Searching for Continuous Gravitational Waves from Compact Objects
Neutron stars, the targets of this research program, are extreme objects formed in supernova explosions. They typically have around 40 % more mass than the Sun, but are only about 20 kilometers in diameter: the only objects that are known to be more compact than this are black holes. Until now, the vast majority of neutron stars have been found via the pulsations that result from their beamed electromagnetic emission periodically sweeping past the Earth; for this reason they are often called pulsars. However, while it is believed that the Milky Way contains about a hundred million neutron stars, fewer than 3000 have been detected so far. Gravitational waves might well be the only way to unveil this invisible population of extreme objects.
The primary goal of our research is now to make the first direct detection of gravitational waves from spinning neutron stars, and thus to observe these stars via a completely different physical mechanism, which would carry important new information about their internal structure and composition.
In 2016, the LIGO Scientific Collaboration announced the first direct observations of short bursts of gravitational waves, emitted during the inspiral and merger of black holes of tens of solar masses. Here we target a different type of gravitational wave signal: the long continuous waveform expected from a rapidly spinning neutron star. Because the star's sky location, spin rate, and deformation from axisymmetry are unknown, there is a large parameter space to search, and the sensitivity is limited by the amount of computing power available. The Einstein@Home volunteer computing project provides the lion's share of our compute cycles and on it we deploy our state-of-the-art search techniques.
The direct detection of gravitational waves has opened a new window on the Universe, providing a new tool for astrophysical observation. The detection of continuous gravitational waves will provide glimpses in the invisible population of neutron stars that inhabits our Galaxy, improve our understanding of stellar evolution and populations and shed light on the internal structure and evolutionary history on these extraordinary objects.