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

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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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paul murdin institute of astronomy cambridge
Paul Murdin

Institute of Astronomy, Cambridge

Closing in on Black Holes – why this conference is important for me
black holes as theoretical entities
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
does nature make stellar black holes
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)
do stellar black holes exist
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
cyg x 1 can we definitively say it is a neutron star or black hole
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?
how close is the evidence to the black hole
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
does nature make massive black holes
Does nature make massive black holes?
  • Martin Rees 1971
    • Routes to a massive black hole
  • Looks like nature can indeed make them
identifications of galactic black holes
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
how close is the evidence to the black hole1
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…
close to a black hole
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

black hole history now a positive view
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

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