INTRODUCTION

Gravitational waves are distortions in the fabric of spacetime that propagate at the speed of light. They are produced by relative motion between gravitating masses. Astrophysical sources of gravitational waves can be classified in terms of the type of the signal they emit. In this regard there are three main types of signals: transient, continuous and stochastic. (More information)

• Transient signals can range from being very short (a few milliseconds) to lasting several minutes. They are generated by events which are intrinsically short in time such as the gravitational wave burst expected from a supernova explosion, as well as events which last very long periods of time but which Advanced Virgo can only observe in the final stage, as compact binary coalescences (CBC), that is mergers of binary systems of black holes or neutron stars. 

• Continuous signals  are long-lived in time, with durations of months or years/ These signals are generally weaker with respect to compact binary coalescence signals; however the fact that they last for long periods of time allows us to accumulate the signal in long stretches of data which increases the chances of detection. Continuous signals can be emitted, for instance, by non-axisymmetric spinning neutron stars: the asymmetric could either be because they depart from a perfect sphere (and thus have some “mountains” at their surface), or because of an oscillating perturbation of their inner structure. 

• Stochastic signals are the result of the incoherent sum of numerous gravitational wave signals too weak to be detected individually. The stochastic background can be generated by the overlap of gravitational signals coming from a myriad of astrophysical sources, close or distant, young or old, too faint or too distant to be detected individually.