Search for continuous gravitational waves: Improving robustness versus instrumental artifacts

The standard multidetector $\mathcal{F}$-statistic for continuous gravitational waves is susceptible to false alarms from instrumental artifacts, for example monochromatic sinusoidal disturbances (``lines''). This vulnerability to line artifacts arises because the $\mathcal{F}$-statistic compares the signal hypothesis to a Gaussian-noise hypothesis, and hence is triggered by anything that resembles the signal hypothesis more than Gaussian noise. Various ad-hoc veto methods to deal with such line artifacts have been proposed and used in the past. Here we develop a Bayesian framework that includes an explicit alternative hypothesis to model disturbed data. We introduce a simple line model that defines lines as signal candidates appearing only in one detector. This allows us to explicitly compute the odds between the signal hypothesis and an extended noise hypothesis, resulting in a new detection statistic that is more robust to instrumental artifacts. We present and discuss results from Monte-Carlo tests on both simulated data and on detector data from the fifth LIGO science run. We find that the line-robust statistic retains the detection power of the standard $\mathcal{F}$-statistic in Gaussian noise. In the presence of line artifacts it is more sensitive, even compared to the popular $\mathcal{F}$-statistic consistency veto, over which it improves by as much as a factor of two in detectable signal strength.