| Abstract: | In this paper we present the results of the first low frequency all-sky
search of continuous gravitational wave signals conducted on Virgo VSR2 and
VSR4 data. The search covered the full sky, a frequency range between 20 Hz and
128 Hz with a range of spin-down between $-1.0 imes 10^{-10}$ Hz/s and $+1.5
imes 10^{-11}$ Hz/s, and was based on a hierarchical approach. The starting
point was a set of short Fast Fourier Transforms (FFT), of length 8192 seconds,
built from the calibrated strain data. Aggressive data cleaning, both in the
time and frequency domains, has been done in order to remove, as much as
possible, the effect of disturbances of instrumental origin. On each dataset a
number of candidates has been selected, using the FrequencyHough transform in
an incoherent step. Only coincident candidates among VSR2 and VSR4 have been
examined in order to strongly reduce the false alarm probability, and the most
significant candidates have been selected. The criteria we have used for
candidate selection and for the coincidence step greatly reduce the harmful
effect of large instrumental artifacts. Selected candidates have been subject
to a follow-up by constructing a new set of longer FFTs followed by a further
incoherent analysis. No evidence for continuous gravitational wave signals was
found, therefore we have set a population-based joint VSR2-VSR4 90$\%$
confidence level upper limit on the dimensionless gravitational wave strain in
the frequency range between 20 Hz and 128 Hz. This is the first all-sky search
for continuous gravitational waves conducted at frequencies below 50 Hz. We set
upper limits in the range between about $10^{-24}$ and $2 imes 10^{-23}$ at
most frequencies. Our upper limits on signal strain show an improvement of up
to a factor of $sim$2 with respect to the results of previous all-sky searches
at frequencies below $80~mathrm{Hz}$. |
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