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Closing in on Black Holes – why this conference is important for me

Paul Murdin Institute of Astronomy, Cambridge. Closing in on Black Holes – why this conference is important for me. Black holes as theoretical entities. John Michell 1783; Pierre Laplace 1796 Posed a theoretical question about a star

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Closing in on Black Holes – why this conference is important for me

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  1. Paul Murdin Institute of Astronomy, Cambridge Closing in on Black Holes – why this conference is important for me

  2. Black holes as theoretical entities • John Michell 1783; Pierre Laplace 1796 • Posed a theoretical question about a star • At what radius of star, like the Sun, would the escape velocity exceed the speed of light? • Karl Schwarzchild 1915; Roy Kerr 1963 • Calculation in general relativity of the gravitational field of a point or spherical mass, with/without rotation • Well-developed solutions looking for their problem

  3. Does nature make stellar black holes? • S. Chandrasekhar 1930 • Maximum mass of (Newtonian) self gravitating stars supported by degenerate electron pressure (white dwarfs) • J. R. Oppenheimer and G. Volkoff 1939 • Ditto for general relativity and a neutron fluid (neutron stars) • “..unlikely that static neutron cores can play any great part in stellar evolution” • (However: Bell 1967: pulsars) • J.R. Oppenheimer and H. Snyder 1939 • Neutron star’s “continued gravitational contraction,” asymptotically to the gravitational radius • → Possibly Nature does make black holes, yes • (but Nature doesn’t make neutron stars! – not a confidence-enhancing, mistaken judgement)

  4. Do stellar black holes exist? • X-ray sources • Luminosities imply accretion • Temperatures imply accretion on to a compact object like a neutron star or black hole • Rocket and balloon-borne detectors: Cygnus XR-1 • Huge positional uncertainty • Uhuru 1971 reduced positional uncertainty of Cygnus X-1 • Hjellming and Wade 1971; Braes and Miley 1971: radio source with precise position • Optical ID with HDE226868 • Correlated variability in X-ray/radio/optical effectively settled the identification • But a post hoc argument

  5. Cyg X-1 - can we definitively say it is a neutron star or black hole? • Uhuru 1971 X-ray fluctuations at ~10 Hz frequencies → neutron star • Webster and Murdin 1971, Bolton 1971 HDE 228868 with a massive companion (>6 Msun), so not a neutron star → BH • Pringle and Rees 1972 → quasi periodic oscillations at inner edge of accretion disc • So black hole found? or some other sort of star with an unexplained small source of X-rays?

  6. How close is the evidence to the black hole? • Cygnus X-1 • Variability at 100 light milliseconds • Companion at 0.2 AU • Evidence is 1,000 to 1,000,000 Schwarzchild radii from the black hole • Evidence connecting Cygnus X-1 to a black hole has weak points • It is credible and consistent to say that Cyg X-1 is a black hole, but not, as far as I can see, unassailable • Perhaps this conference will tell me differently

  7. Does nature make massive black holes? • Martin Rees 1971 • Routes to a massive black hole • Looks like nature can indeed make them

  8. Identifications of galactic black holes • Seyfert 1943 – explosive nuclei • Dent 1965 – variability with 1 year, < 1 l.y. extent • Salpeter, Zeldovich 1964 – powered by accretion onto black hole? • Lynden Bell 1971 – consistent physical model • Redhead, Cohen, Blandford 1978 – aligned jets from radio sources, maintained by rotating black hole • Richstone, Kormendy … 1990-95 • large “black masses” in quiescent galaxies • Miyoshi et al. 1995, many others • NGC 4258 contains a central mass of 3×107 Mo • Genzel 1996, Ghez 1998 • mass of Galaxy’s black hole by motion of a star cluster is 3×106 Mo

  9. How close is the evidence to the black hole? • Galactic centre • S2 is 2×1010 km distant • 1,000 Schwarzchild radii • Evidence connecting these phenomena to a black hole is circumstantial • It is credible and consistent to say that they are, but this evidence is not unassailable • However…

  10. Close to a black hole • High speeds • special relativity • e.g. relativistic beaming • Strong gravitational fields • general relativity • e.g. gravitational redshift • The MCG–6-30-15 Fe line profile provides the direct connection with the BH Schwarzchild radius (1995) • Very strong and convincing evidence, now very robust • Likewise for galactic sources? Not so robust? Tanaka et al., 1995

  11. Black hole history - a cynical view of astronomers, from the 1970’s R McCray 1977

  12. Black hole history now – a positive view Astronomers investigate and their ideas clash Viewing boundary The indifferent and the ignorant Imagination and new technology sees what lies inside Boundary of interest

  13. I hope to learn more…

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