Gravitational Waves

Gravitational waves astrophysics is a new and promising field of research of the Universe [1]. Until recently the observations of the electromagnetic waves (for example the radio waves, visible light, X-rays and gamma) were the prime source of our knowledge: neutron stars were detected as the radio pulsars, X-ray bursters, magnetars etc.

In contrast to the electromagnetic waves, with gravitational waves we ”listen” rather than ”watch” the Universe by registering minor disturbances of the space-time curvature using kilometer scale laser interferometric detectors (Advanced LIGO and Advanced Virgo). Currently, the detectors are sensitive in the range of frequencies similar to the audible range of human ears – between 10 Hz and about 10 kHz. As in the case of an ear, a solitary laser interferometric detector is practically omnidirectional (has a poor angular resolution), and has no imaging capabilities. It registers a coherent signal emitted by a bulk movement of large masses, moving in rapid, non-spherical fashion. Once emitted, gravitational waves are weakly coupled to the surrounding matter and propagate freely without scattering. This has to be contrasted with the electromagnetic emission which originates at the microscopic level, is strongly coupled to the surroundings and often reprocessed; it carries a reliable information from the last scattering surface only. It seems therefore that gravitational wave detectors are the perfect counterpart to the electromagnetic observatories as they may provide us with information impossible to obtain by other means.

Gravitational waves are emitted during the largest cosmic cataclysms: mergers of binary systems of neutron stars or black holes, explosions of supernovae, and by other, more persistent sources, like unstable or deformed rotating neutron stars. The direct detection of gravitational waves allows the study of electromagnetically-dim objects, testing the theory of gravity in the dynamic regime of strong gravitational field, and the *direct* study of the interior of neutron stars which contain the densest and most extreme matter existing
currently in the Universe. These informations cannot be currently obtained with other methods.

Inspiraling binary systems of neutron stars [2] emit characteristic gravitational waves of amplitude increasing with time and frequency, called the chirp signal (named after a similar type of birds’ songs, here [3] for the GW150914, binary system of black holes [4]). The waveform changes because the bodies draw nearer to each other due to the emission of gravitational waves (their non-symmetric orbital movement distorts the spacetime and some energy of the system escapes in the form of gravitational waves). The first phase of the gravitational-wave signal – the inspiral, when the two stars still move around the common center of mass – is crucial for the detection of the merging binary system. This form of the signal is well understood in the language of post-Newtonian approximation to general relativity and thus it can be searched for even when it is deeply buried in the detector’s data stream (one refers to such a data analysis method as the matched-filter method). Once the inspiral waveform is found, one can estimate the masses of the components as well and the distance to the system. The distance measurement is obtained directly from the detector (is inversely proportional to the space-time distortion) *independently* from possible electromagnetic counterpart observations. Chirp ends abruptly after the components merge into one hyper-massive, hot, unstable and pulsating neutron star that some time afterwards collapses and forms a black hole. This is accompanied by a gigantic electromagnetic explosion – a short gamma ray burst. The details of the merger waveform depend on largely unknown microphysical details of the hot and dense neutron-star matter (composition, transport properties, viscosities); such phenomena are modelled on the largest supercomputers on Earth. Comparison of the simulations and observations of the merger waveform will therefore reveal the secrets of the deep neutron-star interiors. In addition, one can imagine measuring the distance to the binary neutron-star merger in two independent ways, using electromagnetic and gravitational waves: a feat quite uncommon in astronomy.

Credit: LIGO
Chirp pattern of gravitational waves detected by LIGO on September 14, 2015.

Another type of cataclysmic sources are core-collapse supernovae [5]: massive stars that reached the phase in which the nuclear fusion in their centres becomes unable to sustain the core against its own gravity. The gravitational collapse of the whole star results in a bright electromagnetic explosion, as well as the creation of a compact remnant. The supernova core is a newly-born, hot neutron star. Gravitational waveforms of collapsing supernovae are complicated and depend on many physical parameters and assumptions: details of the neutrino emission and interaction with matter, convection, existing instabilities etc. As in the case of the binary merger, the waveforms are obtained by means of large-scale computer simulations. Quite different type of gravitational waves related to neutron stars is the quasi-monochromatic radiation emitted by rotating, deformed neutron stars [6]. The deformation of the mass distribution may be caused by the elastic strain, non-axisymmetric distribution of the magnetic field, glitches or processes related to accretion in the binary system [7]. Such an object would emit periodic gravitational waves with a frequency proportional to the frequency of rotation of the star, in a more or less steady fashion for long periods of time (in analogy to radio-pulsars one could called them the gravitational-wave pulsars). Periodic gravitational waves are a probe of the interior of neutron stars: the elastic and viscous properties of the crust, its interaction with the magnetic field and the superfluid component. Fourth type of sources constitute the background gravitational waves, for example emitted by the ensemble of all neutron stars in the Galaxy. Detection of the stochastic background gravitational radiation [8] would contribute to answering the questions pertinent to neutron stars’ populations.

Binary Neutron Star merger simulation. © Bruno Giacomazzo and Luciano Rezzolla (AEI), Ralf Kähler (AEI/ZIB)

Binary Neutron Star merger simulation.
© Bruno Giacomazzo and Luciano Rezzolla (AEI), Ralf Kähler (AEI/ZIB)

[4] https://commons.wikimedia.o/wiki/File:LIGO_measurement_of_gravitational_waves.svg

Author: Michal Bejger


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