Black holes are deep wells in the fabric of space and time. They have such immense gravity that nothing, not even light, can escape them. This makes studying black holes difficult - how can you see something when it does not emit or reflect any form of energy? A black hole seems to be the study of the invisible.
To get around this problem, astrophysicists study other high-energy phenomena that are associated with black holes. A NATO-sponsored Advanced Studies Institute entitled "The Neutron Star - Black Hole Connection" was held on the Greek Island of Crete from June 7-18. Among the Institute organizers were Marshall Space Flight Center researchers Dr. Chryssa Kouveliotou of the Universities Space Research Association and Prof. Jan van Paradijis of the University of Alabama, Huntsville. Several other Marshall scientists also attended the Institute, presenting topics on everything from magnetars to gamma-ray bursts. The close examination of such high-energy phenomena may someday lead to a better understanding of black holes.
"There's very little question about whether black holes exist. They are almost certainly there, and now we need to study them in more detail," said Dr. Martin Weisskopf, chief scientist for X-ray astronomy at Marshall.
How Black Holes are Formed
Einstein's general theory of relativity states that mass distorts the space-time
metric, resulting in gravity. Think of a bowling bowl resting in the middle of a
rubber sheet. The mass of the ball creates a dip in the sheet. Likewise, large
masses such as planets and stars create dips in the space-time metric. The
association between mass, gravity and the space-time metric caused the German
astrophysicist Karl Schwarschild to wonder what happens to the space-time metric
when faced with the greatest mass and gravity possible. His answer: the infinite
density and immense gravity of a black hole.
Neutron stars - stars mainly composed of neutrons - may be a key to understanding how black holes form. Neutron stars are not the most massive stars in the universe, but they are second only to black holes in gravitational intensity. This gravity compresses a neutron star into a sphere of extreme density - comparable to cramming the world's population into a volume the size of a sugar cube. One teaspoon of neutron star would weigh 100 million tons on Earth.
A neutron star actually starts out life as a normal star, with protons, electrons and neutrons. Nuclear fusion in the core generates electromagnetic radiation. The star maintains a balance between outward radiation pressure and the inward pull of gravity, but when the star exhausts its store of nuclear fuel the star's gravitation takes over, causing the star to collapse. At this point, stars follow two paths of evolution. Smaller stars - up to eight times as large as our Sun - become "white dwarfs." A white dwarf is the cooled core of the star, gravitationally contracted to about the size of Earth. A white dwarf doesn't compress further because the pressure of its electrons resists the inward pull of gravity.
Larger stars form larger cores, and when they use up their nuclear fuel they collapse quickly, shedding material in a massive explosion called a supernova. The remaining core forms into a neutron star. The neutrons resist the inward pull of gravity much as a white dwarf's electrons do. If, however, the core exceeds 2 solar masses (twice as massive as our Sun), the neutrons cannot resist the force of gravity. The star becomes a "singularity" of infinite density and collapses into a black hole.
The small size of neutron stars makes them very difficult to detect. Because their lower-energy emissions are absorbed by the interstellar medium, most of our information about neutron stars comes from their emissions of high energy X-rays and gamma rays.
Pulsars and magnetars are two types of neutron stars that were discussed at the NATO Institute. Both pulsars and magnetars are rotating neutron stars with magnetic fields, but the magnetic field of a magnetar is about 100 times greater than a pulsar's.
Neutron stars emit their strongest energy from the poles, so a rotating neutron star appears to "pulse" energy. Like the searchlight of a lighthouse, as one of the poles swings toward the direction of Earth, we receive a short blast of energy that fades as the pole swings away again. Pulsar spin rates cover a wide range from 0.002 seconds to several minutes.
Peter Woods, a University of Alabama/Huntsville graduate student working at Marshall, presented a poster about magnetars at the NATO conference. The strong magnetic fields of magnetars should cause them to spin slower and slower at a nearly constant rate. But last summer, Woods found that one magnetar spun down faster than expected around the same moment the magnetar released an intense burst of gamma energy. Because the two events occurred so closely in time, they may somehow be associated.
"For 3 years, the spin-down of this magnetar was constant, with only small deviations," said Woods. "Then we detected this large flare of gamma energy, and around the same time the magnetar spun down very quickly. Because magnetars are so close to home, the Earth got hit with a huge flare of gamma energy - greater than the gamma energy from any other cosmic source. It actually caused a disturbance in our ionosphere, but no permanent effects were felt. The flare only temporarily disrupted radio communications in the South Pacific, which happened to be facing the source when the gamma rays reached Earth."
