European Pulsar Timing Array limits on an isotropic stochastic gravitational-wave background
We present new limits on an isotropic stochastic gravitational-wave background (GWB) using a six pulsar data set spanning 18 yr of observations from the 2015 European Pulsar Timing Array data release. Performing a Bayesian analysis, we fit simultaneously for the intrinsic noise parameters for each pulsar, along with common correlated signals including clock, and Solar system ephemeris errors, obtaining a robust 95 per cent upper limit on the dimensionless strain amplitude A of the background of A < 3.0 × 10^(−15) at a reference frequency of 1 yr^(−1) and a spectral index of 13/3, corresponding to a background from inspiralling supermassive black hole binaries, constraining the GW energy density to Ω_(gw)(f)h^2 < 1.1 × 10^(−9) at 2.8 nHz. We also present limits on the correlated power spectrum at a series of discrete frequencies, and show that our sensitivity to a fiducial isotropic GWB is highest at a frequency of ∼5 × 10^(−9) Hz. Finally, we discuss the implications of our analysis for the astrophysics of supermassive black hole binaries, and present 95 per cent upper limits on the string tension, Gμ/c^2, characterizing a background produced by a cosmic string network for a set of possible scenarios, and for a stochastic relic GWB. For a Nambu–Goto field theory cosmic string network, we set a limit Gμ/c^2 < 1.3 × 10^(−7), identical to that set by the Planck Collaboration, when combining Planck and high-ℓ cosmic microwave background data from other experiments. For a stochastic relic background, we set a limit of Ω^(relic)_(gw)(f)h^2 <1.2×10^(−9), a factor of 9 improvement over the most stringent limits previously set by a pulsar timing array.
© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 July 8. Received 2015 June 23. In original form 2015 April 14. First published online August 31, 2015. The EPTA is a collaboration between European institutes namely ASTRON (NL), INAF/Osservatorio di Cagliari (IT), Max-Planck-Institut fr Radioastronomie (GER), Nanay/Paris Observatory (FRA), the University of Manchester (UK), the University of Birmingham (UK), the University of Cambridge (UK) and the University of Bielefeld (GER), with the aim to provide high-precision pulsar timing to work towards the direct detection of low-frequency GWs. An Advanced Grant of the European Research Council to implement the Large European Array for Pulsars (LEAP) also provides funding. Part of this work is based on observations with the 100-m telescope of the Max-Planck-Institut für Radioastronomie (MPIfR) at Effelsberg. The Nançay radio Observatory is operated by the Paris Observatory, associated with the French Centre National de la Recherche Scientifique (CNRS). We acknowledge financial support from 'Programme National de Cosmologie and Galaxies' (PNCG) of CNRS/INSU, France. Pulsar research at the Jodrell Bank Centre for Astrophysics and the observations using the Lovell Telescope is supported by a consolidated grant from the STFC in the UK. We thank A. G. Lyne and C. A. Jordan for carrying out the pulsar observations at JBCA. The Westerbork Synthesis Radio Telescope is operated by the Netherlands Institute for Radio Astronomy (ASTRON) with support from The Netherlands Foundation for Scientific Research NWO. This research was performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council, and the Zwicky computer cluster at Caltech is supported by the NSF under MRI-R2 award no. PHY-0960291 and by the Sherman Fairchild Foundation. LL was supported by a Junior Research Fellowship at Trinity Hall College, Cambridge University. ST was supported by appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities through a contract with NASA. CMFM was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme and would like to thank Paul Lasky for useful discussions. AS and JG are supported by the Royal Society. SAS would like to thank Richard Battye for various discussions and comments regarding the cosmic string section in this paper, and acknowledges funding from an NWO Vidi fellowship (PI JWTH). RNC acknowledges the support of the International Max Planck Research School Bonn/Cologne and the Bonn-Cologne Graduate School. KJL is supported by the National Natural Science Foundation of China (Grant no.11373011). RvH is supported by NASA Einstein Fellowship grant PF3-140116. JWTH acknowledges funding from an NWO Vidi fellowship and ERC Starting Grant 'DRAGNET' (337062). PL acknowledges the support of the International Max Planck Research School Bonn/Cologne. KL acknowledges the financial support by the European Research Council for the ERC Synergy Grant BlackHoleCam under contract no. 610058. SO is supported by the Alexander von Humboldt Foundation.
Submitted - 1504.03692v3.pdf
Published - MNRAS-2015-Lentati-2576-98.pdf