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The first direct detections of gravitational waves (GW) from binary black holes and binary neutron stars (BNS) made by Advanced LIGO/Virgo have opened a new era in physics and astronomy. The detection of an EM counterpart, from γ-rays to radio waves, to the BNS merger GW170817 has revealed the full scientific potential of multi-messenger observations of astrophysical compact objects.
Here, I will focus on a class of highly magnetized, millisecond spinning NS, that may form both in the core-collapse of massive stars and in BNS mergers. Such NS were originally proposed as the precursors of a galactic population of peculiar X-ray pulsars (magnetars), allegedly powered by the dissipation of a huge reservoir of magnetic energy. In the last decade, they were suggested as possible central engines of the brightest EM transients (γ-ray bursts, super-luminous supernovae), and their potential as interesting GW sources for the Advanced (and future) detectors was fully recognized.
I will present new results supporting a possible association between gamma-ray bursts and the formation of a millisecond spinning, highly magnetized NS, and will discuss the merits of the secular bar-mode instability and the so-called ``spin-flip” instability, the two main mechanisms by which such NS can produce powerful and distinctive GW signals, carrying pristine information about the physics of their interiors and the equation of state (EoS) of matter at supra-nuclear density.
Because the EM emission associated with these GW signals is also expected to be bright, and to carry its own signatures of the ms-spinning NS, these sources are ideally suited for multi-messenger studies. I will outline ongoing theoretical and observational efforts dedicated to their actual multi-messenger detection, and to the extraction of all the information about their physics.