MG14 - Talk detail

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Gravity is successfully described by Einstein’s theory of general relativity, governing the structure of our entire universe. Yet gravity remains the least understood of all forces in nature, e.g., resisting unification with quantum physics. One of the most fundamental predictions of general relativity are black holes. Their defining feature is the event horizon, the surface that even light cannot escape and where time and space exchange their nature. However, while there are many convincing black hole candidates in the universe, there is no experimental proof for the existence of an event horizon yet. So, does general relativity really hold in its most extreme limit? Do BHs exist or are alternatives needed? The best place to test this is in the center of our own Milky Way. Here a compact radio source with a mass of 4 Million times the mass of the sun, marks the central point of our Galaxy, providing by the far the best evidence for the existence of black holes. Very long baseline radio observations are now probing the smallest scales of this source, making it possible to image the shadow of the event horizon of a black hole for the very first time. Moreover, with the help of advanced numerical general relativistic magneto-hydrodynamic simulations emission and appearance of the source can be successfully modeled almost from first principles. Hence, the Galactic center provides today a unique laboratory for astrophysics and general relativity. The talk will set the stage for the issues discussed at this workshop.

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We here discuss the observational signatures of supramassive pulsar-like neutron stars, i.e. neutron stars with a tenuous magnetosphere, collapsing to a black hole. Such a collapse must eventually occur, if the neutron star is brought above its critical mass for non-rotating neutron stars by either formation, merger, or accretion and is only supported by rapid rotation. Magnetic braking slows down the neutron star and leads to a sudden collapse to a black hole state. Due to the formation of an event horizon any surface emission will be hidden from our view rather quickly and the magnetic-field lines will snap violently at the horizon scale. A massive magnetic wave will form that can turn an almost ordinary pulsar into a bright radio "blitzar": Accelerated electrons from the travelling magnetic shock dissipate a significant fraction of the entire magnetosphere and produce a bright radio burst on millisecond timescales that is observable out to cosmological distances. Such blitzars could be one explanation of the recently discovered Fast Radio Bursts (FRBs), if they are indeed from cosmological distances.

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