The neutron, an elementary particle with neutral charge in the nucleus of atoms, was predicted by Ernest Rutherford in 1920. Twelve years later, James Chadwick found experimental proof of the existence of neutrons and later received the Nobel Prize for his discovery. Only two years after this discovery, astrophysicists predicted the existence of neutron stars. In 1934, after months of rigorous calculations regarding star death and of analysis of the properties of neutrons, Fritz Zwicky and Walter Baade published the paper Supernovae and Cosmic Rays . In this paper, they predict, “With all reserve we advance the view that supernovae represent the transitions from ordinary stars into neutron stars, which in their final stages consist of extremely closely packed neutrons.” They believed that these neutron stars were rapidly spinning, dense remnants of dead
stars.
Getting started on neutron stars:
Neutron stars are the collapsed cores of some massive stars. Neutron stars result from the death of massive stars, which are around 8-15 times more massive than our Sun. When a massive star collapses and explodes in a tremendous supernova explosion, the dense inner core is all that remains.
When the dying star is collapsing, the core gets progressively denser. Eventually, the elementary particles in the core (protons and electrons) cannot withstand the force of gravity, and they fuse together to form neutron. Eventually, all of the material in the core is converted into neutrons and the origin of the object's name becomes obvious.
Properties of Neutron Stars:
Neutron stars have four basic properties:
1. A small diameter: Neutron stars are only several kilometers in diameter, often no more than the size of a city. They are at least forty percent more massive than our Sun, however!
**2. high density: **Consequently, neutron stars are extremely dense, about
1014 (one hundred trillion) times denser than water. A
teaspoonful of neutron star could weigh up to one
billion tons.
3. Strong gravity: The gravitational force of a neutron star is proportional to its mass and inversely proportional to its radius squared. Thus, neutron stars have very strong gravitational fields. All of the particles in a neutron star are pulled together as tightly as possible, leaving a very smooth, spherical surface. If you tried to visit a neutron star, you would immediately be squished into elementary particles. On the surface of a neutron star, the escape speed (the speed at which you would have to travel in order to escape the gravitational field of a neutron star) is 160,000 km/s, over half the speed of light. Compare this to the escape speed on earth, which is only 11.2 km/s.
Neutron Stars pack roughly the mass of our Sun into a region the size of a city. Here's a comparison with New York:
At these incredibly high densities, you could cram all of humanity into a volume the size of a sugar cube. Naturally, the people thus crammed wouldn't survive in their current form, and neither does the matter that forms the neutron star. This matter, which starts out in the original star as a normal, well-adjusted combination of electrons, protons, and neutrons, finds its peace (a lower energy state) as almost all neutrons in the neutron star.
4. Strong magnetic field: These stars also have the strongest
magnetic fields in the known universe. The strongest
inferred neutron star fields are nearly a hundred trillion
times stronger than Earth's fields, and even the feeblest
neutron star magnetic fields are a hundred million times
Earth's, which is a hundred times stronger that any
steady field we can generate in a laboratory. Neutron
stars are extreme in many other ways, too. For example,
maybe you get a warm feeling when you contemplate
high-temperature superconductors, with critical
temperatures around 100 K? Hah! The protons in the
center of neutron stars are believed to become
superconducting at 100 million K, so these are the real
high-T_c champs of the universe.
All in all, these extremes mean that the study of neutron
stars affords us some unique glimpses into areas of
physics that we couldn't study otherwise.
Exploring Neutron Stars:
Neutron stars are believed to form in supernovae such as
the one that formed the Crab Nebula . The stars that eventually become neutron
stars are thought to start out with about 8 to 20-30
times the mass of our sun. These numbers are probably
going to change as supernova simulations become more
precise, but it appears that for initial masses much less
than 8 solar masses the star becomes a white dwarf,
whereas for initial masses a lot higher than 20-30 solar
masses you get a black hole instead (this may have
happened with Supernova 1987A , although detection of
neutrinos in the first few seconds of the supernova
suggests that at least initially it was a neutron star). In
any case, the basic idea is that when the central part of
the star fuses its way to iron, it can't go any farther
because at low pressures iron 56 has the highest
binding energy per nucleon of any element, so fusion or
fission of iron 56 requires an energy input. Thus, the iron
core just accumulates until it gets to about 1.4 solar
masses (the "Chandrasekhar mass"), at which point the
electron degeneracy pressure that had been supporting
it against gravity gives up the ghost and collapses
inward.
At the very high pressures involved in this collapse, it is
energetically favorable to combine protons and electrons
to form neutrons plus neutrinos. The neutrinos escape
after scattering a bit and helping the supernova happen,
and the neutrons settle down to become a neutron star,
with neutron degeneracy managing to oppose gravity.
