Black Holes
NOTE: This section is about stellar-mass black holes. For information about
black holes that measure in the billions of solar masses, see Active Galaxies
& Quasars .
There are many popular myths concerning black holes, many of them
perpetuated by Hollywood. Television and movies have portrayed them as
time-traveling tunnels to another dimension, cosmic vacuum cleaners sucking
up everything in sight, and so on. It can be said that black holes are really just
the evolutionary end point of massive stars. But somehow, this simple
explanation makes them no less mysterious, and no easier to understand.
Black holes: What are they?
Black holes are the evolutionary endpoints of stars at least 10 to 15 times as
massive as the Sun. If a star that massive or larger undergoes a supernova
explosion, it may leave behind a fairly massive burned-out stellar remnant.
With no outward forces to oppose gravitational forces, the remnant will
collapse in on itself. The star eventually collapses to the point of zero volume
and infinite density, creating what is known as a "singularity." Around the
singularity is a region where the force of gravity is so strong that not even light
can escape. Thus, no information can reach us from this region. It is therefore
called a black hole, and its surface is called the "event horizon."
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If
our Sun was suddenly replaced with a black hole of the same mass, Earth's
orbit around the Sun would be unchanged. Of course, Earth's temperature
would change, and there would be no solar wind or solar magnetic storms
affecting us. To be "sucked" into a black hole, one has to cross inside
the Schwarzschild radius. At this radius, the escape speed is equal to
the speed of light, and once light passes through, even it cannot escape.
The Schwarzschild radius can be calculated using the equation for escape
speed:
vesc = (2GM/R)1/2
For photons, or objects with no mass, we can substitute c (the speed of light)
for Vesc and find the Schwarzschild radius, R, to be
R = 2GM/c2
If the Sun were replaced with a black hole that had the same mass as the
Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius
of nearly 700,000 km). Hence the Earth would have to get very close to get
sucked into a black hole at the center of our Solar System.
If we can't see them, how do we know they are there?
Since stellar black holes are small (only a few to a few tens of kilometers in
diameter), and light that would allow us to see them cannot escape, a black
hole floating alone in space would be hard, if not impossible, to see in the
visual spectrum.
However, if a black hole passes through a cloud of interstellar matter, or is
close to another "normal" star, the black hole can accrete matter into itself. As
the matter falls or is pulled towards the black hole, it gains kinetic energy,
heats up and is squeezed by tidal forces. The heating ionizes the atoms, and
when the atoms reach a few million Kelvin, they emit X-rays. The X-rays are
sent off into space before the matter crosses the Schwarzschild radius and
crashes into the singularity. Thus we can see this X-ray emission.
The optical companion of
the black hole candidate Cygnus X-1
Binary X-ray sources are also places to find strong black hole candidates. A
companion star is a perfect source of infalling material for a black hole.
A binary system also allows the calculation of the black hole candidate's
mass. Once the mass is found, it can be determined if the candidate is
a neutron star or a black hole, since neutron stars always have masses of
about 1.5 times the mass of the Sun. Another sign of the presence of a black
hole is its random variation of emitted X-rays. The infalling matter that emits
X-rays does not fall into the black hole at a steady rate, but rather more
sporadically, which causes an observable variation in X-ray intensity.
Additionally, if the X-ray source is in a binary system, and we see it from
certain angles, the X-rays will be periodically cut off as the source
is eclipsed by the companion star. When looking for black hole candidates, all
these things are taken into account. Many X-ray satellites have scanned the
skies for X-ray sources that might be black hole candidates.
Cygnus X-1 (Cyg X-1) is the longest known of the black hole candidates. It is
a highly variable and irregular source, with X-ray emission that flickers in
hundredths of a second. An object cannot flicker faster than the time required
for light to travel across the object. In a hundredth of a second, light travels
3,000 kilometers. This is one fourth of Earth's diameter. So the region emitting
the X-rays around Cyg X-1 is rather small. Its companion star, HDE 226868 is
a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic
observations show that the spectral lines of HDE 226868 oscillate with a
period of 5.6 days. From the mass-luminosity relation, the mass of this
supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have
a mass of about 7 solar masses, or it would not exert enough gravitational pull
to cause the wobble in the spectral lines of HDE 226868. Other estimate put
the mass of Cyg X-1 to as much as 16 solar masses. Since 7 solar masses is
too large to be a white dwarf or neutron star, it must be a black hole.
