Searching for Very High Energy Emission from Pulsars Using the High Altitude Water Cherenkov (HAWC) Observatory

There are currently over 160 known gamma-ray pulsars. While most of them are detected only from space, at least two are now seen also from the ground. MAGIC and VERITAS have measured the gamma ray pulsed emission of the Crab pulsar up to hundreds of GeV and more recently MAGIC has reported emission at $\sim2$ TeV. Furthermore, in the Southern Hemisphere, H.E.S.S. has detected the Vela pulsar above 30 GeV. In addition, non-pulsed TeV emission coincident with pulsars has been detected by many groups, including the Milagro Collaboration. These GeV-TeV observations open the possibility of searching for very-high-energy (VHE, > 100GeV) pulsations from gamma-rays pulsars in the HAWC field of view.Comment: Presented at the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. See arXiv:1508.03327 for all HAWC contribution

Analytical Modeling of a New Compliant Microsystem for Atherectomy Operations

This work offers a new alternative tool for atherectomy operations, with the purpose of minimizing the risks for the patients and maximizing the number of clinical cases for which the system can be used, thanks to the possibility of scaling its size down to lumen reduced to a few tenths of mm. The development of this microsystem has presented a certain theoretical work during the kinematic synthesis and the design stages. In the first stage a new multi-loop mechanism with a Stephenson’s kinematic chain (KC) was found and then adopted as the so-called pseudo-rigid body mechanism (PRBM). Analytical modeling was necessary to verify the synthesis requirements. In the second stage, the joint replacement method was applied to the PRBM to obtain a corresponding and equivalent compliant mechanism with lumped compliance. The latter presents two loops and six elastic joints and so the evaluation of the microsystem mechanical advantage (MA) had to be calculated by taking into account the accumulation of elastic energy in the elastic joints. Hence, a new closed form expression of the microsystem MA was found with a method that presents some new aspects in the approach. The results obtained with Finite Element Analysis (FEA) were compared to those obtained with the analytical model. Finally, it is worth noting that a microsystem prototype can be fabricated by using MEMS Technology classical methods, while the microsystem packaging could be a further development for the present investigation

Supergiant Fast X-ray Transients uncovered by the EXTraS project: flares reveal the development of magnetospheric instability in accreting neutron stars

The low luminosity, X-ray flaring activity, of the sub-class of high mass X-ray binaries called Supergiant Fast X-ray Transients, has been investigated using XMM-Newton public observations, taking advantage of the products made publicly available by the EXTraS project. One of the goals of EXTraS was to extract from the XMM-Newton public archive information on the aperiodic variability of all sources observed in the soft X-ray range with EPIC (0.2-12 keV). Adopting a Bayesian block decomposition of the X-ray light curves of a sample of SFXTs, we picked out 144 X-ray flares, covering a large range of soft X-ray luminosities (1e32-1e36 erg/s). We measured temporal quantities, like the rise time to and the decay time from the peak of the flares, their duration and the time interval between adjacent flares. We also estimated the peak luminosity, average accretion rate and energy release in the flares. The observed soft X-ray properties of low-luminosity flaring activity from SFXTs is in qualitative agreement with what is expected by the application of the Rayleigh-Taylor instability model in accreting plasma near the neutron star magnetosphere. In the case of rapidly rotating neutron stars, sporadic accretion from temporary discs cannot be excluded.Comment: Accepted for publication in MNRAS (accepted 2019 May 1; received 2019 April 30; in original form 2019 February 25). 22 pages, 16 figures, 3 tables

A candidate optical counterpart to the middle-aged gamma-ray pulsar PSR J1741-2054

We carried out deep optical observations of the middle-aged $\gamma$-ray pulsar PSR J1741-2054 with the Very Large Telescope (VLT). We identified two objects, of magnitudes $m_v=23.10\pm0.05$ and $m_v=25.32\pm0.08$, at positions consistent with the very accurate Chandra coordinates of the pulsar, the faintest of which is more likely to be its counterpart. From the VLT images we also detected the known bow-shock nebula around PSR J1741-2054. The nebula is displaced by \sim 0\farcs9 (at the $3\sigma$ confidence level) with respect to its position measured in archival data, showing that the shock propagates in the interstellar medium consistently with the pulsar proper motion. Finally, we could not find evidence of large-scale extended optical emission associated with the pulsar wind nebula detected by Chandra, down to a surface brightness limit of $\sim 28.1$ magnitudes arcsec$^{-2}$. Future observations are needed to confirm the optical identification of PSR J1741-2054 and characterise the spectrum of its counterpart.Comment: 8 pages, 3 figures, Astrophysical Journal, in pres