Since the supernova rate is around 1 per 30 years, and
because most supernovae probably make neutron stars
instead of black holes, in the 10 billion year lifetime of
the galaxy there have probably been 10^8 to 10^9
neutron stars formed. One other way, maybe, of forming
neutron stars is to have a white dwarf accrete enough
mass to push over the Chandrasekhar mass, causing a
collapse. This is speculative, though, so I won't talk
about it further.
The Rise of Neutron Star:
We'll talk about neutron star evolution in a bit, but let's
say you take your run of the mill mature neutron star,
which has recovered from its birth trauma. What is its
structure like? First, the typical mass of a neutron star is
about 1.4 solar masses, and the radius is probably about
10 km. By the way, the "mass" here is the gravitational
mass (i.e., what you'd put into Kepler's laws for a
satellite orbiting far away). This is distinct from the
baryonic mass, which is what you'd get if you took
every particle from a neutron star and weighed it on a
distant scale. Because the gravitational redshift of a
neutron star is so great, the gravitational mass is about
20% lower than the baryonic mass.
Anyway, imagine starting at the surface of a neutron star
and burrowing your way down. The surface gravity is
about 10^11 times Earth's, and the magnetic field is
about 10^12 Gauss, which is enough to completely mess
up atomic structure: for example, the ground state
binding energy of hydrogen rises to 160 eV in a 10^12
Gauss field, versus 13.6 eV in no field. In the atmosphere
and upper crust, you have lots of nuclei, so it isn't
primarily neutrons yet. At the top of the crust, the nuclei
are mostly iron 56 and lighter elements, but deeper down
the pressure is high enough that the equilibrium atomic
weights rise, so you might find Z=40, A=120 elements
eventually. At densities of 10^6 g/cm^3 the electrons
become degenerate, meaning that electrical and thermal
conductivities are huge because the electrons can travel
great distances before interacting.
Deeper yet, at a density around 4x10^11 g/cm^3, you
reach the "neutron drip" layer. At this layer, it becomes
energetically favorable for neutrons to float out of the
nuclei and move freely around, so the neutrons "drip"
out. Even further down, you mainly have free neutrons,
with a 5%-10% sprinkling of protons and electrons. As
the density increases, you find what has been dubbed
the "pasta-antipasta" sequence. At relatively low (about
10^12 g/cm^3) densities, the nucleons are spread out
like meatballs that are relatively far from each other. At
higher densities, the nucleons merge to form spaghetti-
like strands, and at even higher densities the nucleons
look like sheets (such as lasagna). Increasing the
density further brings a reversal of the above sequence,
where you mainly have nucleons but the holes form (in
order of increasing density) anti-lasagna, anti-
spaghetti, and anti-meatballs (also called Swiss
cheese).
When the density exceeds the nuclear density 2.8x10^14
g/cm^3 by a factor of 2 or 3, really exotic stuff might be
able to form, like pion condensates, lambda hyperons,
delta isobars, and quark-gluon plasmas. Here's a
gorgeous figure (from http://www.astroscu.unam.mx/
neutrones/NS-picture/NStar/NStar-I.gif ) that shows the
structure of a neutron star:
Yes, you may say, that's all very well for keeping nuclear
theorists employed, but how can we possibly tell if it
works out in reality? Well, believe it or not, these things
may actually have an effect on the cooling history of the
star and their spin behavior! That's part of the next
section.
The Decline and Fall of A Neutron Star:
At the moment of a neutron star's birth, the nucleons
that compose it have energies characteristic of free fall,
which is to say about 100 MeV per nucleon. That
translates to 10^12 K or so. The star cools off very
quickly, though, by neutrino emission, so that within a
couple of seconds the temperature is below 10^11 K and
falling fast. In this early stage of a neutron star's life
neutrinos are produced copiously, and since if the
neutrinos have energies less than about 10 MeV they
sail right through the neutron star without interacting,
they act as a wonderful heat sink. Early on, the easiest
way to produce neutrinos is via the so-called "URCA"
processes: n->p+e+(nu) [where (nu) means an
antineutrino] and p+e->n+nu. If the core is composed of
only "ordinary" matter (neutrons, protons, and
electrons), then when the temperature drops below
about 10^9 K all particles are degenerate and there are
so many more neutrons than protons or electrons that
the URCA processes don't conserve momentum, so a
bystander particle is required, leading to the "modified
URCA" processes n+n->n+p+e+(nu) and n+p+e->n+n
+nu. The power lost from the neutron stars to neutrinos
due to the modified URCA processes goes like T^8, so as
the star cools down the emission in neutrinos drops
sharply.
When the temperature has dropped far enough (probably
between 10 and 10,000 years after the birth of the
neutron star), processes less sensitive to the
temperature take over. One example is standard thermal
photon cooling, which has a power proportional to T^4.