An illustration of Cygnus X-1, showing the companion star HDE 226868,
the black hole, material streaming from the companion to the black hole,
and the emission of X-rays near the black hole.
There are now about 20 X-ray binaries (as of early 2009) with known black
holes (from measurements of the black hole mass). The first of these, an Xray transient called A0620-00, was discovered in 1975, and the mass of the
compact object was determined in the mid-1980's to be greater than 3.5 solar
masses. This very clearly excludes a neutron star, which has a mass near 1.5
solar masses, even allowing for all known theoretical uncertainties. The best
case for a black hole is probably V404 Cygni, whose compact star is at least
10 solar masses. There are an additional 20 X-ray binaries which are likely to
contain black holes - their behavior is the same as the confirmed black holes,
but mass measurements have not been possible.
Active Galaxies and Quasars
Active galaxies are galaxies which have a small core of emission embedded
in an otherwise typical galaxy. This core may be highly variable and very
bright compared to the rest of the galaxy. Models of active galaxies
concentrate on the possibility of a supermassive black hole which lies at the
center of the galaxy. The dense central galaxy provides material
which accretes onto the black hole releasing a large amount
of gravitational energy. Part of the energy in this hot plasma is emitted as xrays and gamma rays.
For "normal" galaxies, we can think of the total energy they emit as the sum of
the emission from each of the stars found in the galaxy. For the "active"
galaxies, this is not true. There is a great deal more emitted energy than there
should be... and this excess energy is found in the infrared, radio, UV, and Xray regions of the electromagnetic spectrum. The energy emitted by an active
galaxy (or AGN) is anything but "normal". So what is happening in these
galaxies to produce such an energetic output?
There are several types of active galaxies: Seyferts, quasars, and blazars.
Most scientists believe that, even though these types look very different to us,
they are really all the same thing viewed from different directions! Quasars are
active galaxies which are all very, very, very far away from us. Some of the
quasars we have seen so far are 12 billion light-years away! Blazars are very
bright in the radio band, which results from looking directly down a jet which is
emitting in synchrotron radiation. On the other hand, if the jet is not pointing
toward you at all, and the dusty disk of material which lies in the plane of the
galaxy is in the way, you would see just what we see from the Seyferts. By
measuring their redshifts, we find that Seyferts are much closer to us than
quasars or blazars.
Active galaxies are intensely studied at all wavelengths. Since they can
change their behavior on short timescales, it is useful to study them
simultaneously at all energies. X-ray and gamma-ray observations have
proven to be important parts of this multi-wavelength approach since many
high-energy quasars emit a large fraction of their power at such energies. Xrays can penetrate outward from very near the center of a galaxy. Since that
is where the "engines" of AGN are located, X-rays provide scientists with
unique insights into the physical processes occurring there. In addition,
gamma-ray observations alone can provide valuable information on the nature
of particle acceleration in the quasar jet, and clues as to how the particles
interact with their surroundings.
A diagram of an active galaxy, showing the primary components.
A Monster in the Middle
Most large galaxies have ~1011 Mo of stars, ~109-10 Mo of interstellar gas, and
~1012 Mo of dark matter. But at least 5% of galaxies — though it may be all of
them — also have something else lurking inside: a monster in the middle. This
monster is a supermassive black hole, which ejects tremendous amounts of
energy from jets at its top and bottom. How can this incredible objects be
explained?
Long ago, when galaxies were young, the stars in the central regions of the
galaxies were very closely packed. Star collisions and mergers occurred,
giving rise to a single massive black hole (MBH) with perhaps 106 to 109 Mo.
Gas from the galaxy's interstellar medium, from a cannibalized galaxy, or from
a star that strays too close, falls onto the MBH. As in X-ray binary star
systems, an accretion disk forms, emitting huge amounts of light across
the electromagnetic spectrum (infrared to gamma rays). The MBH
plus accretion disk produces the phenomena seen in active galactic nuclei
(AGN). Below are optical and radio images of the active galaxy NGC 4261.
