P. Leaci

Search for continuous gravitational waves: improving robustness versus instrumental artifacts

The standard multidetector 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 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 F-statistic in Gaussian noise. In the presence of line artifacts it is more sensitive, even compared to the popular F-statistic consistency veto, over which it improves by as much as a factor of two in detectable signal strength.

Directed searches for continuous gravitational waves from binary systems: parameter-space metrics and optimal Scorpius X-1 sensitivity

We derive simple analytic expressions for the (coherent and semi-coherent) phase metrics of continuous-wave sources in low-eccentricity binary systems, both for the long-segment and shortsegment regimes (compared to the orbital period). The resulting expressions correct and extend previous results found in the literature. We present results of extensive Monte-Carlo studies comparing metric mismatch predictions against the measured loss of detection statistic for binary parameter osets. The agreement is generally found to be within 10% 30%. As an application of the metric template expressions, we estimate the optimal achievable sensitivity of an Einstein@Home directed search for Scorpius X-1, under the assumption of suciently small spin wandering. We nd that

Fully coherent follow-up of continuous gravitational-wave candidates: an application to Einstein@Home results

We characterize and present the details of the follow-up method used on the most significant outliers of the Hough Einstein@Home all-sky search for continuous gravitational waves [J. Aasi et al Phys. Rev. D 87, 042001 (2013)]. This follow-up method is based on the two-stage approach introduced by [M. Shaltev and R. Prix, Phys. Rev. D 87, 084057 (2013)], consisting of a semicoherent refinement followed by a fully coherent zoom. We quantify the efficiency of the follow-up pipeline using simulated signals in Gaussian noise. This pipeline does not search beyond first-order frequency spin-down, and therefore we also evaluate its robustness against second-order spin-down. We present the details of the Hough Einstein@Home follow-up [J. Aasi et al Phys. Rev. D 87, 042001 (2013)] on three hardware-injected signals and on the eight most significant search outliers of unknown origin.

Antenna pattern of DUAL detectors of gravitational waves and its exploitation in a network of advanced interferometers

We investigate the directional sensitivity to plane gravitational waves (GWs) of DUAL detectors of cylindrical shape. Calculations make use of the finite elementmethodtosimulatetheresponsestotheGWRiemanntensorofasinglemass DUAL (SMD) and of a tapered cylinder (TC) in their wide sensitivity bandwidth. We show that one SMD or a pair of TCs is able to cover both GW polarization amplitudes from almost all incoming directions. We discuss the achievable enhancement in tackling the inverse problem for high frequency [∼(2‐5) kHz] GWs by adding a TC detector to the future advanced LIGO‐ VIRGO network.

OctApps: a library of Octave functions for continuous gravitational-wave data analysis

Gravitational waves are minute ripples in spacetime, first predicted by Einsteins general theory of relativity in 1916. Gravitational waves from rapidly-rotating neutron stars, whose shape deviates from perfect axisymmetry, are a potential astrophysical source of gravitational waves, but which so far have not been detected. The search for this type of signals, also known as continuous waves, presents a significant data analysis challenge, as their weak signatures are expected to be buried deep within the instrumental noise of the LIGO and Virgo detectors. The OctApps library provides various functions, written in Octave, intended to aid research scientists who perform searches for continuous gravitational waves.

New prospects for continuous gravitational wave detection from binary systems in the advanced detector era

Gravitational wave detection is eagerly expected as one of the major discoveries of the next decade. A worldwide effort is currently underway, building ever-more-sensitive detectors and developing more powerful data-analysis tools. Continuous gravitational waves (CWs) are a promising class of signals for advanced detectors, and a particularly interesting type of CW sources are neutron stars in low-mass X-ray binaries, with Scorpius X-1 being the most remarkable representative. We describe the details of a project aimed to perform directed searches for CWs from binary systems. We use a search algorithm based on coherently computing a maximum likelihood statistic for a bank of signal templates, and then incoherently summing this statistic over the segments in which the entire data set has been previously split. The current strategy strongly relies on the derivation of precise analytic expressions for the (coherent and semicoherent) phase metrics of CW sources in low-eccentricity binary systems, for the two regimes of long and short segments compared to the orbital period. As an application of the metric template expressions, we estimate the optimal achievable sensitivity of an Einstein@Home directed search for Scorpius X-1, under the assumption of sufficiently small spin wandering, and using data from the upcoming advanced detectors. We find that such a search would be able to beat the torque-balance level up to a frequency of 500–600 Hz, if orbital eccentricity is well-constrained, and up to a frequency of ∼160–200 Hz for more conservative assumptions about the uncertainty on orbital eccentricity.

Searching for continuous gravitational wave signals using LIGO and Virgo detectors

Direct and unequivocal detection of gravitational waves represents a great challenge of contemporary physics and astrophysics. A worldwide effort is currently operating towards this direction, building ever sensitive detectors, improving the modelling of gravitational wave sources and employing ever more sophisticated and powerful data analysis techniques. In this paper we review the current status of LIGO and Virgo ground based interferometric detectors and some data analysis tools used in the continuous wave searches to extract the faint gravitational signals from the interferometric noise data. Moreover we discuss also relevant results from recent continuous wave searches.

Using a cleaning technique for the search of continuous gravitational waves in LIGO data

High frequency short events, due for instance to delta-like spurious disturbances, may affect the broad band noise level and thus produce a loss in the efficiency of detection of continuous gravitational waves. We identify such events, remove them from a set of LIGO fifth science run (S5) data and characterize the resulting sensitivity improvements. We use the same parameter values as used in a previous pilot study on Virgo data, but do not observe on LIGO S5 data the same sensitivity improvements.