Another example is thermal pair bremsstrahlung in the
crust, where an electron passes by a nucleus and,
instead of emitting a single photon as in standard
bremsstrahlung, emits a neutrino-antineutrino pair. This
has a power that goes like T^6, but its importance is
uncertain. In any case, the qualitative picture of
"standard cooling" that has emerged is that the star first
cools by URCA processes, then by modified URCA, then
by neutrino pair bremsstrahlung, then by thermal photon
emission. In such a picture, a 1,000 year old neutron star
(like the Crab pulsar) would have a surface temperature
of a few million degrees Kelvin.
But it may not be that simple...
Near the center of a neutron star, depending on the
equation of state the density can get up to several times
nuclear density. This is a regime that we can't explore
on Earth, because the core temperatures of 10^9 K that
are probably typical of young neutron stars are actually
cold by nuclear standards, since in accelerators when
high densities are produced it's always by smashing
together particles with high Lorentz factors. Here, the
thermal energies of the particles are much less than
their rest masses. Anyway, that leaves us with only
theoretical predictions, which (as you might expect
given the lack of data to guide us) vary a lot. Some
people think that strange matter, pion condensates,
lambda hyperons, delta isobars, or free quark matter
might form under those conditions, and it seems to be a
general rule that no matter what the weird stuff is, if you
have exotic matter then neutrino cooling processes
proportional to T^6 can exist, which would mean that
the star would cool off much faster than you thought. It
even appears possible in some equations of state that
the proton and electron fraction in the core may be high
enough that the URCA process can operate, which would
really cool things down in a hurry. Adding to the
complication is that the neutrons probably form a
superfluid (along with the protons forming a
superconductor!), and depending on the critical
temperature some of the cooling processes may get cut
off.
So how do we test all this? We expect that after a
hundred years or so the core will become isothermal
(because it is then superfluid), and we can estimate
thermal conductivities in the crust, so if we could
measure the surface temperatures of many neutron stars,
then we could estimate their core temperatures, which
combined with age estimates and an assumption that all
neutron stars are basically the same would tell us about
their thermal evolution, which in turn would give us a
hint about whether we needed exotic matter.
Unfortunately, neutron stars are so small that even at the
10^6 K or higher temperatures expected for young
neutron stars we can just barely detect them. Adding to
the difficulty is that at those temperatures the peak
emission is easily absorbed by the interstellar medium,
so we can only see the high-energy tail clearly.
Nonetheless, ROSAT has detected persistent X-ray
emission from several young, nearby neutron stars, so
now we have to interpret this emission and decide what
it tells us about the star's temperature.
This ain't easy. The first complication is that the X-ray
emission might not be thermal. Instead, it could be
nonthermal emission from the magnetosphere. That
could carry information of its own, but it makes
temperature determinations difficult; basically, we have
to say that, strictly, we only have upper limits on the
thermal emission. Even if it were all thermal, we need to
remember that we only see a section of the spectrum
that is observable by an X-ray satellite, so we could be
fooling ourselves about the bolometric luminosity. In
fact, some early simulations of radiation transfer through
a neutron star atmosphere indicated that a neutron star
of effective temperature T_eff would yield far more
observed counts than a blackbody at T_eff. Thus, a
blackbody fit would overestimate the true temperature.
These simulations used opacities computed for zero
magnetic field. Thus, especially for low atomic number
elements such as helium, there weren't any opacity
sources at 500 eV (where the detectors operate), so in
effect we would be seeing deeper into the atmosphere
where it was hotter. Such simulations may be relevant
for millisecond pulsars, which have magnetic fields in
the 10^8 G to 10^10 G range.
Most pulsars, though, have much stronger fields, on the
order of 10^12 G. In fields this strong, the binding
energies of atoms go up (as mentioned before, the
ground state binding energy of hydrogen in 10^12 G is
160 eV), meaning that the opacity at those higher
energies rises as well. Thus, the X-ray detectors don't
see as far down into the atmosphere, and the inferred
temperature is less than in the nonmagnetic case. The
details of the magnetic calculations are very difficult to
do accurately, as they require precise computations of
ionization equilibrium and polarized radiative transfer,
and these are nasty in strong fields and dense, hot,
matter. It seems, though, that when magnetic effects are
included a blackbody isn't too bad an approximation.
Stay tuned.
So what does all this mean with respect to neutron star
composition? Yep, you guessed it, we don't have
enough data. If you squint and look sideways at a graph
of estimated temperature versus age, you might
convince yourself that there is some evidence of rapid
cooling, which wouldn't fit with the standard cooling
scenario. But, unfortunately, the error bars are too large
to be definite. We really need a large area detector that
can pick up more stars. Features in the spectra would be
nice, too, but at the moment that's just a dream. In the
meantime, here's some recent data, plotted against
several representative cooling curves that make various
assumptions about the internal composition (this graph
is from www.physik.uni-muenchen.de/sektion/
suessmann/astro/cool/ :
Spin history
Neutron stars rotate very rapidly, up to 600 times per
second. But how are they spinning when they are born?
