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  • Relativistic Phenomena around Black Holes

    Prof Shiho Kobayashi

    EM counterparts of GW Sources: Gravitational waves are one of the greatest predictions of Einstein's theory of general relativity (Einstein 1916). In several years, gravitational waves are very likely to be detected for the first time ever, with the Advanced LIGO detector. However, the first detections are expected to be made around the detection threshold level, additional info (timing/sky location) significantly enhances the effective sensitivity of the detectors. Furthermore, electromagnetic counterparts will help us to understand the whole picture of the events (what exactly happened) and astrophysics.

    Compact stellar mergers (NS-NS, NS-BH) are strong GW sources (and progenitors of short gamma-ray bursts) and the primary targets of LIGO/Virgo, so it is very likely that the first detections would be associated with these merger events. Once a GW detection is reported, all telescopes in the world on ground/space will point to the direction. It is very interesting to study any EM signals from NS-NS, NS-BH mergers.

    Hypervelocity Stars: Hypervelocity stars (HVSs) are stars with velocities great enough to allow their escape from the Milky Way Galaxy. Ordinary stars in the Galaxy have velocities on the order of 100 km/s, while HVSs (especially those near the centre of the Galaxy) have velocities on the order of 1000 km/s. The existence of HVSs was first predicted in 1988, and their existence confirmed in 2005. They are a natural consequence of the existence of the super-massive black hole at the heart of our galaxy. When a binary swings too close to the central black hole, the tidal force can tear the binary apart, capturing one star while violently flinging the other outward at enormous speed. HVSs provide a strong test of the model for injection of the stars into the Galactic potential. Coupled with observations of the stellar population at the Galactic centre, observations of HVSs provide promising probes of the binary population, the stellar mass function, and the central potential of the Milky Way. Since dark matter's gravitational pull can be measured by the shape of the HVSs' trajectories out of the Milky Way, HVSs would reveal the dark matter distribution surrounding our galaxy and its nature.

    Tidal Disruption of Stars by Massive Black Holes: When a star comes very close to a mass black hole, the star itself is disrupted by the strong tidal force of the black hole. Future surveys like Large Synoptic Survey Telescope (LSST) are expected to discover thousands of such tidal disruption events. Slides related to the project are at here

    The student will have opportunities to work with collaborators in the UK and abroad.