Limits on the strength of individual gravitational wave sources using high-cadence observations of PSR B1937+21

We present the results of a search for gravitational waves (GWs) from individual sources using high-cadence observations of PSR B1937+21. The data were acquired from an intensive observation campaign with the Lovell telescope at Jodrell Bank, between 2011 June and 2013 May. The almost daily cadence achieved allowed us to be sensitive to GWs with frequencies up to 4.98 x 10(-6) Hz, extending the upper bound of the typical frequency range probed by pulsar timing arrays. We used observations taken at three different radio frequencies with the Westerbork Synthesis Radio Telescope in order to correct for dispersion measure effects and scattering variances. The corrected timing residuals exhibited an unmodelled periodic noise with an amplitude of 150 ns and a frequency of 3.4 yr(-1). As the signal is not present in the entire data set, we attributed it to the rotational behaviour of the pulsar, ruling out the possibilities of being either due to a GW or an asteroid as the cause. After removing this noise component, we placed limits on the GW strain of individual sources equalling to h(s) = 1.53 x 10(-11) and hs = 4.99 x 10(-14) at 10(-7) Hz for random and optimal sources locations, respectively.