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| Unveiling Black Holes in a Supernova Cauldron | |
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Mercury, November/December 1999 Table of Contents
Shmuel
Balberg Monica
Colpi Stuart
L. Shapiro Luca
Zampieri Identifying black holes has been one of the greatest challenges in astronomy over the last few decades. Black holes are a theoretical consequence of Albert Einstein's general theory of relativity, where the entire mass of an object has collapsed to a size approaching that of a mathematical point. As a result, the gravitational field very close to such an object is so great that nothing—not even light—can escape; it appears as a "black hole" in space. In the absence of any direct emission of light, astronomers can only hope to identify black holes indirectly, through the effect of their gravitational pull on gas and stars in their vicinity. Over the past decade significant progress in observational techniques and theoretical modeling has led to the identification of over twenty "black hole candidates." These can be divided into two distinct groups: stellar-mass black holes, with masses several times that of our Sun (commonly denoted M°, where 1 M° = 2 x 1033 grams); and supermassive black holes, with masses of the order of millions of M°. The latter are associated with the centers of galaxies, including our own Milky Way, and quasars. The most compelling evidence for a supermassive black hole is a disk of hydrogen observed to be rotating around the center of the galaxy NGC 4258. Applying Kepler's laws to the orbital radius and velocity requires a compact central object with a mass of about 36 million M°. The stellar-mass black hole candidates are detected when they accrete material, supplied by a binary companion star. The first and best known candidate of this type is found in the constellation Cygnus of our galaxy, and is denoted as Cyg X-1. In essence, a fraction of the gravitational potential energy released in the accretion is converted into internal energy as the material spiraling into the black hole compresses and heats. Some of this heat escapes as radiation, providing the observational signature of the accreting object. In the case of the black holes, the temperature of the material is so great that its typical emission is in the x-ray band of the spectrum. Classifying these objects as black hole "candidates" relies mostly on a mass estimate—since no other object is known to be both massive and compact enough to account for the observations. It also has been suggested that an identification could be made on the basis of the emitted radiation. Gas accreting onto a black hole will produce a different radiative luminosity and spectrum from those of gas accreting onto other compact objects (such as neutron stars) which have hard surfaces. A closely related challenge has been the gathering of observational evidence to identify astronomical scenarios where black holes are created. Theorists have suggested several such scenarios, spanning from creation of black holes during the birth of the Universe via the Big Bang, to a secular merger of many stars in the centers of galaxies, to a catastrophic collapse of a massive or supermassive star, or even a relativistic cluster of stars. Here we focus on the formation of a stellar-mass black hole in the explosion of an evolved massive star, a supernova. After the explosion, some of the debris will gradually fall back onto the black hole, giving rise to a source of radiant energy. Can we detect this energy source and thereby uncover a black hole formed in the midst of such an explosion? It seems that nature may finally provide us with an opportunity to do so. When Big Stars Go Boom Supernovae are one of nature's grandest spectacles: when a massive star explodes, it emits about as much energy in a matter of seconds as it did throughout its entire life of tens of millions of years. For a few weeks, the exploding star out-shines its entire host galaxy (see Figure 1)! This fantastic display of power allows us to observe supernovae in very distant galaxies; with present day satellites and telescopes astronomers detect several dozens of supernovae a year. The best studied supernova was observed to explode in the Large Magellanic Cloud, a satellite galaxy of the Milky Way Galaxy, in February 1987, giving its name-SN1987A (previous page). The proximity of this supernova, only 160,000 lightyears away, has made it the brightest supernova observed on Earth since Kepler's time (1604), and has provided astronomers with ample opportunity for study over the last twelve years. Why do stars explode and how might a black hole form? The theory of stellar evolution suggests that stars with masses larger than 8 M° can go through all the stages of thermonuclear burning, or fusion. Close to the end of its life, a massive star is expected to resemble a cosmic onion, composed of an iron core, surrounded by concentric shells of hot, fusing material: silicon and magnesium rich, oxygen and carbon rich, helium rich, and an envelope of unburned hydrogen (see Figure 2). Each inner layer forms when conditions in the star allow the lighter element above it to initiate fusion. Iron is the most tightly bound of all nuclei; it cannot produce energy by thermonuclear fusion. Hence, the iron core cannot generate thermal energy to support itself against the gravitational load of its own mass, and when it is large enough—theoretical estimates give a critical mass somewhat larger than 1 M°—it becomes unstable and collapses. Within a few seconds the inner part of the collapsing iron core contracts from its original, roughly Earth size, to a radius of a few tens of kilometers. The density reaches about 1014 grams per cubic centimeter—the density of atomic nuclei. The collapsed core becomes a gigantic nucleus: a mixture of neutrons, protons, and electrons, supported against further collapse by nuclear forces. These forces resist the compression of the matter, and are able to halt the collapse of the inner part of the core (about 0.6 M°). A shock wave forms and slows the rest of the core material that continues to rain inward. At least temporarily, the dying star is saved. The gravitational potential energy made available by the collapse of the core is fantastic—several times 1053 ergs—similar to the power output of the entire observable Universe in the course of one day, and several thousand times the energy required to detach the envelope of the star from the collapsed core. Initially much of this energy is stored in the very dense core as heat. The core can gradually radiate this energy away in the form of neutrinos because only these very weakly interacting particles can escape the extreme densities of the core. Most of the neutrinos escape without any further effect, but a small fraction scatter off particles in the outer part of the collapsing core, heating the material and increasing its pressure, like a hot plate boiling a column of water ascending onto it. Theoretical simulations show that within about one second, the neutrinos deposit about one percent of their total energy in the infalling material and induce a pressure that is large enough to stop the inflow of matter. The inflow motion is then reversed and material is pushed outward, which subsequently leads to the explosion of the star. 
