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

Missing cosmic metals revealed by X-ray absorption towards distant sources

The census of heavy elements (metals) produced by all stars through cosmic times up to present-day is limited to ~50%; of these only half are still found within their parent galaxy. The majority of metals is expelled from galaxies into the circumgalactic (or even more distant, intergalactic) space by powerful galactic winds, leaving unpleasant uncertainty on the amount, thermal properties and distribution of these key chemical species. These dispersed metals unavoidably absorb soft X-ray photons from distant sources. We show that their integrated contribution can be detected in the form of increasing X-ray absorption with distance, for all kinds of high-energy cosmic sources. Based on extensive cosmological simulations, we assess that $\sim$ 10\% of all cosmic metals reside in the intergalactic medium. Most of the X-ray absorption arises instead from a few discrete structures along the line of sight. These extended structures, possibly pin-pointing galaxy groups, contain million degree, metal-enriched gas, 100-1,000 times denser than the cosmic mean. An additional ~10% of cosmic metals could reside in this phase.Comment: Accepted for publication in Astronomy & Astrophysics. 9 pages, 4 figures, 1 tabl

Strong gravitational field light deflection in binary systems containing a collapsed star

Large light deflection angles are produced in the strong gravitational field regions around neutron stars and black holes. In the case of binary systems, part of the photons emitted from the companion star towards the collapsed object are expected to be deflected in the direction of the earth. Based on a semi-classical approach we calculate the characteristic time delays and frequency shifts of these photons as a function of the binary orbital phase. The intensity of the strongly deflected light rays is reduced by many orders of magnitude, therefore making the observations of this phenomenon extremely difficult. Relativistic binary systems containing a radio pulsar and a collapsed object are the best available candidates for the detection of the strongly deflected photons. Based on the accurate knowledge of their orbital parameters, these systems allow to predict accurately the delays of the pulses along the highly deflected path, such that the sensitivity to very weak signals can be substantially improved through coherent summation over long time intervals. We discuss in detail the cases of PSR 1913+16 and PSR 1534+12 and find that the system geometry is far more promising for the latter. The observation of the highly deflected photons can provide a test of general relativity in an unprecedented strong field regime as well as a tight constraint on the radius of the collapsed object.Comment: 7 pages, uuencoded, gzip'ed, postscript file with figures included. Accepted for pubblication in MNRA

Anelastic relaxation and 139^{139}La NQR in La2−x_{2-x}Srx_xCuO4_4 around the critical Sr content x=0.02

Anelastic relaxation and $^{139}$La NQR relaxation measurements in La$_{2-x}$Sr$_x$CuO$_4$ for Sr content x around 2 and 3 percent, are presented and discussed in terms of spin and lattice excitations and ordering processes. It is discussed how the phase diagram of La$_{2-x}$Sr$_x$CuO$_4$ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previous thought, with a transition to a quasi-long range ordered state at T = 150 K, as indicated by recent neutron scattering data. On the other hand, the $^{139}$La NQR spectra are compatible with a transition to a conventional AF phase around T = 50 K, in agreement with the phase diagram commonly accepted in the literature. In this case the relaxation data, with a peak of magnetic origin in the relaxation rate around 150 K at 12 MHz and the anelastic counterparts around 80 K in the kHz range, yield the first evidence in La$_{1.98}$Sr$_{0.02}$CuO$_4$ of freezing involving simultaneously lattice and spin excitations. This excitation could correspond to the motion of charged stripes.Comment: 10 pages, 8 figure

A new flaring high energy gamma-ray source

We report the detection of a new gamma-ray source in the Fermi-LAT sky using a source detection tool based on the Minimal Spanning Tree algorithm. The source, not reported in previous LAT catalogues but very recently observed in the X-rays and optical bands, is characterized by an increasing gamma-ray activity in 2012 June-September, reaching a weekly peak flux of (3.3+-0.6)*10^-7 photons cm^-2 s^-1. A search for a possible counterpart provides indication that it can be associated with the radio source NVSS J141828+354250 whose optical SDSS colours are typical of a blazar.Comment: 4 pages, 3 figures. Accepted for publication in Astronomy & 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