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 x 10(-15) < A < 1.5 x 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 approximate to 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(9) M-circle dot out to a distance of about 25 Mpc, and with M-c > 10(10) M-circle dot out to a distance of about 1Gpc (z approximate to 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.'Programme National de Cosmologie and Galaxies' (PNCG) of CNRS/INSU, France; STFC in the UK; Netherlands Foundation for Scientific Research NWO; Higher Education Funding Council for England; Science and Technology Facilities Council; Vulcan cluster of MPIfG-AEI (Golm-Germany); National Science Foundation [PHYS-1066293]; Junior Research Fellowship at Trinity Hall College, Cambridge University; NASA Postdoctoral Program at the Jet Propulsion Laboratory; NASA; Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme; Royal Society; NWO Vidi fellowship; International Max Planck Research School Bonn/Cologne; Bonn-Cologne Graduate School; National Natural Science Foundation of China [11373011]; NASA Einstein Fellowship [PF3-140116]; ERC [337062]; Alexander von Humboldt FoundationSCI(E)ARTICLEstba@aei.mpg.de; petiteau@apc.in2p3.fr; asesana@star.sr.bham.ac.uk21665-167945