A Stress/Displacement Virtual Element Method for Plane Elasticity Problems

The numerical approximation of 2D elasticity problems is considered, in the framework of the small strain theory and in connection with the mixed Hellinger-Reissner variational formulation. A low-order Virtual Element Method (VEM) with a-priori symmetric stresses is proposed. Several numerical tests are provided, along with a rigorous stability and convergence analysis

Quantitative characterisation of deltaic and subaqueous clinoforms

AbstractClinoforms are ubiquitous deltaic, shallow-marine and continental-margin depositional morphologies, occurring over a range of spatial scales (1–104m in height). Up to four types of progressively larger-scale clinoforms may prograde synchronously along shoreline-to-abyssal plain transects, albeit at very different rates. Paired subaerial and subaqueous delta clinoforms (or ‘delta-scale compound clinoforms’), in particular, constitute a hitherto overlooked depositional model for ancient shallow-marine sandbodies. The topset-to-foreset rollovers of subaqueous deltas are developed at up to 60m water depths, such that ancient delta-scale clinoforms should not be assumed to record the position of ancient shorelines, even if they are sandstone-rich.This study analyses a large dataset of modern and ancient delta-scale, shelf-prism- and continental-margin-scale clinoforms, in order to characterise diagnostic features of different clinoform systems, and particularly of delta-scale subaqueous clinoforms. Such diagnostic criteria allow different clinoform types and their dominant grain-size characteristics to be interpreted in seismic reflection and/or sedimentological data, and prove that all clinoforms are subject to similar physical laws.The examined dataset demonstrates that progressively larger scale clinoforms are deposited in increasingly deeper waters, over progressively larger time spans. Consequently, depositional flux, sedimentation and progradation rates of continental-margin clinoforms are up to 4–6 orders of magnitude lower than those of deltas. For all clinoform types, due to strong statistical correlations between these parameters, it is now possible to calculate clinoform paleobathymetries once clinoform heights, age spans or progradation rates have been constrained.Muddy and sandy delta-scale subaqueous clinoforms show many different features, but all share four characteristics. (1) They are formed during relative sea-level stillstands (e.g., Late Holocene); (2) their stratigraphic architecture and facies character are dominated by basinal processes, and are quite uniform; (3) their plan-view morphology is shore-parallel and laterally extensive; (4) their sigmoidal cross-sectional geometry contrasts with the oblique profiles of most subaerial deltas. Holocene-age, delta-scale, sand-prone subaqueous clinoforms occur on steep (≥0.26°) and narrow (5–32km) shelves, at typical distances of 0.6–7.2km from the shoreline break. That contrasts with mud-prone subaqueous deltas, which form clinoforms on gently-sloping (0.01–0.38°), wide (23–376km) shelves, at usual distances of 7.5–125km from the shoreline. Delta-scale sand-prone subaqueous clinoforms have diagnostically steep foresets (0.7–23°). Similarly steep gradients were observed in much larger shelf-prism- and continental-margin-scale clinoforms. Gentler foreset gradients are shown by sand-prone subaerial deltas (0.1–2.7°), and mud-prone subaqueous and subaerial deltas (0.03–1.50°). Due to the lack of connections with river mouths, Holocene delta-scale sand-prone subaqueous clinoform deposits have progradation rates (1–5×102km/Myr) and unit-width depositional flux (1–15km2/Myr) that are up to 3–4 and 2–3 orders of magnitude lower, respectively, than age-equivalent input-dominated subaerial deltas and muddy subaqueous deltas. Lower progradation/aggradation ratios are reflected in a larger spread of clinoform trajectory angles (from −0.4° to +3.5°) than the very low values displayed by age-equivalent subaerial and muddy subaqueous deltas.As slowly prograding, steep, sigmoidal clinoforms are strongly suggestive of sand-prone subaqueous deltas, the Sognefjord Formation and Bridport Sand are likely Jurassic examples of this clinoform type, and host hydrocarbon reservoirs. In contrast, the Campanian Blackhawk Formation is an outcrop example of delta-scale compound clinoforms with a muddy subaqueous component

