We search for continuous gravitational waves ( CGWs ) produced by individual super-massive black-hole binaries ( SMBHBs ) in circular orbits using high-cadence timing observations of PSR J1713 + 0747 .
We observe this millisecond pulsar using the telescopes in the European Pulsar Timing Array ( EPTA ) with an average cadence of approximately 1.6 days over the period between April 2011 and July 2015 , including an approximately daily average between February 2013 and April 2014 .
The high-cadence observations are used to improve the pulsar timing sensitivity across the GW frequency range of 0.008 - 5 \upmu Hz .
We use two algorithms in the analysis , including a spectral fitting method and a Bayesian approach .
For an independent comparison , we also use a previously published Bayesian algorithm .
We find that the Bayesian approaches provide optimal results and the timing observations of the pulsar place a 95 per cent upper limit on the sky-averaged strain amplitude of CGWs to be \lesssim 3.5 \times 10 ^ { -13 } at a reference frequency of 1 \upmu Hz .
We also find a 95 per cent upper limit on the sky-averaged strain amplitude of low-frequency CGWs to be \lesssim 1.4 \times 10 ^ { -14 } at a reference frequency of 20 nHz .