There are only 4 known magnetars, all lying within our galaxy or in the Large Magellanic Cloud just outside our galaxy. Because magnetars also emit enormous bursts of X-ray energy, the Chandra X-ray Observatory could be used to search for more of them.
"The Chandra Observatory will be important in nailing down magnetar theory," said Woods. "We'll be able to look at these objects in much greater detail than ever before."
Weisskopf described the capabilities of the Chandra X-ray Observatory at the NATO Institute. Scientists hope Chandra - due to be launched on July 20 - will greatly improve detection of X-ray emissions, including those near black holes.
"We can detect X-rays almost right up to a black hole's event horizon," said Weisskopf, referring to the region where matter is irresistibly pulled into a black hole.
As matter is pulled from nearby stars and swirls down into a black hole, an accretion disk forms around the black hole's event horizon. Because the matter gets very hot as it falls closer to the black hole, it emits X-rays. But how do scientists know if the X-rays are coming from an accretion disk, and not some other phenomena like supernovae, or even stars?
If an object is hot enough to emit X-rays, it usually also emits light. Therefore we can see our Sun, other stars and supernovae - they are all visible X-ray emitters. The emission of X-rays from an unseen object could indicate the existence of a black hole.
Scientists only have indirect evidence for black hole accretion disks; no one has actually seen such a phenomenon. One of the indications of an accretion disc is a pattern of repeating X-rays. The timing of X-rays can indicate the presence of material orbiting around a mass. By measuring that orbit speed, scientists can also estimate the mass of the central object . Masses of a certain size would normally be visible, so invisible objects of a certain large mass are assumed to be black holes.
The combination of X-rays from an invisible source, a large mass, and the development of an accretion disk lend strong circumstantial evidence for the existence of a black hole.
"One of the goals is to unscramble the X-ray signatures from what's coming from the accretion disks and separate it from other sources of emission," said Weisskopf. "Someday we hope to improve X-ray telescope resolution a million times better than it is today. That sounds like science fiction, but it is theoretically possible."
The study of gamma rays is just as important as X-rays in improving our understanding of neutron stars and black holes. Magnetars, in fact, were originally called soft-gamma repeaters (SGRs) because they were first seen as sporadic bursts of gamma energy that repeated over time.
Gamma-ray bursts (GRBs) were a topic of extreme interest at the NATO conference. These brief bursts of gamma ray energy are different from SGRs because they are extremely far away, have much more energy, and only flash once - never to be seen again.
The Burst and Transient Source Experiment (BATSE), which was designed to detect gamma energy throughout the Universe, has been recording about one gamma-ray burst a day since its launch in 1990. Dr. Gerald Fishman, chief scientist at Marshall for gamma-ray astronomy and principal investigator on BATSE, gave an overview at the NATO conference of the history and observations of gamma-ray bursts observed by BATSE.
Gamma ray bursts have recently been measured to be at the very limit of the universe - what scientists call "cosmological" distances. Their distance and brief life span - only a few seconds - make them difficult to detect. It is thought that the energy from a GRB is greater than any other energy source in the Universe.
"I was surprised by how much gamma-ray burst discussion there was at the conference," said Woods. "The two main models for what creates the bursts are the hypernova and the compact merger model - where a neutron star colliding with a black hole creates a gamma-ray burst. But many other people had their own unique theories of what creates gamma-ray bursts. Because it is still such a mystery, everyone wants to throw their two cents in."
"I've been working so hard on Chandra, I haven't been in touch with X-ray results as much as I would have liked," said Weisskopf. "I was surprised by how much and how little we've progressed! There's clearly been some progress - before, we used mathematical models to describe the data, and now, there are physical models. So there's been real progress in the physics. But some of the fundamental questions of 25 years ago are still questions today. For instance, we now know how far away gamma-ray bursts are - once a big question - but we still don't know what causes them!"
Still, both Weisskopf and Woods say the NATO Institute was helpful in inspiring new thoughts and in suggesting new projects to pursue. Although many questions about high-energy objects like black holes and gamma-ray bursts remain, such conferences provide fuel to scientists in their quest to better understand the Universe.
This NATO Advanced Studies Institute was the fifth of a series focused on neutron stars. The aim of the Institute is to provide a systematic introduction and overview of neutron stars and black hole systems.
Topics covered
- Radio pulsar phenomenology and theory
- Compact X-ray binaries and their evolution
- X-ray binary variability
- Comparison of accreting neutron stars and black holes
- Neutron star magnetic field evolution
- X-ray observations of single neutron stars
- Gamma ray bursts and afterglows
- Accretion discs and advective flows
- Magnetars: soft gamma repeaters, anomalous X-ray pulsars