arXiv subjects

A. Sartori

Publications and source records attributed to A. Sartori.

9 recordsLinked to original sources

arXiv2014

Persistent currents in two-component condensates in a toroidal trap

The stability of persistent currents in a two-component Bose-Einstein condensate in a toroidal trap is studied both in the miscible and immiscible regimes. In the miscible regime we show that superflow decay is related to linear instabilities of the spin-density Bogoliubov mode. We find a region of partial stability, where the flow is stable in the majority component while it decays in the minority component. We also characterize the dynamical instability appearing for a large relative velocity between the two components. In the immiscible regime the stability criterion is modified and depends on the specific density distribution of the two components. The effect of a coherent coupling between the two components is also discussed.

cond-mat.quant-gas↗

arXiv2011

A time-variable, phase-dependent emission line in the X-ray spectrum of the isolated neutron star RX J0822-4300

RX J0822-4300 is the Central Compact Object associated with the Puppis A supernova remnant. Previous X-ray observations suggested RX J0822-4300 to be a young neutron star with a weak dipole field and a peculiar surface temperature distribution dominated by two antipodal spots with different temperatures and sizes. An emission line at 0.8 keV was also detected. We performed a very deep (130 ks) observation with XMM-Newton, which allowed us to study in detail the phase-resolved properties of RX J0822-4300. Our new data confirm the existence of a narrow spectral feature, best modelled as an emission line, only seen in the `Soft' phase interval - when the cooler region is best aligned to the line of sight. Surprisingly, comparison of our recent observations to the older ones yields evidence for a variation in the emission line component, which can be modelled as a decrease in the central energy from ~0.80 keV in 2001 to ~0.73 keV in 2009--2010. The line could be generated via cyclotron scattering of thermal photons in an optically thin layer of gas, or - alternatively - it could originate in low-rate accretion by a debris disk. In any case, a variation in energy, pointing to a variation of the magnetic field in the line emitting region, cannot be easily accounted for.

astro-ph.HE↗

arXiv2010

HST and VLT observations of the neutron star 1E 1207.4-5209

1E 1207.4-5209, the peculiar Central Compact object in the G296.5+10.0 supernova remnant, has been proposed to be an "anti-magnetar" - a young neutron star born with a weak dipole field. Accretion, possibly of supernova fallback material, has also been invoked to explain a large surface temperature anisotropy as well as the generation of peculiar cyclotron absorption features superimposed to its thermal spectrum. Interestingly enough, a faint optical/infrared source was proposed as a possible counterpart to 1E 1207.4-5209, but later questioned, based on coarse positional coincidence. Considering the large offset of 1E 1207.4-5209 with respect to the center of its host supernova remnant, the source should move at ~70 mas/yr. Thus, we tested the association by measuring the proper motion of the proposed optical counterpart. Using HST observations spanning 3.75 years, we computed a 3 sigma upper limit of 7 mas/yr. Absolute astrometry on the same HST data set also places the optical source significantly off the 99% confidence Chandra position. This allows us to safely rule out the association. Using the HST data set, coupled to ground-based observations collected at the ESO/VLT, we set the deepest limits ever obtained to the optical/infrared emission from 1E 1207.4-5209. By combining such limits to the constraints derived from X-ray timing, we rule out accretion as the source of the thermal anisotropy of the neutron star.