They may be born rotating very fast, with periods
comparable to a millisecond (although evidence is
ambiguous). After that, they spin down ever after
because of magnetic torques. This seems to be
supported by the fact that some of the youngest pulsars,
such as the Crab pulsar (33 ms) and the Vela pulsar (80
ms) have unusually short periods. After a pulsar is born,
its magnetic field will exert a torque and slow it down,
with typical spindown rates of 10^-13 s/s for a young
pulsar like the Crab.
Although overall the tendency is for isolated pulsars to
slow down, they can undergo very brief periods of
spinup. These events are called "glitches", and they can
momentarily change the period of a pulsar by up to a few
parts in a million. The effects of glitches decay away in a
few days, and then the pulsar resumes its normal
spindown. In current models of glitches, the superfluid
core and normal crust are presumed to couple
impulsively, and since the crust had been spun down by
the magnetic field while the superfluid kept rotating at
its original rate, this coupling would speed up the crust,
leading to the observed spinup. It is very difficult to
treat this process from first (nuclear) principles, because
the critical angular velocity difference at which the crust
and superfluid finally couple depends sensitively on
various ill-determined properties of neutron superfluids,
and since these properties aren't directly accessible by
experiments we may have to be satisfied by our current
phenomenological description. Incidentally, the glitch
should also heat up the crust, and late in the lifetime of
the neutron star heating by rotational dissipation can
actually become a significant source of heat and affect
the temperature evolution.
Fine, so that's an isolated neutron star. If the star has a
companion, it can accrete from the companion and have
its rotational frequency altered that way. If the
companion is a low-mass star, say half the mass of our
Sun or lower, accretion tends to proceed by Roche lobe
overflow (more on that later). This type of flow has a lot
of angular momentum, so the matter forms a disk around
the star. The radius of the inner edge of the disk is
determined by the strength of the magnetic field; the
stronger the field, the farther out it can control the
accretion flow (for a given accretion rate). The star then
(more or less) tries to come to equilibrium with the
Keplerian angular velocity of the matter at the inner edge
of the accretion disk. This means that neutron stars with
relatively small (10^8 to 10^9 Gauss) magnetic fields can
be spun up to high frequencies, and this is the accepted
picture of how we get millisecond pulsars.
If the companion of the neutron star is a high-mass star
(over 10 solar masses) instead, then the matter that
makes it onto the neutron star goes in the form of a low
angular momentum wind. Therefore, the neutron star
isn't spun up to such high frequencies; in fact, some
pulsars that are in high-mass systems have periods
longer than 1000 seconds. The process of wind accretion
is a very complicated one, and numerical simulations of
the process push the limits of computers. It appears
that, in some circumstances, a disk may form briefly
around the neutron star, only to be dissipated and
replaced by a disk going the other way. One barrier to
understanding this kind of accretion is that, even with
today's computers, high-resolution 3D simulations just
aren't feasible now, so we have to derive what insight
we can from good two-dimensional calculations.
Misanthropic (aka isolated) neutron stars
Neutrons were discovered in 1932, and very shortly
afterward (in 1934) a suggestion was made by Walter
Baade and Fritz Zwicky that neutron stars were formed in
supernovae. But for many decades after that, neutron
stars were just hypothetical phenomena that didn't
attract much interest. Since the stars are so small,
people felt that the prospects for observing them were
minimal, and thus little effort was expended on theory or
observation of neutron stars.
This changed dramatically in 1967, due to serendipity
and the diligence of an Irish graduate student by the
name of Jocelyn Bell. Bell and her advisor, Anthony
Hewish, were working on radio observations of quasars,
which had been discovered in 1963. Bell and some other
graduate students constructed a scintillation array for
the observations, then she got down to examining the
charts of data produced (she had to analyze the miles of
charts by hand, since this was in the days before
powerful computers!). One day she noticed a bit of
"scruff" that appeared on the charts every second and a
third. The scruff was so regular that she first thought it
must be artificial. However, careful checking showed that
indeed the signal was extraterrestrial, and in fact that it
must be from outside the solar system. This source, CP
1919, was the first radio pulsar to be discovered.
The discovery initiated a storm of activity that has still
not abated. A number of other pulsars were discovered,
including one in the Crab Nebula, site of a famous
supernova in the year 1054 that was observed by
Chinese, Arabic, and North American astronomers (but
not recorded, as far as we know, by Europeans). Within a
year or so of the initial discovery, it became clear that
(1) pulsars are fast, with periods known in 1968 from
0.033 seconds (the Crab pulsar) to about 2 seconds, (2)
the pulsations are very regular, with a typical rate of
change of only a second per ten million years, and (3)
over time, the period of a pulsar always increased
slightly.
