Transcript Closing in on Black Holes

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 confidenceenhancing, 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 HDE226868
Correlated variability in Xray/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
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?
• 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 … 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 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
• High speeds
– special relativity
• e.g. relativistic beaming
• Strong gravitational fields
– general relativity
• e.g. gravitational redshift
Tanaka et al., 1995
• 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?
Black hole history - a cynical view
of astronomers, from the 1970’s
R McCray 1977
Black hole history now – a
positive view
The indifferent
and the ignorant
Viewing
boundary
Astronomers
investigate and
their ideas clash
Imagination
and new
technology
sees what
lies inside
Boundary of
interest
• I hope to learn more…