Very hard states in neutron star low-mass X-ray binaries

We report on unusually very hard spectral states in three confirmed neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed power-law model, we found photon indices of \Gamma ~ 1, significantly lower than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so called hard state. For individual sources very hard spectra were already previously identified but here we show for the first time that likely our sources were in a distinct spectral state (i.e., different from the hard state) when they exhibited such very hard spectra. It is unclear how such very hard spectra can be formed; if the emission mechanism is similar to that operating in their hard states (i.e., up-scattering of soft photons due to hot electrons) then the electrons should have higher temperatures or a higher optical depth in the very hard state compared to those observed in the hard state. By using our obtained \Gamma as a tracer for the spectral evolution with luminosity, we have compared our results with those obtained by Wijnands et al. (2015). We confirm their general results in that also our sample of sources follow the same track as the other neutron star systems, although we do not find that the accreting millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA

Discovery of coherent millisecond X-ray pulsations in Aql X-1

We report the discovery of an episode of coherent millisecond X-ray pulsation in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for slightly more than 150 seconds, during which the pulse frequency is consistent with being constant. No X-ray burst or other evidence of thermonuclear burning activity is seen in correspondence with the pulsation, which can thus be identified as occurring in the persistent emission. The pulsation frequency is 550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from burst oscillations in this source. Hence we identify this frequency with the neutron star spin frequency. The pulsed fraction is strongly energy dependent, ranging from 10% (16-30 keV). We discuss possible physical interpretations and their consequences for our understanding of the lack of pulsation in most neutron star low-mass X-ray binaries. If interpreted as accretion-powered pulsation, Aql X-1 might play a key role in understanding the differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions. Slightly extended discussion. One author added. Uses emulateapj.cl

X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary

We present XMM-Newton and Chandra observations of the low-mass X-ray binary XSS J12270--4859, which experienced a dramatic decline in optical/X-ray brightness at the end of 2012, indicative of the disappearance of its accretion disk. In this new state, the system exhibits previously absent orbital-phase-dependent, large-amplitude X-ray modulations with a decline in flux at superior conjunction. The X-ray emission remains predominantly non-thermal but with an order of magnitude lower mean luminosity and significantly harder spectrum relative to the previous high flux state. This phenomenology is identical to the behavior of the radio millisecond pulsar binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray emission is produced in an intra-binary shock driven by the pulsar wind. This further demonstrates that XSS J12270-4859 no longer has an accretion disk and has transformed to a full-fledged eclipsing "redback" system that hosts an active rotation-powered millisecond pulsar. There is no evidence for diffuse X-ray emission associated with the binary that may arise due to outflows or a wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical Journa

A Chandra observation of the millisecond X-ray pulsar IGR J17511-3057

IGR J17511-3057 is a low mass X-ray binary hosting a neutron star and is one of the few accreting millisecond X-ray pulsars with X-ray bursts. We report on a 20ksec Chandra grating observation of IGR J17511-3057, performed on 2009 September 22. We determine the most accurate X-ray position of IGR J17511-3057, alpha(J2000) = 17h 51m 08.66s, delta(J2000) = -30deg 57' 41.0" (90% uncertainty of 0.6"). During the observation, a ~54sec long type-I X-ray burst is detected. The persistent (non-burst) emission has an absorbed 0.5-8keV luminosity of 1.7 x 10^36 erg/sec (at 6.9kpc) and can be well described by a thermal Comptonization model of soft, ~0.6keV, seed photons up-scattered by a hot corona. The type-I X-ray burst spectrum, with average luminosity over the 54sec duration L(0.5-8keV)=1.6 x 10^37 erg/sec, can be well described by a blackbody with kT_(bb)~1.6keV and R_(bb)~5km. While an evolution in temperature of the blackbody can be appreciated throughout the burst (average peak kT_(bb)=2.5(+0.8/-0.4)keV to tail kT_(bb)=1.3(+0.2/-0.1)keV), the relative emitting surface shows no evolution. The overall persistent and type-I burst properties observed during the Chandra observation are consistent with what was previously reported during the 2009 outburst of IGR J17511-3057.Comment: 6 pages, 4 figures, accepted for publication in ApJ (2012-06-08

The Advanced X-ray Timing Array (AXTAR)

AXTAR is an X-ray observatory mission concept, currently under study in the U.S., that combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR's main instrument is a collimated, thick Si pixel detector with 2-50 keV coverage and 8 square meters collecting area. For timing observations of accreting neutron stars and black holes, AXTAR provides at least an order of magnitude improvement in sensitivity over both RXTE and Constellation-X. AXTAR also carries a sensitive sky monitor that acts as a trigger for pointed observations of X-ray transients and also provides continuous monitoring of the X-ray sky with 20 times the sensitivity of the RXTE ASM. AXTAR builds on detector and electronics technology previously developed for other applications and thus combines high technical readiness and well understood cost.Comment: 4 pages with 1 figure, to appear in the proceedings of "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands et al. (AIP Conf. Proc.). Footnote and references adde