astro-ph.HE↗

arXiv2010

HST/WFPC2 observations of the LMC pulsar PSR B0540-69

The study of the younger, and brighter, pulsars is important to understand the optical emission properties of isolated neutron stars. PSRB0540-69, the second brightest (V~22) optical pulsar, is obviously a very interesting target for these investigations. The aim of this work is threefold: constraining the pulsar proper motion and its velocity on the plane of the sky through optical astrometry, obtaining a more precise characterisation of the pulsar optical spectral energy distribution (SED) through a consistent set of multi-band, high-resolution, imaging photometry observations, measuring the pulsar optical phase-averaged linear polarisation, for which only a preliminary and uncertain measurement was obtained so far from ground-based observations. We performed high-resolution observations of PSRB0540-69 with the WFPC2 aboard the HST, in both direct imaging and polarimetry modes. From multi-epoch astrometry we set a 3sigma upper limit of 1 mas/yr on the pulsar proper motion, implying a transverse velocity <250 km/s at the 50 kpc LMC distance. Moreover, we determined the pulsar absolute position with an unprecedented accuracy of 70 mas. From multi-band photometry we characterised the pulsar power-law spectrum and we derived the most accurate measurement of the spectral index (0.70+/-0.07) which indicates a spectral turnover between the optical and X-ray bands. Finally, from polarimetry we obtained a new measurement of the pulsar phase-averaged polarisation degree (16+/-4%),consistent with magnetosphere models depending on the actual intrinsic polarisation degree and depolarisation factor, and we found that the polarisation vector (22+/-12deg position angle) is possibly aligned with the semi-major axis of the pulsar-wind nebula and with the apparent proper motion direction of its bright emission knot.

astro-ph.SR↗

arXiv2002

Critical wetting in power-law wedge geometries

We investigate critical wetting transitions for fluids adsorbed in wedge-like geometries where the substrate height varies as a power-law, $z(x,y) \sim |x| ^γ$, in one direction. As $γ$ is increased from 0 to 1, the substrate shape is smoothly changed from a planar-wall to a linear wedge. The continuous wetting and filling transitions pertinent to these limiting geometries are known to have distinct phase boundaries and critical singularities. We predict that the intermediate critical wetting behaviour occurring for $0<γ< 1$ falls into one of {\it{three}} possible regimes depending on the values of $γ$, p and q. The unbinding behaviour is characterised by a high degree of non-universality, strongly anisotropic correlations and enhanced interfacial roughness. The shift in phase boundary and emergence of universal critical behaviour in the linear wedge limit is discussed in detail.

cond-mat.soft↗

arXiv1999

Wetting at Non-Planar Substrates: Unbending & Unbinding

We consider fluid wetting on a corrugated substrate using effective interfacial Hamiltonian theory and show that breaking the translational invariance along the wall can induce an 'unbending' phase transition in addition to unbinding. Both first order and second order unbending transitions can occur at and out of coexistence. Results for systems with short-ranged and long-ranged forces establish that the unbending critical point is characterised by hyperuniversal scaling behaviour. We show that, at bulk coexistence, the adsorption at the unbending critical point is a universal multiple of the adsorption for the correspondent planar system.

cond-mat.soft↗

arXiv1997

Searching the Higgs with the Neurochip TOTEM

We show that neural network classifiers can be helpful in discriminating Higgs production events from the huge background at LHC, assuming the case of a mass value $M_H \sim 200$ GeV. We use the high performance neurochip TOTEM, trained by the Reactive Tabu Search algorithm (RTS), which could be used for on-line purposes. Two different sets of input variables are compared.

hep-ex↗

arXiv1997

Role of Neural Networks in the Search of the Higgs Boson at LHC

We show that neural network classifiers can be helpful to discriminate Higgs production from background at LHC in the Higgs mass range M= 200 GeV. We employ a common feed-forward neural network trained by the backpropagation algorithm for off-line analysis and the neural chip Totem, trained by the Reactive Tabu Search algorithm, which could be used for on-line analysis.

hep-ex↗

arXiv1997

Totem: a case study in HEP

It is being proved that the neurochip \Totem{} is a viable solution for high quality and real time computational tasks in HEP, including event classification, triggering and signal processing. The architecture of the chip is based on a "derivative free" algorithm called Reactive Tabu Search (RTS), highly performing even for low precision weights. ISA, VME or PCI boards integrate the chip as a coprocessor in a host computer. This paper presents: 1) the state of the art and the next evolution of the design of \Totem{}; 2) its ability in the Higgs search at LHC as an example.

hep-ex↗