Narrow-band search of continuous gravitational-wave signals from Crab and Vela pulsars in Virgo VSR4 data

In this paper we present the results of a coherent narrow-band search for continuous gravitationalwave
signals from the Crab and Vela pulsars conducted on Virgo VSR4 data. In order to take into
account a possible small mismatch between the gravitational wave frequency and two times the
star rotation frequency, inferred from measurement of the electromagnetic pulse rate, a range of
0.02 Hz around two times the star rotational frequency has been searched for both the pulsars. No
evidence for a signal has been found and 95% confidence level upper limits have been computed
both assuming polarization parameters are completely unknown and that they are known with some
uncertainty, as derived from X-ray observations of the pulsar wind torii. For Vela the upper limits
are comparable to the spin-down limit, computed assuming that all the observed spin-down is due
to the emission of gravitational waves. For Crab the upper limits are about a factor of two below
the spin-down limit, and represent a significant improvement with respect to past analysis. This is
the first time the spin-down limit is significantly overcome in a narrow-band search.