The pulse profile and spin evolution of the accreting pulsar in Terzan 5, IGR J17480-2446, during its 2010 outburst

(abridged) We analyse the spectral and pulse properties of the 11 Hz transient accreting pulsar, IGR J17480-2446, in the globular cluster Terzan 5, considering all the available RXTE, Swift and INTEGRAL observations performed between October and November, 2010. By measuring the pulse phase evolution we conclude that the NS spun up at an average rate of =1.48(2)E-12 Hz/s, compatible with the accretion of the Keplerian angular momentum of matter at the inner disc boundary. Similar to other accreting pulsars, the stability of the pulse phases determined by using the second harmonic component is higher than that of the phases based on the fundamental frequency. Under the assumption that the second harmonic is a good tracer of the neutron star spin frequency, we successfully model its evolution in terms of a luminosity dependent accretion torque. If the NS accretes the specific Keplerian angular momentum of the in-flowing matter, we estimate the inner disc radius to lie between 47 and 93 km when the luminosity attains its peak value. Smaller values are obtained if the interaction between the magnetic field lines and the plasma in the disc is considered. The phase-averaged spectrum is described by thermal Comptonization of photons with energy of ~1 keV. A hard to soft state transition is observed during the outburst rise. The Comptonized spectrum evolves from a Comptonizing cloud at an electron temperature of ~20 keV towards an optically denser cloud at kT_e~3 keV. At the same time, the pulse amplitude decreases from 27% to few per cent and becomes strongly energy dependent. We discuss various possibilities to explain such a behaviour, proposing that at large accretion luminosities a significant fraction of the in-falling matter is not channelled towards the magnetic poles, but rather accretes more evenly onto the NS surface.Comment: To appear in MNRA

Picosecond q-switched 1064/532 nm laser in tattoo removal. our single center experience

Background: Tattoo removal is becoming increasingly popular, and Q-switched lasers represent the gold standard in the treatment of this condition. In this study, we report our experience with a new Q-switched picosecond laser device, evaluating its effectiveness and safety. Methods: A total of 34 patients asking for tattoo removal were consecutively enrolled in this open study. The clinicians decided on operating settings based on the Fitzpatrick phototype, the type of tattoo, and the tattoo location. A maximum of seven sessions, with a minimum interval of eight weeks between each session, were performed. At the six month follow-up visit following the last treatment session, patient satisfaction was assessed using a visual analogue scale and two dermatologists evaluated the aesthetic outcome based on pictures taken before and after treatment. Results: A total of 34 patients were included and analyzed: 17 females (50%) and 17 males (50%). The mean patient age was 43.6 ± 11 years. Participants’ Fitzpatrick skin type ranged from II to IV. The mean number of treatment sessions performed was 3.3 ± 2.0 per patient. Over 40% of patients showed complete removal of the tattoo, with most of the patients indicating satisfaction with the treatment. Conclusions: The Q-switched 1064/532 nm laser may be considered the gold standard treatment for tattoo removal. Picosecond pulses seem to guarantee fewer sessions and excellent results when compared to other laser systems in tattoo removal

Direct imaging of defect formation in strained organic flexible electronics by Scanning Kelvin Probe Microscopy

The development of new materials and devices for flexible electronics depends crucially on the understanding of how strain affects electronic material properties at the nano-scale. Scanning Kelvin-Probe Microscopy (SKPM) is a unique technique for nanoelectronic investigations as it combines non-invasive measurement of surface topography and surface electrical potential. Here we show that SKPM in non-contact mode is feasible on deformed flexible samples and allows to identify strain induced electronic defects. As an example we apply the technique to investigate the strain response of organic thin film transistors containing TIPS-pentacene patterned on polymer foils. Controlled surface strain is induced in the semiconducting layer by bending the transistor substrate. The amount of local strain is quantified by a mathematical model describing the bending mechanics. We find that the step-wise reduction of device performance at critical bending radii is caused by the formation of nano-cracks in the microcrystal morphology of the TIPS-pentacene film. The cracks are easily identified due to the abrupt variation in SKPM surface potential caused by a local increase in resistance. Importantly, the strong surface adhesion of microcrystals to the elastic dielectric allows to maintain a conductive path also after fracture thus providing the opportunity to attenuate strain effects