Today in Astronomy 102:
black hole observations, v.4
 Active galaxies
(conclusion): accretion
disks around the black
holes in active galactic
nuclei, their formation
and workings.
 Gamma-ray bursters: a
longtime mystery, and
the greatest of all
natural disasters, now Image: the NASA Compton Gamma-Ray
Observatory (GRO), shortly after
seen as black-hole
deployment in 1991 by the crew of the space
formation.
shuttle Atlantis (NASA/Marshall Space
Flight Center).
6 November 2001
Astronomy 102, Fall 2001
1
Matter falling into AGN black holes:
large accretion disks
The disk-shaped collection of matter surrounding the black
hole in an AGN arises rather naturally from the influence of
the black hole on stars and other material in the galactic
center.
 Stars in a galaxy perpetually interact with each others’
gravity as well as the gravity of the galaxy at large.
 These interactions - long-range collisions - usually result
in transfers of energy and momentum between stars. Two
stars, originally in similar orbits and undergoing such a
collision, will usually find themselves pushed to different
orbits, one going to a smaller-circumference orbit, and one
going to a larger orbit.
6 November 2001
Astronomy 102, Fall 2001
2
Matter falling into AGN black holes: large
accretion disks (continued)
Thus some stars are pushed to the very center of the galaxy
after a number of these encounters. What happens if there is a
black hole there?
 The star begins to fall in, but the spin of its orbital motion,
and the tidal forces that tend to rip the star apart, keep
this from happening all at once.
 Stellar material spreads out into a rotating, flat
distribution around the black hole: the beginnings of an
accretion disk.
6 November 2001
Astronomy 102, Fall 2001
3
How accretion disks form: tidal disruption of stars
Black
hole
Star
View from high above, along orbit’s axis.
Simulation by P. Quinn and G. Sussman, 1985
6 November 2001
Astronomy 102, Fall 2001
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How accretion disks form (continued)
Eventually the tidally-disrupted material from many stellar encounters
settles down into a flattened disk. Collisions among particles in the disk
cause material to lose its spin and become accreted by the black hole.
The disk is thus gradually consumed.
Black hole horizon
Perspective view
Accretion
disk
Rotation
of disk
6 November 2001
Astronomy 102, Fall 2001
5
Operation of accretion disks
 Recall that for non-spinning black holes, orbits with
circumference less than 3CS are unstable, and no orbits
exist with circumference less than 1.5CS. Within this
volume the disk structure breaks down and material
tends to stream in toward the horizon.
 A large amount of power, mostly in the form of X rays
and  rays, is emitted by the infalling material. Pressure
exerted by this light slows down the rate at which
accretion takes place.
 Much of this high-energy light is absorbed by the disk,
which heats up and re-radiates the energy as longer
wavelength light.
• Heated disk = compact central object seen in radio
images of radio galaxies and quasars.
6 November 2001
Astronomy 102, Fall 2001
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Operation of accretion disks (continued)
 Some of the particles absorbing the highest-energy light
are accelerated to speeds approaching that of light. If their
velocity takes them into the disk, they just collide with
disk material and lose their energy to heat. If their velocity
takes them perpendicular to the disk, they may escape
(Blandford and Rees, 1975).
• High-speed particles escaping perpendicular to the
disk = jets seen in radio and visible images of radio
galaxies and quasars. Their high speeds (approaching
c) explain the one-sideness and “faster than light”
motion of quasar jets.
• Several other possibilities exist for jet acceleration; see
Thorne’s figure 9.7 (pg. 349).
6 November 2001
Astronomy 102, Fall 2001
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Structure of an accretion disk
Jet
Not drawn to scale!
Innermost stable orbit
Ingoing: matter,
being accreted
Outgoing: X and  rays,
heating disk and
accelerating jets
Horizon
6 November 2001
Astronomy 102, Fall 2001
Accretion
disk (crosssection view)
8
Direct observation of an accretion disk in an AGN:
radio galaxy NGC 4261
88,000 light years
400 light years
Left: radio (red) and visible (white) view with ground-based telescopes.
Right: visible light image by the Hubble Space Telescope (NASA/STScI).
