Southwest University

About: Southwest University is a education organization based out in Chongqing, China. It is known for research contribution in the topics: Gene & Population. The organization has 29772 authors who have published 27755 publications receiving 409441 citations. The organization is also known as: Southwest University in Chongqing & SWU.

A study of spatial correlations in pulsar timing array data

Abstract: Mon. Not. R. Astron. Soc. 000, 1–13 () Printed 9 October 2015 (MN L A TEX style file v2.2) arXiv:1510.02363v1 [astro-ph.IM] 8 Oct 2015 A study of spatial correlations in pulsar timing array data C. Tiburzi 1,2,3,4⋆ , G. Hobbs 5 , M. Kerr 5 , W. A. Coles 6 , S. Dai 5,7 , R. N. Manchester 5 , A. Possenti 1 , R. M. Shannon 5,8 , X. P. You 9 1 INAF - Osservatorio Astronomico di Cagliari, Via della Scienza, 09047 Selargius (CA), Italy di Fisica, Universit`a di Cagliari, Cittadella Universitaria 09042 Monserrato (CA), Italy 3 Max-Planck-Institut f¨ ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany 4 Fakult¨ at f¨ur Physik, Universit¨at Bielefeld, Postfach 100131, 33501 Bielefeld, Germany 5 CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia 6 Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093-0407, USA 7 Department of Astronomy, School of Physics, Peking University, Beijing, 100871, China 8 Australia and International Centre for Radio Astronomy Research, Curtin University, Bentley WA 6102, Australia 9 School of Physical Science and Technology, Southwest University, Chongqing 400715, China 2 Dipartimento Accepted on 2015 September 9 ABSTRACT Pulsar timing array experiments search for phenomena that produce angular correlations in the arrival times of signals from millisecond pulsars. The primary goal is to detect an isotropic and stochastic gravitational wave background. We use simulated data to show that this search can be affected by the presence of other spatially correlated noise, such as errors in the reference time standard, errors in the planetary ephemeris, the solar wind and instrumentation issues. All these effects can induce significant false detections of gravitational waves. We test mitigation routines to account for clock errors, ephemeris errors and the solar wind. We demonstrate that it is non-trivial to find an effective mitigation routine for the planetary ephemeris and emphasise that other spatially correlated signals may be present in the data. Key words: pulsar: general – gravitational waves – methods: numerical 1 INTRODUCTION The pulsar timing method provides a way to study the physics and astrometry of individual pulsars, the properties of the intervening interstellar medium and to search for un-modeled phenomena. Pul- sar times-of-arrival (ToAs) are first measured at radio observato- ries and then converted to the reference frame of the solar-system barycenter (SSB). The extreme rotational stability of pulsars en- able these barycentric ToAs to be predicted using a simple model (known as the “pulsar ephemeris”) for the pulsar’s rotational and orbital motion. Differences between the measured and predicted ToAs are referred to as “timing residuals”. Features in the timing residuals (which imply a temporal correlation between the ToAs of an individual pulsar) indicate that the model with which we are describing the pulsar is not complete. It may not account for some events, such as glitches (e.g., Wang et al. 2012), variations in the interstellar medium (e.g., Keith et al. 2013) or intrinsic instabilities in the pulsar rotation (e.g., Shannon & Cordes 2010). The power E-mail:ctiburzi@oa-cagliari.inaf.it c RAS spectrum of such residuals is often “red”, that is, characterized by an excess of power at lower fluctuation frequencies. In contrast, “white” power spectra have statistically the same amount of power for each frequency. An iterative process is followed to improve the pulsar model in order to minimize the residuals. Numerous stud- ies have already been carried out using this technique, including tests of theories of gravity (e.g., Kramer et al. 2006), analyses of the Galactic pulsar population (e.g., Lorimer et al. 2006) and the interstellar medium (e.g., Keith et al. 2013). Pulsar Timing Array (PTA) projects 1 are employing this tech- nique to study phenomena that simultaneously affect multiple pul- sars. A main goal of PTA experiments is the direct detection of 1 There are three collaborations in the world that are leading PTA experiments: the Parkes PTA (PPTA) in Australia (Manchester et al. 2013), the European PTA (EPTA) in Europe (Kramer & Champion 2013) and the North America Nanohertz Observatory for gravitational waves (NANOgrav) in North America (McLaughlin 2013). These teams have joined together to establish the International PTA (IPTA; Hobbs et al. 2010; Manchester & IPTA 2013).

Ultra-broadband linear polarization converter based on anisotropic metasurface

Abstract: Through the manipulation of co-polarized reflection and cross-polarized reflection from a periodic array of metal-dielectric-metal resonators, a plethora of unprecedented metamaterial devices have been successfully demonstrated, such as perfect absorber and polarization converter. Recently, some broadband absorbers based on anisotropic resonators have been reported, which are actually poor absorbers when the cross-polarized reflection is considered. Here, we demonstrate that an ultra-wideband and high-efficiency reflective cross-polarization convertor can be achieved by breaking the symmetry of the resonator unit of a perfect absorber. Simulation results show that the polarization conversion ratio of the proposed metasurface is above 90% in the frequency range from 6.67 to 17.1 GHz and the relative bandwidth reaches 87.7%. The experimental results are in good agreement with the simulation results. The method paves a new way for the design of broadband polarization convertor, which can also be extended to the terahertz band.

One-pot green synthesis of graphene oxide/gold nanocomposites as SERS substrates for malachite green detection

Abstract: In this contribution, graphene oxide/gold nanoparticle (GO/AuNPs) hybrids were in situ fabricated through a green one-pot procedure by using tyrosine as an environment friendly and biocompatible reducing agent, which can be used as highly efficient surface enhanced Raman scattering (SERS) substrates with the enhancement factor at 3.8 × 103. The as-prepared GO/AuNPs hybrids have good biocompatibility, providing the prospect of applications for biomedicine determinations. In addition, taking the advantages of the electromagnetic and chemical enhancement mechanism and the high affinity of GO and AuNPs towards positive dyes, a sensitive, selective and label-free malachite green (MG) detection method was demonstrated. The SERS measurement showed that the minimum detection concentration of MG in water was as low as 2.5 μmol L−1 with a linear response range from 2.5 to 100 μmol L−1 (R2 = 0.996). Moreover, this method can be applied to detect MG in a fishery water sample with satisfactory results.