Nitrogen and oxygen abundances in the Local Universe

We present chemical evolution models aimed at reproducing the observed (N/O) vs. (O/H) abundance pattern of star forming galaxies in the Local Universe. We derive gas-phase abundances from SDSS spectroscopy and a complementary sample of low-metallicity dwarf galaxies, making use of a consistent set of abundance calibrations. This collection of data clearly confirms the existence of a plateau in the (N/O) ratio at very low metallicity, followed by an increase of this ratio up to high values as the metallicity increases. This trend can be interpreted as due to two main sources of nitrogen in galaxies: i) massive stars, which produce small amounts of pure primary nitrogen and are responsible for the (N/O) ratio in the low metallicity plateau; ii) low- and intermediate-mass stars, which produce both secondary and primary nitrogen and enrich the interstellar medium with a time delay relative to massive stars, and cause the increase of the (N/O) ratio. We find that the length of the low-metallicity plateau is almost solely determined by the star formation efficiency, which regulates the rate of oxygen production by massive stars. We show that, to reproduce the high observed (N/O) ratios at high (O/H), as well as the right slope of the (N/O) vs. (O/H) curve, a differential galactic wind - where oxygen is assumed to be lost more easily than nitrogen - is necessary. No existing set of stellar yields can reproduce the observed trend without assuming differential galactic winds. Finally, considering the current best set of stellar yields, a bottom-heavy initial mass function is favoured to reproduce the data.FV thanks the Cavendish Astrophysics Group at the University of Cambridge for kindly supporting his visit during 2014 September. FB acknowledges funding from the United Kingdom Science and Technology Facilities Council (STFC). RM acknowledges funding from the United Kingdom STFC through grant ST/M001172/1. FM acknowledges financial support from PRIN-MIUR 2010-2011 project ‘The Chemical and Dynamical Evolution of the Milky Way and Local Group Galaxies’, prot. 2010LY5N2T.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw53

X-ray observations and the search for Fermi-LAT gamma-ray pulsars

The Large Area Telescope (LAT) on Fermi has detected ~150 gamma-ray pulsars, about a third of which were discovered in blind searches of the $\gamma$-ray data. Because the angular resolution of the LAT is relatively poor and blind searches for pulsars (especially millisecond pulsars, MSPs) are very sensitive to an error in the position, one must typically scan large numbers of locations. Identifying plausible X-ray counterparts of a putative pulsar drastically reduces the number of trials, thus improving the sensitivity of pulsar blind searches with the LAT. I discuss our ongoing program of Swift, XMM-Newton, and Chandra observations of LAT unassociated sources in the context of our blind searches for gamma-ray pulsars.Comment: 6 pages, 5 Figures, submitted to AN, proceedings of the workshop "The Fast and the Furious: Energetic Phenomena in Isolated Neutron Stars, Pulsar Wind Nebulae and Supernova Remnants" ESAC, Madrid, Spain 22nd - 24th May 201

Multi-wavelength observations of 3FGL J2039.6-5618: a candidate redback millisecond pulsar

We present multi-wavelength observations of the unassociated gamma-ray source 3FGL J2039.6-5618 detected by the Fermi Large Area Telescope. The source gamma-ray properties suggest that it is a pulsar, most likely a millisecond pulsar, for which neither radio nor $\gamma$-ray pulsations have been detected yet. We observed 3FGL J2039.6-5618 with XMM-Newton and discovered several candidate X-ray counterparts within/close to the gamma-ray error box. The brightest of these X-ray sources is variable with a period of 0.2245$\pm$0.0081 d. Its X-ray spectrum can be described by a power law with photon index $\Gamma_X =1.36\pm0.09$, and hydrogen column density $N_{\rm H} < 4 \times 10^{20}$ cm$^{-2}$, which gives an unabsorbed 0.3--10 keV X-ray flux of $1.02 \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. Observations with the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) discovered an optical counterpart to this X-ray source, with a time-average magnitude $g'\sim 19.5$. The counterpart features a flux modulation with a period of 0.22748$\pm$0.00043 d that coincides, within the errors, with that of the X-ray source, confirming the association based on the positional coincidence. We interpret the observed X-ray/optical periodicity as the orbital period of a close binary system where one of the two members is a neutron star. The light curve profile of the companion star, with two asymmetric peaks, suggests that the optical emission comes from two regions at different temperatures on its tidally-distorted surface. Based upon its X-ray and optical properties, we consider this source as the most likely X-ray counterpart to 3FGL J2039.6-5618, which we propose to be a new redback system.Comment: 35 pages, 8 figures, accepted for publication on Astrophysical Journa

Nuclear data uncertainty quantification on PWR spent nuclear fuel as a function of burnup

Nuclear data uncertainty analysis on the spent nuclear fuel inventory was performed on the Takahama-3 NT3G23 assembly, where the sample SF95-4 was irradiated up to a burnup of approximately 36 GWd/ t according to the SFCOMPO benchmark. The cross-section covariance matrices stored in the ENDF/B-VIII.0, JEFF-3.3 and JENDL-4.0u evaluated nuclear data libraries were propagated with the stochastic sampling algorithms implemented in the SANDY code. A comparison of the concentration uncertainty differences obtained using data from the three libraries is reported. Similarities were found with the fuel composition uncertainty results obtained for the Calvert Cliffs MKP109 sample P SFCOMPO benchmark. Such a similarity was also found when comparing concentration uncertainties along the sample irradiation. Therefore, the main contributors to the concentration uncertainty of a number of nuclides were identified at different burnup levels in the two samples. To complement the similarity analysis, a correlation study of the concentration distributions predicted by the two models was performed. The reported results hint a dominance of the common uncertainty propagation mechanisms over the model differences in the determination of concentration uncertainty