6 November 2001
Astronomy 102, Fall 2001
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Direct observation of an accretion disk in an AGN:
Seyfert galaxy NGC 4258
The same things happen around the
central black holes in Seyfert
galaxies as happen in radio galaxies
and quasars. But Seyfert galaxies are
spirals, and have a lot more
interstellar gas and dust than the
elliptical hosts of RGs and quasars,
which can stop the jets.
In the center of NGC 4258, Moran et
al. (1995) detected a molecular disk
about 1 ly in diameter, rotating at
1000 km/sec near the outer edge.
This implies a black hole mass of
3.6107 M.
(Radio and visible-light images: NRAO)
6 November 2001
Astronomy 102, Fall 2001
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Seyfert galaxy NGC 4258
(continued)
A jet is seen emerging
perpendicular to the disk, but
is entrained by interstellar
material in NGC 4258 and
appears as an extra set of
spiral features.
(Artwork and images by
Inoue, Kagaya, Greenhill and
De Pree.)
6 November 2001
Astronomy 102, Fall 2001
11
Mid-lecture Break.
Exam #2 is on Thusday, 2-3:15 PM; Homework #5 is due just
after midnight tonight.
 Remember, there’s a practice exam, with solutions, on the
AST 102 Web site.
 There’s even going to be a review session: 6 PM,
tomorrow evening, right
here in Hubbell
Auditorium.
The Chandra X-ray Observatory, launched in 1999 (CfA/NASA).
6 November 2001
Astronomy 102, Fall 2001
12
Gamma-ray bursters
In the mid 1960s, after ratification of the Nuclear Test-Ban
Treaty, the US and USSR each put up satellites with X- and ray detectors to monitor the other’s compliance with the
treaty.
 Immediately these instruments detected many brief,
bright bursts of  rays, similar to the expectations for
above-ground nuclear detonations. Naturally, this
worried all concerned, even though the bursts were not
correlated with seismic events.
 The satellites could not determine very well the direction
from which the  rays came, so it took a while to
determine that they actually came from outer space rather
than Earth. (Even then, the data remained top secret.)
6 November 2001
Astronomy 102, Fall 2001
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Typical gamma-ray burst (CGRO/NASA)
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Astronomy 102, Fall 2001
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Gamma-ray bursters (continued)
Soon it became possible to measure the directions of the 
rays well enough to show that the bursts came from locations
spread randomly and uniformly all over the sky. This is very
different from non-burst  ray sources.
 Bright sources of  rays:
neutron stars or black
holes?
 The nearest stars also
appear to be randomly
and uniformly spread
all over the sky. Are the
 bursters just remnants
of nearby dead stars?
6 November 2001
Astronomy 102, Fall 2001
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Gamma-ray bursters (continued)
Still, it was not possible to measure the position of any of the
-ray bursters precisely enough to observe them at any other
wavelength.
 One can’t really make  ray telescopes with which this
could be done.  rays do not reflect or refract significantly.
 The original determinations of -ray burster locations on
the sky were made by triangulation among several
satellite detectors, using the different arrival times
of the burst at each satellite.
 Typical bursts only last about 30 or 40 seconds, and it
takes hours to notify ground-based observers that a burst
has occurred.
So years passed without any explanation of their nature.
6 November 2001
Astronomy 102, Fall 2001
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The BATSE experiment on the Compton GammaRay observatory
In 1991, the NASA Compton Gamma-Ray Observatory was
deployed. It included the Burst and Transient Source
Experiment (BATSE), designed to detect more and fainter 
ray bursts, and measure their locations more precisely, than
was possible hitherto.
 The expectation was that the distribution of fainter  ray
bursters would look more like the Milky Way.
• Just like stars: the brighter nearby ones are evenly
distributed in the sky, but the more distant, fainter
ones comprise the Milky Way.
 The expectation was not borne out, though -- the  ray
bursters still looked uniform on the sky, even at faint
levels.
6 November 2001
Astronomy 102, Fall 2001
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A BATSE gamma-ray burster sky map.
This is a map of the whole sky, displayed so that the Milky
Way lies along the equator. The positions of 1776 BATSE
detections are plotted. Note that there is no tendency for the
-ray bursters to cluster in the Milky Way.
Image:
Michael
Briggs and
the BATSE
team, NASA
MSFC.
