Living Black Holes
Shuang Nan Zhang,
Physics Department and Center for Astrophysics
Tsinghua University, Beijing, 100084, China
Except for mini- or quantum black holes predicted but not observed so far, all other black holes start up as dead-bodies of massive stars or clusters of stars. Yet, those black holes we can detect and study today are living and interacting with their environments, and sometimes even grow together with their host galaxies if found in centers of galaxies. A stellar mass black hole found in company with a normal star may accrete matter from its companion and produce strong X-ray/gamma-ray radiation, as well as relativistic outflows/jets. Similar phenomena also occur for supermassive black holes that are 6-9 orders of magnitudes more massive and found in centers galaxies, as they are supplied with abundant material from their host galaxies. In fact the feedback from the radiation and outflows from the central black holes also have a strong bearing on the evolution of their hosts.
Despite of the apparent importance of black holes’ playing in the cosmic history, several burning questions still remain unanswered: (1) How are stellar mass black holes formed when massive stars exhaust their nuclear fuel and gravitationally collapse? A massive star may collapse quietly to become a black hole, or experience a supernova, or even a hypernova, which are the most energetic explosions since the Big Bang; (2) How does a stellar mass black grow to become a supermassive black hole within the cosmic age? Perhaps some exotic modes of mass accretion take place, or the seed black hole may even accrete dark matter surrounding it to grow quickly in the beginning and then starts to accrete normal matter in its host galaxy; (3) Observationally how to determine the demographics of black holes? Perhaps determining their masses and maybe also their angular momenta will help us understand many of the above questions; (4) What happens when a neutron star and a black hole, or two black holes, or two neutron stars merge together to form another black hole? Perhaps strong gravitational radiations are produced. It is often argued that direct detection of gravitational waves from black holes may provide ultimate proof of the existence of black holes, and thus the unambiguous verification of Einstein’s General Relativity.
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