The central object, accretion disk and lobes are all visible.
Ground-based and Hubble Space Telescope images
of the Active Galaxy NGC 4261
The different types of AGN are variations on this theme. Many galaxies may
have a quiet MBH that has not recently accreted gas. Seyfert galaxies exhibit
accretion onto a moderate-mass MBH, while the more luminous quasi-stellar
objects, like quasars, exhibit accretion onto a high-mass MBH.
In approximately 10% of the AGN, the MBH + accretion disk somehow
produces narrow beams of energetic particles and magnetic fields, and eject
them outward in opposite directions away from the disk. These are
the radio jets, which emerge at nearly the speed of light. Radio
galaxies, quasars, and blazars are AGN with strong jets that can travel
outward into large regions of intergalactic space. Many of the apparent
differences between types of AGN are due to our having different orientations
with respect to the disk. With blazars and quasars, we are looking down the
jet. For Seyferts, we are viewing the jet broadside.
Considerable uncertainties remain. Exactly how are jets produced and
accelerated? Where do the clouds producing the broad emission lines come
from? Can we empirically confirm that a MBH is actually present?
An artist's conception of an AGN
Seyfert Galaxies
Of the two types of Active Galactic Nuclei (AGN) which emit gamma rays,
Seyfert galaxies are the low-energy gamma-ray sources.
Seyfert galaxies typically emit most of their gamma rays up to energies of
about 100 keV and then fade as we observe them at higher energies. Early
gamma-ray observations of Seyfert galaxies indicated that photons were
detected up to MeV energies, but more sensitive observations have cast
doubt on this possibility. At these low gamma-ray energies, the emission is
usually a smooth continuation of the X-ray emission from such objects. This
generally indicates that the physical processes creating the gamma rays are
thermal processes similar to those responsible for emission from galactic
black hole sources. As a result, gamma-ray studies of the high-energy
spectrum and variability can give scientists important information about the
physical environment in the AGN.
Observations of Seyfert galaxies in gamma rays are also important for studies
of the cosmic gamma-ray background. Even in regions of the sky where there
are no point sources, a faint gamma-ray glow is detectable. It may be that this
glow is the sum of many faint galaxies or perhaps a more exotic process.
Studies of individual Seyfert galaxies can be combined with a model of how
such objects are distributed in the Universe to compare to the diffuse gammaray background. In this way, astronomers not only learn about the interesting
AGN phenomena, but also learn more about the general nature of the
Universe as a whole.
An artists concept of an active galactic nucleus
Consider NGC 4151, a spiral galaxy 15 Mpc away. Photographs by Carl
Seyfert in the 1940s showed a bright point-like nucleus. Its spectrum is
unusual: in addition to a continuum and absorption lines from normal stars,
Seyfert galaxy nuclei have strong emission lines. Some are commonly found
lines, such as hydrogen (e.g. the Balmer series H-alpha, H-beta lines). Others
are not as common, or even rare, like the lines for twice-ionized oxygen, in
which the oxygen atom has lost two of its electrons, and whose formation
requires extremely hot gas. The lines are broad, requiring that the gas
be Doppler shifted in all directions up to ~20,000 km/s. The nuclei vary in
brightness on timescales of months, requiring them to be < 1 parsec in size.
The total luminosity can be equivalent to 1010 Lo.
What is this bizarre object in the center of Seyfert's spiral galaxies?
Later in the 1940s, astronomers began scanning the skies with radio
telescopes. They found strange radio structures on opposite sides of radio
galaxies, plus a tiny source of radio emission at the nucleus. The nuclei of
these radio galaxies shoot out narrow beams of extremely
energetic electrons and magnetic fields, producing radio synchrotron
radiation. The radio components include: the compact core at the galaxy
nucleus, jets, lobes, and a hot spot where the jet slams into the interstellar
medium.