With this data, it was realized quickly that pulsars had
to be rotating neutron stars. With certain exceptions that
don't apply in this case, if a source varies over some
time t , then its size must be less than the distance light
can travel in that time, or ct (otherwise the variation
would be happening faster than the speed of light).
Thus, these objects had to be less than 300,000 km/s
times 0.033 seconds, or 10,000 km, in size. This restricts
us to white dwarfs, neutron stars, or black holes. You
can get a periodic signal from such objects via
pulsation, rotation, or a binary orbit. White dwarfs are
large enough that their maximum pulsational, rotational,
or orbital frequencies are more than a second, so this is
ruled out. Black holes don't have solid surfaces to
which to attach a beacon, so rotation or vibration of
black holes is eliminated. Black holes or neutron stars
in a binary could produce the required range of periods,
but the binary would emit gravitational radiation, the
stars would get closer together, and the period would
decrease, not increase (and would do so very quickly,
too!). Pulsations of neutron stars typically have periods
of milliseconds, not seconds. The only thing left is
rotating neutron stars, and this fits all of the
observations admirably. Here's an animated gif of a
pulsar.
There have now been more than 1000 radio pulsars
discovered, with periods from about 1.4 milliseconds to
more than 5 seconds. Their discovery is considered one
of the three most important astronomical discoveries in
the latter half of the twentieth century (along with
quasars and the microwave background), and in part for
his role in the discovery of pulsars Anthony Hewish
shared the 1974 Nobel Prize in physics.
Friendship between Neutron Stars:
Not all neutron stars are destined to lead a life of
isolation. Some of them are born in binaries that survive
the supernova explosion that created the neutron star,
and in dense stellar regions such as globular clusters
some neutron stars may be able to capture companions.
In either case, mass may be transferred from the
companion to the neutron star, as mentioned in the spin
evolution section above.
If the companion star has less than the mass of our Sun,
the mass transfer occurs via Roche lobe overflow. If part
of the companion star's envelope is close enough to the
neutron star, the neutron star's gravitational attraction
on that part of the envelope is greater than the
companion star's attraction, with the result that the gas
in the envelope falls onto the neutron star. However,
since the neutron star is tiny, astronomically speaking,
the gas has too much angular momentum to fall on the
star directly and therefore orbits around the star in an
accretion disk. Within the disk, magnetic or viscous
forces operate to allow the gas in the disk to drift in
slowly as it orbits, and to eventually reach the stellar
surface. If the magnetic field at the neutron star's
surface exceeds about 10^8 G, then before the gas gets
to the stellar surface the field can couple strongly to the
matter and force it to flow along field lines to the
magnetic poles. The friction of the gas with itself as it
spirals in towards the neutron star heats the gas to
millions of degrees, and causes it to emit X-rays. Some
characteristic dimensions of this sort of system are
displayed in the figure.
Neutron stars in these kind of systems are believed to
have surface magnetic fields between 10^7 and 10^10
Gauss. This means that the accreting gas can spiral very
close to the neutron star before it is grabbed by the
magnetic field. At such a close distance, the orbital
frequency is very high (hundreds of Hertz), so the
neutron star is spun up rapidly. As mentioned earlier,
this is how we think we get millisecond pulsars. Those
millisecond pulsars, by the way, are extremely stable
rotators; the best are at least as stable as atomic clocks!
There have been suggestions that using millisecond
pulsars as cosmic clocks could tell us about all sorts of
exotic things, such as the presence of a background of
gravitational radiation left over from the Big Bang.
Another fun phenomenon associated with neutron stars
that have low-mass companions is X-ray bursts. These
typically last a few seconds to a few minutes, and have a
peak luminosity nearly a hundred thousand times our
Sun's luminosity. The model for these bursts is that as
hydrogen and helium is tranferred to the neutron star
from the companion, it builds up in a dense layer.
Eventually, the hydrogen and helium have been packed
in a layer so dense and hot that thermonuclear fusion
starts, which then converts most or all of the gas into
iron, releasing a tremendous amount of energy. This is
the equivalent of detonating the entire world's nuclear
arsenal on every square centimeter of the neutron star's
surface within a minute! Some of these binaries can be
amazingly close to one another. Here's an artist's
conception (from heasarc.gsfc.nasa.gov/Images/exosat/
slide_gifs/exosat18.gif ) of one particularly extreme case,
that of 4U~1820-30, which has a binary period of just
over eleven minutes! Too bad the distances are in
miles...
If the companion to the neutron star has a mass between
one and ten times our Sun's mass, the mass transfer is
unstable and doesn't last very long, so there are few
objects in this category.
