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Astrophysical Sources of Gravitational Waves

Artist’s rendition of a mixed neutron star – black hole coalescence © Carl Knox, OzGrav – Swinburne University

Astrophysical sources of gravitational waves can be classified in terms of the type of signal they emit. In this regard there are three main types of signals: transient, continuous and stochastic.
Transient signals can range from being very short (a few milliseconds) to lasting several minutes. They include signals which are intrinsically short in time such as the GW burst expected from a supernova explosion, as well as signals which last very long periods of time but which Advanced Virgo can observe only in the final stage, as compact binary coalescences (CBC), mergers of binary systems of black holes or neutron stars. 

Signals which are long-lived in time, with durations of months or years, can be classified as continuous. These signals are generally weaker with respect to CBC 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 spinning neutron stars, either 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.
This last type of signal has been never detected so far; however they are anticipated to be detectable by the Advanced detectors in the upcoming runs, or by one of the upcoming third-generation detectors.
Quasi-periodic signals could also be emitted by ultra-light boson-clouds, a possible constituent of dark matter, orbiting around stellar-mass black holes.

A third type of signal is the stochastic background: this is 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 overlapping 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. Another kind of similar gravitational background could also have a cosmological origin, i.e. generated soon after the Big Bang according to the theory of inflation. The global network of Advanced detectors is not expected to be sensitive enough to the primordial background from standard inflation and other types of detectors are better suited;
In addition the astrophysical background could be at reach of the network of Advanced Detectors in the next observing periods. Detection of this background would help elucidate the star formation history and the evolution of astrophysical sources.
Below a more in-depth description of some of the sources listed above.

Coalescence of compact binary systems

Artist’s rendition of a black hole merger © Raul Rubio

A compact binary system is composed of two compact stellar objects: examples are a Binary system of Neutron Stars (BNS) or a Binary system of two Black Holes (BBH) or a mixed binary system of a neutron star and a black hole (NSBH), orbiting around each other; this is a typical source of GWs. As the system evolves with time, the two compact objects inspiral faster around one another and get closer and closer until they eventually merge, because the system loses energy through the emission of GWs. This phenomenon is known as coalescence. As the two bodies approach one another, the GWs that are generated increase in frequency and amplitude. This type of signal is called a “chirp”.

During the final stage of the orbital decay and before the merging of the two bodies, the GWs are strong enough to be observed in the frequency sensitive band of Advanced Virgo: depending on their masses, we can observe from a few up to many hundreds orbits of the coalescing bodies around one another
The first direct observation of gravitational waves was GW150914, the coalescence of two black holes of masses of the order of about 30 times that of our Sun, merging at a distance of 1.3 billion light-years from Earth. The first detection involving data from the Virgo detector was GW170814, a signal from a similar system. The signal arrived at the detector on August, 14, 2017: this event was also the first one detected by a network of three detectors (Virgo, LIGO Hanford and LIGO Livingston).

The Unexpected

Every time humans have observed the sky with a new instrument, explored new ranges of electromagnetic wavelengths or utilized additional messengers from the cosmos, unexpected objects and phenomena have been discovered.
When looking for gravitational waves, we are also expecting surprises!
For instance cosmic strings are “defects” in the fabric of spacetime which might have formed during the evolution of the Universe. These strings have never been observed, but are expected to emit GWs if they exist. Maybe Advanced Virgo will allow us to detect such phenomena for the first time. Cosmic strings are just an example of unexpected GW sources. There are many other theoretical GW sources that might reveal an exotic astrophysical source: GWs leaking out from black hole mimickers, ultra-light boson clouds forming around spinning black holes and others.