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

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? 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? 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 HDE 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? Uhuru 1971 X-ray fluctuations at ~10 Hz frequencies → neutron star Webster and Murdin 1971, Bolton 1971 HDE with a massive companion (>6 M sun ), 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? 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? Martin Rees 1971 –Routes to a massive black hole Looks like nature can indeed make them

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 … –large “black masses” in quiescent galaxies Miyoshi et al. 1995, many others –NGC 4258 contains a central mass of 3×10 7 M o Genzel 1996, Ghez 1998 –mass of Galaxy’s black hole by motion of a star cluster is 3×10 6 M o

How close is the evidence to the black hole? Galactic centre –S2 is 2×10 10 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 High speeds – special relativity e.g. relativistic beaming Strong gravitational fields –general relativity e.g. gravitational redshift The MCG– 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

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

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

I hope to learn more…