If the companion to the neutron star has a mass more
than about ten times our Sun's mass, the companion
naturally produces a stellar wind, and some of that wind
falls on the neutron star. The neutron stars in these
systems have strong magnetic fields, around 10^12
Gauss (similar to typical isolated pulsars). At field
strengths this high, almost all the accreting gas is
forced to flow along field lines to the magnetic poles.
This means that the X-rays primarily come from the
resulting hot spots on the poles. It also means that if
the magnetic axis and rotation axis of the star aren't co-
aligned, the radiation sweeps past us once per rotation
and we see X-ray pulsations. These systems are
therefore called "accretion-powered pulsars", to
distinguish them from the "rotation-powered pulsars"
that Jocelyn Bell discovered.
For some recent results on accreting neutron stars,
check out a poster from a science fair for grownups held
at the University of Chicago.
What the hell makes gamma-ray bursts?
Gamma-ray bursts have been known for more than 25
years, but there are still a lot of uncertainties about their
origins. They were first discovered in the late 1960's as
part of nuclear test ban verification; US satellites picked
up bursts of gamma rays and there was a lot of concern
that these might be due to Soviet nuclear explosions,
but it was determined that the bursts originated outside
the atmosphere. The "official" discovery came in 1973
(by Klebsedal, Olsen, and Strong). Since then, more than
2500 bursts have been detected, over 1800 by BATSE
(the Burst and Transient Source Experiment aboard the
Compton Gamma-Ray Observatory). Before tackling the
question of what gamma-ray bursts are, we need to
establish what they are observationally.
Loosely speaking, gamma-ray bursts are, well, bursts of
energy that appear mostly in gamma rays and come from
outside the Earth. The flux at earth is between 10^-8
erg/cm^2/s and 10^-3 erg/cm^2/s, the duration of the
bursts is between 10 ms and 1000 s, and the photons
typically have energies between 100 keV and 2 MeV,
although energies down to 5 keV and up to 18 GeV have
been seen from some bursts. The flux as a function of
time varies from burst to burst, but often a spike within
a burst follows the "fred" profile (fast rise, exponential
decay). All in all, gamma-ray bursts are extremely heterogeneous, so it is
tough to extract characteristic behaviors that would lead
to easy classification (see a typical time profile for
Can we at least tell how far away gamma-ray bursts are?
Until recently, the answer was "no", not with any
certainty. From the early 1970s it has been apparent that
gamma-ray bursts come from all parts of the sky with
approximately equal probability. Since other aspects of
gamma-ray bursts (such as the fast rise time [<1 ms in
some cases] and high photon energies) seemed
consistent with a neutron star origin, most people prior
to 1990 believed that gamma-ray bursts came from
galactic neutron stars, and that instruments simply
hadn't had the sensitivity to probe deeply enough to see
a bias towards the galactic center and plane. However,
since 1990 the Burst and Transient Source Experiment
(BATSE) aboard the Compton Gamma-Ray Observatory
has seen nearly one gamma-ray burst per day, and
these too are nearly isotropic (click for the sky map of
the first 921 bursts). It is believed that, if galactic
neutron stars really are the sources of gamma-ray
bursts, BATSE should be able to see them far enough
away that the distribution should be more like a pancake
than a sphere. Another piece of evidence comes from the
number of sources seen with at least a given flux. If the
universe were Euclidean and the sources were spread
out uniformly, then out to a distance r there would be a
number of sources proportional to r^3, and the dimmest
sources would have fluxes proportional to 1/r^2. Thus,
in a Euclidean universe with uniformly distributed
sources of a given intrinsic luminosity, a plot of log N
(N=number of sources at a flux greater than F) versus
log F should have a slope of -3/2. At the highest fluxes
this slope is seen, but at lower fluxes the slope
becomes smaller, exhibiting a continuous rollover and
becoming about -0.8 at the lowest fluxes BATSE can
see.
What does that mean? The dropoff at lower fluxes, which
corresponds to greater distances if the intrinsic
luminosity is constant, means that in some sense there
is an edge to the distribution. For example, if the
sources were distributed in a thin plane instead of a
sphere, the slope would be -1, and for sources in a line
the slope is -0.5. Even if the source distribution is
spherical, the slope will roll over if the sources become
less dense at greater distances, or if the flux drops off
faster than 1/r^2. Because of the isotropy of the
distribution, many people believed that gamma-ray
bursts are cosmological, at typical redshifts z=1, where
the redshift would decrease the flux in about the right
way to account for the log N - log F rollover. But until
1997 there wasn't any "smoking gun" to tell us for sure
that the bursts were cosmological, and actually there
were plausible models of gamma-ray bursts in which the
bursts came from an extended halo around our own
galaxy.
