The electromagnetic counterparts of compact binary mergers
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Abstract
Mergers of binaries consisting of two neutron stars, or a black hole and a\nneutron star, offer a unique opportunity to study a range of physical and\nastrophysical processes using two different and almost orthogonal probes -\ngravitational waves (GW) and electromagnetic (EM) emission. The GW signal\nprobes the binary and the physical processes that take place during the last\nstages of the merger, while the EM emission provides clues to the material that\nis thrown out following the merger. The accurate localization, which only the\nEM emission can provide, also indicates the astrophysical setting in which the\nmerger took place. In addition, the combination of the two signals provides\nconstraints on the nature of gravity and on the expansion rate of the Universe.\nThe first detection of a binary neutron star merger by the LIGO-Virgo\ncollaboration, GW170817, initiated the era of multi-messenger GW-EM\nastrophysics and demonstrated the great promise it holds. The event produced an\nunprecedented data set, and although it was only a single event, it provided\nremarkable results that revolutionized our knowledge of neutron star mergers.\nGW170817 is especially exciting since we know that it is not one of a kind and\nthat many more events will be detected during the next decade. In this review,\nI summarize, first, the theory of EM emission from compact binary mergers,\nhighlighting the unique information that the combined GW-EM detection provides.\nI then describe the entire set of GW and EM observations of GW170817, and\nsummarize the range of insights that it offers. This includes clues about the\nrole that similar events play in the r-process elements budget of the Universe,\nthe neutron star equation of state, the properties of the relativistic outflow\nthat followed the merger, and the connection between neutron star binary\nmergers and short gamma-ray bursts.\n