A study of spatial correlations in pulsar timing array data

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).