European Pulsar Timing Array limits on continuous gravitational waves from individual supermassive black hole binaries
We have searched for continuous gravitational wave (CGW) signals produced by individually resolvable, circular supermassive black hole binaries (SMBHBs) in the latest European Pulsar Timing Array (EPTA) data set, which consists of ultraprecise timing data on 41-ms pulsars. We develop frequentist and Bayesian detection algorithms to search both for monochromatic and frequency-evolving systems. None of the adopted algorithms show evidence for the presence of such a CGW signal, indicating that the data are best described by pulsar and radiometer noise only. Depending on the adopted detection algorithm, the 95 per cent upper limit on the sky-averaged strain amplitude lies in the range 6 × 10^(−15) < A < 1.5 × 10^(−14) at 5 nHz < f < 7 nHz. This limit varies by a factor of five, depending on the assumed source position and the most constraining limit is achieved towards the positions of the most sensitive pulsars in the timing array. The most robust upper limit – obtained via a full Bayesian analysis searching simultaneously over the signal and pulsar noise on the subset of ours six best pulsars – is A ≈ 10^(−14). These limits, the most stringent to date at f < 10 nHz, exclude the presence of sub-centiparsec binaries with chirp mass M_c>10^9M_⊙ out to a distance of about 25 Mpc, and with M_c>10^(10)M_⊙ out to a distance of about 1Gpc (z ≈ 0.2). We show that state-of-the-art SMBHB population models predict <1 per cent probability of detecting a CGW with the current EPTA data set, consistent with the reported non-detection. We stress, however, that PTA limits on individual CGW have improved by almost an order of magnitude in the last five years. The continuing advances in pulsar timing data acquisition and analysis techniques will allow for strong astrophysical constraints on the population of nearby SMBHBs in the coming years.
© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 September 6. Received 2015 August 31. In original form 2015 June 24. First published online November 19, 2015. 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. 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 several supercomputers: the CCIN2P3 computer cluster of the CNRS-IN2P3 (Lyon-France), the ARAGO computer cluster of the François Arago Centre (Paris-France), 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 Vulcan cluster of MPIfG- AEI (Golm-Germany). The authors acknowledge the support of VirtualData from LABEX P2IO for providing computing resources through its StratusLab cloud. This work was supported in part by the National Science Foundation under grant no. PHYS-1066293 and the hospitality of the Aspen Center for Physics. 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. AS and JG are supported by the Royal Society. SAS 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. SO is supported by the Alexander von Humboldt Foundation.
Submitted - MNRAS-2016-Babak-1665-79.pdf