All this changed in 1997, when researchers using the
Italian-Dutch satellite BeppoSAX made a tremendous
breakthrough. A sticking point in our understanding of
gamma-ray bursts was that they were always a "one and
done" type of phenomenon, in which, after a brief flash
of gamma-rays lasting a few seconds, that was all she
wrote. The lack of any detectable emission in other
frequencies (such as optical and radio) meant that these
sources couldn't be localized with any certainty. This is
where BeppoSAX came in. This satellite has the
capability of detecting X-ray emission from half a dozen
to a dozen gamma-ray bursts per year and localizing the
emission to within about two arcminutes (a thirtieth of a
degree, or a little less than the apparent size of a billiard
ball at the far end of a football field). This is about 100
times better than the localization possible with BATSE.
It allowed people to discover that most of the gamma-
ray bursts observed with BeppoSAX have x-ray
afterglows; here's an image of the first one, showing a
bright spot (left) that fades away later (right). Many have
optical and radio afterglows as well! The afterglows in
optical and radio allowed the position to be nailed down
to an arcsecond or better (the apparent size of an
eyelash at the far end of a football field!). Further
observations showed that, at least in projection and
probably in reality, the gamma-ray bursts are in
galaxies. Not only that, but redshifts have been
measured for many of these galaxies, and some of them
are really big: one was measured at a redshift greater
than 3.4! So that settles at least part of the question: the
bursts observed with BeppoSAX are definitely
cosmological. If gamma-ray bursts are cosmological,
their energy release must be gigantic. It is now thought
that most bursts are tightly beamed (like a lighthouse)
and emit a good 10^51 ergs in just gamma rays. It is still
really puzzling how this could happen. The constraints
on these models are really tight, and no detailed model
yet suggested can circumvent all the constraints. No
matter what these are, though, the energy release by
itself guarantees that the central engine is one of the
biggest blasts around! The two most popular current
ideas are the bursts are caused by the inspiral and
merger of two neutron stars or a neutron star and a
black hole, or (2) the bursts are caused by the collapse
of a massive star (maybe 20 solar masses or more) into a
rapidly spinning massive black hole. General opinion is
that the longer bursts are of type (2), but there is still a
question about the shorter bursts. Either way, it seems
unavoidable that all that energy sails out into interstellar
space and produces a serious blast wave.
Soft gamma-ray repeaters and whopping magnetic fields.
Another (somewhat less) mysterious type of bursting
events believed to come from neutron stars is the soft
gamma-ray repeater bursts. These typically last from 0.1
seconds to 3 seconds and have spectral peaks in the 10
keV to 30 keV range. Soft gamma-ray repeaters have in
the past been identified with supernova remnants, but
with the possible exception of the single source in the
Large Magellanic Cloud (SGR 0525-66) these
identifications are now considered dubious (see
Gaensler et al. 2001, ApJ, 559, 963). Caution is especially
appropriate because there are only four (!) SGRs known
(SGR 0525-66, SGR 1900+14, SGR 1806-20, and SGR
1627-41, where the numbers give the right ascension
and declination in B1950 coordinates). Despite the
paucity of these sources, interest has focused on them
because (1) they have observational properties distinct
from that of any other known astronomical phenomenon,
(2) they have some tantalizing links to gamma-ray
bursts, and (3) one current model of SGRs involves
neutron stars with 10^14 Gauss to 10^15 Gauss surface
magnetic fields, in which a variety of exotic
microphysical processes could be important.
One particular burst from SGR 0525-66, which occurred
on March 5, 1979, has attracted so much attention that it
is usually called just the "March 5 event". This was the
highest intensity gamma-ray event seen up to that
point. It started with a hard spike that lasted a quarter of
a second and had a rise time less than a millisecond,
then continued emitting softer radiation for another 200
seconds. The emission during this extended tail had a
clear period of 8 seconds, and was consistent with
rotational modulation. Because of the high intensity and
rapid onset of this event, nine different satellites
throughout the Solar System recorded this event, and
the relative timing between the satellites allowed the
direction of the event to be determined very accurately.
It was determined that the event came from a direction
consistent with the N49 supernova remnant in the Large
Magellanic Cloud, putting it at a distance of somewhat
more than 50 kiloparsecs. At this distance, the initial
hard spike had a peak luminosity of more than 10^45
ergs per second. That is to say, in the first quarter
second of the burst, this source put out as much energy
as the Sun radiates in 3000 years! This is also the event
that makes some astronomers think that SGRs are
related to classical gamma-ray bursts. If the hard spike
is analyzed by itself, then its duration, light curve, and
energy spectrum are indistinguishable from classical
GRBs. Indeed, if the event had occured ten times as far
away as it did (so that we would have missed the
extended soft emission), we would have considered this
another ho-hum gamma-ray burst.
