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Electromagnetic counterpart to gravitational wave emission detected

Just a few weeks after the annoucement of the Nobel Prize awarded to Thorne, Weiss and Barish for the discovery of gravitational waves with the LIGO interferometer, gravitational waves make the headlines again, in a spectacular way. On the 16th of October 2017, the american National Science Foundation and the European Southern Observatory have announces the discovery of a simultaneous detection of a gravitational wave event (GW170817) and a gamma-ray burst (GRB170817A), observed in several electromagnetic bands (from X-rays, to optical, down to infrared). The discovery, that led to the publication of dozens of papers on Nature, Science and the Astrophysical Journal (that has a focus issue dedicated to the event), was made possible by a combination of events. The inclusion of the italian Virgo detector allowed for an efficient sky localization of the event, that occurred on the 17th of August 2017. less than two second after the GW detection, a short gamma-ray burst has been detected from the same patch of the sky. After this, most telescopes in the world have observed the galaxy NGC 4993, the host of the event, detecting it at essentially all wavelengths in the electromagnetic spectrum.

This discovery is a landmark for several reasons. First of all it marks the beginning of a new branch of physics, gravitational wave astronomy, that allows to combine multi-messenger signals, such as gravitational and electromagnetic waves. It is a combined effort that brings together experimental physicists, such as the LIGO/Virgo researchers, togwther with astronomers. It has implications on fundamental physics, astronomy, nuclear physics and even cosmology.

At its first discovery, this new way of observing the Universe has already fully demonstrated its potential, solving one of the great mysteries in modern astronomy: the origin of short gamma-ray bursts. It was long suspected that they originated from the merger of two neutron stars. Now, the discovery of gravitational waves demonstrates beyond any doubt that this theory is correct.

From the point of view of fundamental physics, for the first time the polarization of the gravitational wave has been measured, allowing to obtain information on the orientation of the binary orbit in the plane of the sky.

From the point of view of nuclear physics, optical observations have revealed the presence of heavy elements, such as gold and platinum, showing that the nucleosynthesis of these elements occurs during neutron star mergers.

Finally, the event also has cosmological implications. The simultaneous detection of gravitational and electromagnetic emission offers the possibility to calibrate the distance-redshift relation, and thus use these events as "standard sirens" to measure cosmological parameters. The measurement of the Hubble constant thorugh this new method, although still with large uncertainties, is in good agreements with previous methods obtained for example from the cosmic microwave background, but has been obtained in a completely independent way.

The discovery has seen a strong participation of italian researchers, and in particular from the Milan area. The contribution of the Virgo detector, located in Cascina near Pisa, as mentioned above has been fundamental to localize the event in the sky. To this we need to mention the fundamental role of italian astronomers in the electromagnetic detection, and in particular of several researchers of the Astronomical Observatory of Brera/Merate.

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17 October 2017
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