LIGO Document P1500030-v24

First low frequency all-sky search for continuous gravitational wave signals

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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 and a range of spin-down between
\( -1.0 \times 10^{-10} \) Hz/s and \( +1.5 \times 10^{-11} \) Hz/s, and was based on a hierarchical approach. The starting point was a set of short FFTs, of length 8192 seconds.
Aggressive data cleaning, both in time and frequency domain, 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 as incoherent step. Coincident candidates among the two runs have been found in order to strongly reduce the false alarm
probability and the most significant have been selected. Both the selection of candidates and in the coincidence step have been done in such a way to avoid being overwhelmed by candidates due to 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, still based on the FrequencyHough transform. No evidence for continuous gravitational wave signals was found, therefore we have set a population-based joint
VSR2-VSR4 upper limit over the full analyzed frequency band.
This is the first time an all-sky search for continuous gravitational waves is conducted at frequencies below 50 Hz.
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