate O atom (μ1-O). The O atoms of two di-methyl-formamide (DMF) and one methanol mol-ecule complete the Zn coordination spheres. The methanol ligand inter-acts with the noncoordinating DMF mol-ecule via an O-H⋯O hydrogen bond of medium strength. Carbazoles between the layers inter-digitate through weak C-H⋯.π inter-actions to form a laminar solid stacked along [010]. Two kinds of C-H⋯π inter-actions are present, both with a distance of 2.64 Å, between the H atoms and the centroids, and a third C-H⋯π inter-action, where the aromatic H atom is located above the carbazole N-atom lone pair (H⋯N = 2.89 Å). Several C-H⋯O inter-actions occur between the coordinating DMF mol-ecule, the DMF solvent mol-ecule, and ligating carboxyl-ate O atoms

CAD-CAE methods to support restoration and museum exhibition of bronze statues: the “Principe Ellenistico”

Ancient bronze statues mainly require material integrity assessment and restoration. Restoration may include also the update of the museum exhibition, defining new structural frames and fragment re-composition to preserve the statue and improve the interpretation of the original aspect. This paper proves how engineering methods (such as Finite Element Analysis, Computer Aided Design modelling, Reverse Engineering) may assist cultural heritage experts and restorers in these tasks. It presents the activities made together with the Museo Nazionale Romano and the Istituto Superiore per la Conservazione e il Restauro, on the so-called “Principe Ellenistico” (Hellenistic Prince). This bronze was found in pieces (body, left arm and right leg), at the end of 19th century during an excavation made in Rome. No visual or reference sources can say its origin and its final posture was defined by restorers at the end of the 19th century according to their hypothesis and studies. In the 20th century, a further restoration was made on the critical areas of the surface, together with some structural improvement of the inner frame. Nowadays, after a review of its position inside the Museum, new experimental and numerical analyses have been carried out to better understand surface weakness and correct left arm positionin

Virtual prototyping of medieval weapons for historical reconstruction of siege scenarios starting from topography and archaeological investigations

Chronicles of sieges to castles or fortresses, using “machinae”, can often be found in historical sources. Moreover, archaeological excavations of castles or fortresses has brought to light rocks or projectiles whose carving suggests a military usage. Nevertheless, chronicles and discoveries alone, are seldom enough to propose a faithful reconstruction of these machines. Therefore, the aim of this research is the development of methodologies for reconstructing virtual scenarios of sieges, starting from the scarce information available. In order to achieve it, a procedure for the virtual reconstruction of the siege machine has been set up, focusing on typology and dimensions of the machines, also investigating possible fire positions according to topography. The entire procedure has been developed using the siege of Cervara di Roma’s Rocca as a case study. Late medieval chronicles (end of 13th Century) report the siege brought by the papal army in order to restore the jurisdiction on the Cervara’s stronghold, following the insurrection of a group of vassals headed by a monk named Pelagio. The discovery, in the area of the Rocca, of a stone that could have been used as a projectile confirms what reported. The proposed methodology is composed of two parts. The first one is connected to the study of the “internal ballistics”, to understand the performances and to build virtual models of siege machines. The second part is the study of the “external ballistics”, then to the positioning and shooting ability of possible machines, analysing the topography of the area. In this paper, we present the feasibility of this methodology through the preliminary results achieved correlating internal and external ballistics

Optical and infrared polarimetry of the transient LMXB Cen X-4 in quiescence

We present the first optical and infrared polarimetric study of the low mass transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed to search for an intrinsic linear polarisation component in the system emitted radiation that might be due, e.g., to synchrotron emission from a compact jet, or to Thomson scattering with free electrons in an accretion disc. Multiband (BVRI) optical polarimetric observations were obtained during two nights in 2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These observations cover about the 30% of the 15.1 hours orbital period. J-band observations were obtained in 2007 with the NICS (TNG) instrument at La Palma, for a totality of 1 hour observation. We obtained 3-sigma upper limits to the polarisation degree in all the optical bands, with the most constraining one being in the I-band (P<0.5%). No phase-correlated variability has been noticed in all the filters. The J-band observations provided a 6% upper limit on the polarisation level. The constraining upper limits to the polarisation in the optical allowed us to evaluate the contribution of the possible emission of a relativistic particles jet to the total system radiation to be less then the 10%. This is in agreement with the observation of a spectral energy distribution typical of a single black body of a K-spectral type main sequence star irradiated from the compact object. Due to the low S/N ratio it was not possible to investigate the possible dependency of the polarisation degree from the wavelength, that could be suggestive of polarisation induced by Thomson scattering of radiation with free electrons in the outer part of the accretion disc. Observations with higher S/N ratio are required to examine in depth this hypothesis, searching for significant phase-correlated variability.Comment: 7 pages, 9 figures, accepted for publication in section 7. Stellar structure and evolution of Astronomy and Astrophysic

The quiescent X-ray emission of three transient X-ray pulsars

We report on BeppoSAX and Chandra observations of three Hard X-Ray Transients in quiescence containing fast spinning (P<5 s) neutron stars: A 0538-66, 4U 0115+63 and V 0332+53. These observations allowed us to study these transients at the faintest flux levels thus far. Spectra are remarkably different from the ones obtained at luminosities a factor >10 higher, testifying that the quiescent emission mechanism is different. Pulsations were not detected in any of the sources, indicating that accretion of matter down to the neutron star surface has ceased. We conclude that the quiescent emission of the three X-ray transients likely originates from accretion onto the magnetospheric boundary in the propeller regime and/or from deep crustal heating resulting from pycnonuclear reactions during the outbursts.Comment: Accepted for publication on ApJ (5 pages and 2 figures