Observations of other bursts from SGR 0525-66 (none as
spectacular as the March 5 event) and bursts from SGR
1900+14 and SGR 1806-20 suggested initially that all are
associated with supernova remnants, but as mentioned
above this has been challenged. Even if they are
associated with the remnants, the sources are not at the
center of the remnants; instead, they are off to the side,
by distances that would imply a velocity of 500-1500
kilometers per second. The typical peak luminosity of a
SGR burst is 10^40 to 10^42 ergs per second. This
information can be put together as follows:
The March 5 event displayed an 8 second rotational
period. Black holes don't have solid surfaces to give
such a coherent rotational period, therefore it must be
a neutron star.
SGRs may (or may not!) be associated with supernova
remnants. If they aren't, most bets are off. On the
other hand, if they are:
Supernova remnants leave behind neutron stars or
black holes, so SGRs must be related to neutron stars
or black holes.
If the supernova remnant were more than about
100,000 years old, it would have dissipated so we
couldn't see it. We can, thus the compact object
producing the SGR must be relatively young.
Thus, if SGRs are associated with supernova remnants
then they come from young neutron stars.
What is the energy source for the bursts?
One naturally thinks of accretion or rotation, but strong
magnetic fields have also been considered.
If SGRs are associated with supernova remnants they are
moving at high speeds, because they are not at the
center of the remnants. Accretion then has serious
problems, because the high velocities inferred for all
three SGRs mean that the neutron star can't pick up
enough mass from the interstellar medium. Also, it turns
out that accretion from, e.g., asteroids would be
expected to last tens of thousands of seconds instead of
the observed tenths of seconds. Rotation has even
greater problems. A neutron star spinning at an 8
second period, such as the one that produced the March
5 event, has only about 3 times 10^44 ergs in rotational
energy available. But the March 5 event itself released
about 4 times 10^44 ergs, and the X-ray energy released
since then in persistent emission is another 3 times
10^44 ergs, so there isn't enough rotational energy to do
the job.
Starting about 1992, Chris Thompson and Rob Duncan
started proposing another energy source, that of very
strong magnetic fields. They were drawn to this in part
because the March 5 event implies a very long rotational
period (8 seconds) compared to the expected birth spin
period of neutron stars (less than a second). If, as
usually thought, the neutron star spins down by
magnetic braking, then to get to that long period in the
5,000 year age of the N49 supernova remnant requires
that the field be nearly 10^15 Gauss! Thompson and
Duncan noticed that this would imply a total magnetic
energy in the star of about 10^47 ergs, which is easy
enough. They also found that this model is consistent
with the other properties of SGR bursts.
So, maybe some neutron stars have magnetic fields of
10^15 Gauss. So what? Given that we're sure that some
neutron stars have fields of 10^12 to 10^13 Gauss, which
already sounds unbelievably large, what's the big deal
with another two orders of magnitude?
The difference comes at the subatomic level. In a
magnetic field, a charged particle such as an electron or
proton will spiral around the field at a preferred
frequency, the cyclotron frequency, that is proportional
to the strength of the field. This principle is used in
magnetic resonance imaging, where the preferred
frequency (of nuclei) is in the radio wavelengths. When
magnetic fields of neutron star strength are introduced,
the electron cyclotron frequency is in the X-rays, and
when the field is 4.414 times 10^13 Gauss the electron
cyclotron energy (the cyclotron frequency times Planck's
constant) equals the electron rest mass energy. This
field turns out to be a critical field in quantum
electrodynamics, such that (essentially) above that field
there are a number of bizarre processes (e.g., single
photon pair production, photon splitting) that can be
very important, whereas below the critical field those
processes are negligible. We don't have a prayer of
accessing this regime of ultrastrong fields in the
laboratory, and we only have our quantum mechanical
predictions to guide us. So, if we can establish that
such fields exist in astronomy, then by studying those
objects we can test our quantum mechanical theories in
a new physical regime.
But first, we have to get more direct evidence that such
high fields exist. Recent supporting evidence arrived in
1998, when several soft gamma-ray repeaters were
active and when it was finally possible to measure (1) a
spin period and (2) a rate of change of the spin period,
which in the simplest approximation allow an estimate
of the magnetic field of these sources. You guessed it, it
seems like magnetic fields greater than about 10^14
Gauss are required, although there are still some
subtleties. What would really be nice would be a
signature in the energy spectrum of these ultrastrong
fields. We don't have it yet, but there has been a lot of
effort by many people. Tomek Bulik and I have proposed
the one that I think is most promising. It relates to
something called the vacuum resonance, and the
spectral signature is a dip in the X-ray spectrum that
moves to lower energies when the intensity is higher. If
such a signature is seen, we will then have strong and
direct evidence for the existence of these superstrong
fields, and theorists such as myself will have a
wonderful new playground!