6 November 2001
Astronomy 102, Fall 2001
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Analysis of the BATSE results
Obviously -ray bursters are not numerously distributed
throughout our galaxy, as stars are. What other explanations
are there?
 Very nearby objects that are evenly distributed on the
sky, like the very nearest stars, or the cloud of comets
surrounding the Solar system.
• But how would these objects emit  rays?
 Very distant objects. Distant galaxies and galaxy clusters
are evenly distributed on the sky.
• But if the  ray bursters are that far away, their
luminosities are (problematically) enormous.
6 November 2001
Astronomy 102, Fall 2001
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The BeppoSAX satellite
In 1996 the Italian and Dutch space agencies launched
BeppoSAX, a satellite observatory designed (in part) to detect
X rays from some  ray bursters.
 X-ray telescopes can be made, though with difficulty. (As
you know, X-rays are good at passing unhindered
through matter, too, so they’re hard to reflect or refract.)
 The hope was that for each  ray burst they could find a
corresponding, bursting or fading, X-ray source and
measure its position.
 It worked. About 1 out of every 20  ray bursters found by
BATSE is also detected and localized by BeppoSAX, and
the position is made available to observers on the ground
within hours.
6 November 2001
Astronomy 102, Fall 2001
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Results of visible-light observations of BeppoSAX
positions:  ray bursters live in distant galaxies
Image of the  ray burst of 28 February 1997, taken with the
STIS instrument on the Hubble Space Telescope on 5
September 1997 (Andy Fruchter, STScI/NASA).
6 November 2001
Astronomy 102, Fall 2001
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 ray bursters live in distant galaxies (continued)
Image of the  ray burst of 8 May
1997, taken with the STIS instrument
on the Hubble Space Telescope in
early June (upper) and early August
(lower) 1998, by Andy Fruchter,
STScI/NASA).
Note that as the bright star-like object
fades, a faint, fuzzy galaxy remains.
6 November 2001
Astronomy 102, Fall 2001
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 ray bursters live in distant galaxies (continued)
Recent image of the  ray burst
of 14 December 1997, taken
with the STIS instrument on
the Hubble Space Telescope
(Steve Odewahn, Caltech).
The spectrum of this galaxy
indicates that its distance is 12
billion light years. At that
distance, the  ray burst
amounted to an energy of
3x1053 erg in  rays alone, if it
emitted its energy in all
directions.
6 November 2001
Astronomy 102, Fall 2001
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So what are  ray bursters?
3x1053 erg is an awful lot of energy to emit in 40 seconds.
 100 supernovae account for this much energy, but take
months and do so at longer wavelengths.
 The entire Milky Way emits this much light in 250 years,
but does so with much longer wavelength light.
Note that mc2 for a 2 M object (the heaviest neutron 2star) is

2
33
10 cm 
E  mc  2  2  10 gm   3  10

sec 

 3.6  10 54 erg
Accretion of a neutron star by a black hole at 10% efficiency
of conversion of rest energy to light would do it. Formation of
a black hole much more massive than this -- say, by merger
of two neutron stars – would also do.
6 November 2001
Astronomy 102, Fall 2001
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Some of the possibilities for  ray bursters
 Binary neutron stars, coalescing to form a black hole?
• Involves formation of a hot accretion disk, and burst
composed of  rays and relativistic, massive particles
(cosmic rays).
 Neutron star-black hole binary, with the neutron star
captured by the black hole?
• Also seems to work, theoretically.
 Hypernova: collapse of a 50-100 M star to form a black
hole, accompanied by a supernova-like explosion?
• Currently favored model, despite problems getting the
collapsing star to do the right sort of “bounce.”
Maybe all three mechanisms are represented among the
bursters. All involve black hole formation or growth.
6 November 2001
Astronomy 102, Fall 2001
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 ray bursters: the greatest of all natural disasters
A  ray burst like that on 14 December
1997 would destroy all life within
several thousand light years of the
burster. At 3000 ly:
  rays would ionize Earth’s
atmosphere; the gas would
recombine to form nitric oxides,
which in turn would eliminate the
ozone layer.
 If the  rays are followed by a
month-long blast of cosmic rays (as
models predict), everything within
200 m of the surface would receive a
lethal dose of radiation.
6 November 2001
Astronomy 102, Fall 2001
Sky and Telescope,
February 1998
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