Multimessenger Transients in the X-Ray Light

Dear Colleagues,

Galactic X-ray sources hosting a compact object (neutron star, black hole, X-ray binaries or XRBs, white dwarf, and cataclysmic variables or CVs) and a donor star are characterised by marked X-ray variability, which reflects variability in the accretion process. In addition, a more erratic variability is observed, such as X-ray bursts or flares caused by thermonuclear burning events on the neutron star's surface. Recently, a few X-ray binaries have been found to change from an accretion-powered state to a rotation-powered state, from the almost complete ablation of the X-ray-irradiated companion. When accretion stops, the neutron star becomes an active radio pulsar, albeit with a lower X-ray luminosity than in the accretion-powered state. In the rotation-powered state, the X-ray emission is powered either by the pulsar itself or by an intra-binary shock between the pulsar wind and that of the companion. Not to forget the mysterious Tidal Disruption Events, where a possible donor star is close enough to be completely disrupted by the gravitational pull of the compact object, producing strong X-ray flashes. On the other side of the time scale, microvariability is observed, apart from X-ray pulsations and quasiperiodic variability in the accretion disk. In many cases, all the aforementioned variability is accompanied by variability at other wavelengths, notably in optical and infrared wavelengths. The recent discovery of gamma-ray emissions from X-ray binaries has allowed us to connect variability in the low and very high energy regimes. These are only a few examples of multiwavelength variability in Galactic X-ray sources. Whether gravitational wave (GW) emission can be observed in these systems is debatable. With two objects spiralling in, this must exist, but it is probably quenched by the accretion flow. Of course, GW detectability depends on the companion mass and density. Therefore, neutron star–neutron star or neutron star–black hole systems have a higher chance.

To summarise, all Galactic X-ray sources alternate low- and high-emission states, resulting in a clear transient behaviour.

In this Special Issue, we aim to present state-of-the-art multiwavelength studies of Galactic X-ray sources and make the case for possible GW emissions from such systems.

Prof. Dr. Roberto Mignani
Guest Editor

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• X-ray
• galactic sources
• emission characteristics
• variability
• multi-wavelength counterparts
• gravitational wave detection