Figure 1. An image of the galaxy NGC 1536 before (left) and after the explosion of supernova 1997D. SN1997D was serendipitously discovered on 14 January 1997 by De Mello and Benetti (IAU Circular no. 6537). The parent galaxy NGC 1536 is located about 14 Mpc from our Solar System. Images courtesy of the authors. Neutrinos' Surprising Role This amazing hypothesis, that the weakly interacting neutrinos are the drivers of the Universe's grandest explosions, has been the subject of intense investigation. Researchers use computer simulations to study the complex competition of neutrinos and gravity. It is no easy task: one must model the physics of neutrino interactions and nuclear forces, both of which are not well known, and couple them with the turbulent hydrodynamics of matter falling onto the core in a strong gravitational field. Although the different research groups performing such simulations are not without disagreements, the physical underpinnings of the explosion mechanism are commonly accepted. Our confidence in the fundamentals of supernova theory was significantly strengthened when a pulse of neutrinos was detected by terrestrial detectors in coincidence with the initial optical burst from SN1987A, yielding a neutrino number and energy consistent with theory. And what of the compact core left behind as a remnant of the explosion? In most cases it remains a giant nucleus held together by the force of gravity: a neutron star. To date several hundred neutron stars have been detected, mostly as radio "pulsars"-rotating neutron stars which produce beamed electromagnetic emission, due to an extremely strong surface magnetic field. If the beam is not aligned with the spin axis we see a pulse, like from a lighthouse, every time the beam sweeps Earth. Neutron stars are the only known objects that can provide such rapid, steady pulses, some with periods as low as milliseconds.
 Figure 2. Interior cut-away of a massive progenitor star, showing its characteristic "onion" structure close to the end of its life. A Hydrogen plus Helium envelope surrounds internal concentric layers rich in heavier elements (Oxygen and Silicon); at the center, the iron core (Fe). Illustration courtesy of authors. One crucial feature of neutron stars is that they have a maximum mass beyond which the gravitational load is too great even for the immense power of the strong interactions. The exact value of this mass is unknown, due to the uncertainties in our theory of the nuclear forces. However, theoretical studies suggest that it is about 2 M°, and most likely no more than 3 M°. Should a neutron star accrete enough matter to push its mass beyond this maximum mass, it must collapse into a black hole. Since there exists such a maximum mass, the nature of the compact remnant of a supernova is uncertain. First, the explosion itself is not guaranteed: The neutrinos must deposit enough energy in the incoming matter to reverse its flow before the collapsed inner core accretes enough mass to crush it into a black hole. If a black hole forms too early, the neutrino stream is cut off, and no more pressure can be built and the entire star will collapse onto the new black hole. Such an event is often referred to as a "failed supernova." The rate at which material falls onto the core is therefore critical to the explosion. This rate tends to depend on the initial mass of the iron core, which in turn is larger when the total mass of the progenitor star is larger. According to current computer simulations, the critical limit lies at a progenitor mass of about 40 M°. The iron cores in stars with larger mass will lead to an infall rate that will defeat the neutrinos, forming a black hole before an explosion can be driven. Stars even more massive, which have reached this evolutionary stage, end up entirely as black holes. Watching Out for Fallback Neutrinos are expected to triumph and drive a successful explosion in lower mass stars, but the details depend strongly on the amount of kinetic energy the envelope can obtain. If the rate of infalling material is large, more energy is required to reverse its motion, and less energy remains available to drive the explosion. Insufficient kinetic energy means that some of the material originally close to the core will not be able to fully escape its gravitational pull, but will instead reach some maximum distance and then reverse and accrete onto it. This process is generally referred to as "fallback." Most of this fallback occurs very early in the course of the explosion, so that even as the supernova progresses, the core may find that it has exceeded the maximum mass it can support and collapses to a black hole. In this case, a successful supernova will leave behind a black hole, not a neutron star. Larger mass stars tend to experience larger fallback rates as the neutrinos build up the pressure, and so, they usually give rise to weaker explosions. Furthermore, the explosion energy must overcome the gravitational binding energy, which is naturally larger when the star is more massive. This combination implies that more massive stars experience a larger amount of fallback, even when a successful explosion does take place. Computer simulations indicate that stars with masses between 8 M° and about 20-25 M° will undergo rather little fallback and are likely to produce a neutron star. The exact upper limit depends on the maximum stable mass of neutron stars and the details of neutrino-interaction physics. In fact, several pulsars have been associated with sites of known supernovae, like the Vela and Crab nebulae, confirming that they were indeed born in supernovae. The rarer stars with masses in the range 25-40 M° are also expected to explode successfully, but the rate of fallback will be sufficient to induce collapse and the formation of a black hole. It is evidence for this particular scenario that we are currently pursuing. Clues to Black Hole Formation We do indeed expect that fallback will be a continuous process: at any given instant of time, there is always some material that has just inverted its outward motion and has started to fall back onto the black hole, even as the bulk of the envelope continues to stream outward. This late fallback will continue for many years, at an ever decreasing rate, generating energy in a similar fashion to |
In
the next year, observations of a distant, newly discovered supernova
may provide us an opportunity to identify a black hole in the debris
of such a stellar explosion. For the first time, we mayh obtain
unmistakable evidence that supernova explosions can give birth to
black holes.