Quasars
One of the most remarkable trends in gamma-ray astronomy in recent years
has been the emergence of high-energy gamma-ray quasars as an important
component of the gamma-ray sky. At gamma-ray energies, these active
galaxies are bright; they are highly variable at all energies. Unlike the Seyfert
type AGN, most of these sources are preferentially detected at high energies,
usually 100 MeV or more. In fact, they have been detected above 1 GeV, and
some up to several TeV! Given the large distances to these objects and the
strong emission of high-energy gamma rays, these are the most powerful
particle accelerators in the Universe. Over 50 high-energy quasars are known
at this time. Some appear as fuzzy stars that can be seen with large amateur
telescopes.
Many astronomers believe that Seyfert galaxies and high-energy quasars are
basically the same type of objects, but we are simply viewing them differently.
Radio observations of AGN often show powerful jets, streams of particles
coming from the central source -- like water from a spigot. Charged particles
are accelerated to nearly the speed of light in these jets. In the unified view of
active galaxies, high-energy quasars are being viewed with the jet pointed
towards us which allows us to see the resulting energetic radiation. With
Seyfert galaxies, we are viewing from the side and do not see the very highenergy radiation which is traveling down the jet.
The region of the sky containing one of the high-energy quasars,
PKS 0528+134, is shown at two different times using the EGRET
instrument on the Compton Gamma-Ray Observatory. These active
galaxies are highly variable, strongly emitting gamma-rays
sometimes, disappearing at other times.
In the 1960s, some radio sources seemed to be associated with "stars," and
were called quasi-stellar radio sources or quasars. However, they had spectra
similar to Seyfert galaxy nuclei. It became clear that they are Seyferts, and
radio galaxies where the nucleus out shines all of the stars by factors of 10 to
1,000. The luminosity of quasars can reach 1012 Lo. They also tend to be
farther away than either Seyfert galaxies or blazars.
In the 1970-80s, findings include:
1. X-ray satellite telescopes found strong and very rapidly variable X-ray
emission from Seyferts and quasars. Timescales for these variations
were as short as days, hours, or even minutes.
2. Rare BL Lac objects and blazars were discovered. These are radio
galaxies with jets pointing directly at us, ejected by the active nucleus at
velocities near the speed of light.
3. Optical astronomers find thousands of faint distant quasars which are
not radio-loud. Strangely, there were many more quasars early in
the Universe than there are today.
4. In 1993, the Compton Gamma-Ray Observatory discovers incredibly
intense gamma-rays from the jets of some blazars: Stronger than X-ray,
optical, radio emission combined.
Blazars
The AGNs observed at higher energies form a subclass of AGNs known as
blazars; a blazar is believed to be an AGN which has one of its relativistic jets
pointed toward the Earth so that what we observe is primarily emission from
the jet region. They are thus similar to quasars, but are not observed to be as
luminous. The visible and gamma-ray emission from blazars is variable on
timescales from minutes to days. Although theories exist as to the causes of
this variability, the sparse data do not yet allow any of the ideas to be tested.
To date more than 60 blazars have been detected by the EGRET experiment
aboard the Compton Gamma-Ray Observatory. All these objects appear to
emit most of their bolometric luminosity at gamma-ray energies and, in
addition, are strong extragalactic radio sources.
AGNs observed at high (>100 MeV) energies form a subclass known as
blazars, which is thought to be an AGN that has one of its relativistic jets
pointed toward Earth so the emission we observe is dominated by
phenomena occurring in the jet region. Among all AGNs, blazars emit over the
widest range of frequencies, and have been detected from radio to gammaray.
Specifically, to be classified as a blazar an AGN must be seen with one of the
following properties:



high radio-brightness accompanied by flatness of the radio spectrum
high optical polarization,
strong optical variability on very short timescales (less than few days).
In the class of objects selected according to these criteria, there appear to be
two subgroups:


Sources showing strong and broad emission lines, such as those of
quasars (called Flat Spectrum Radio Quasars)
Sources showing a featureless optical spectrum (called BL Lac
objects).
There are additional important differences between these subclasses. For
example, blazars show different luminosity and redshift distributions, and a
different morphology of the extended radio emission.
In its first year of operation, the Fermi Gamma-Ray Space Telescope detected
709 active galaxies, most of which are blazars. Of these, 300 are BL Lac
objects, nearly 300 are Flat Spectrum Radio Quasars (FSRQ), 41 are other
types of AGN, and 72 are of unknown types.