.. inclination i of the system, and several important things can be calculated. The mass function f(M) = M2^3 sin i / (M1 +M2)^2 gives a relation between the masses of the two bodies, and the semi-major axis a1=AM2/(M1+M2)^2 sin i (where A is the separation of the centers of mass) gives the size of the orbit, which can also be related to the rotational velocities of the stars. A spectroscopic binary with no visible companion would be a candidate for a black hole, and if the dim star’s mass is determined to be greater than that of the visible star, it would be a promising candidate. However, this method consists of many uncertainties.
Although there were no hard cases for black holes any scientists search, there arose another way a black hole might show itself. If the black hole were in a gaseous nebula, the gas would fall into the black hole. The inherent magnetic fields of the gas create turbulence, generating heat, which is in turn transformed into electromagnetic radiation. The luminosity of the gas could oscillate rapidly due to the turbulence, and such rapid oscillations would give the black hole away. Another Soviet scientist, Schwarzmann, developed the Multichannel Analyzer of Nanosecond Pulses of Brightness Variation in an effort to detect these oscillations, but that method also proved fruitless.
X-ray novas are a special class of X-ray binaries where the system contains a late-type optical companion (a star near the end of its life) and a compact object, which can be either a neutron star or a black hole . Usually the spectrum of the companion in this type of system is very weak compared to that of the gas, but in X-ray novae the fraction of light from X-ray heating is negligible, and we have an excellent opportunity to study the system in detail. If the accretion disk is due to a black hole, then understanding the companion star in detail will also allow understanding of the processes of X-ray emission. Several X-ray satellites detected Muscae 1991 and calculations began to pinpoint an optical companion. To do this, the exact position of the X-ray source must be known.
If there is a star in the visible range at that same position, it is most likely related to the X-ray star, and the light curve can then be studied in detail. In this case, a companion was found. The similarities of Muscae 1991 with one of the best black hole candidates, V616 Mon, make it seem realistic that it might be a black hole. The evolution of the light curves, the decay rate in magnitude of the novae, and variations in brightness on the order of a day are all similar in the two systems. The spectrum of the nova, its various emission lines and other spectroscopic details, also does not resemble a classical nova in the same stages, but instead resembles that of the black hole candidates Cen X-4 and V616 Mon.
As it is not a classical nova, the distance to Muscae 1991 must be estimated from a known linear relation of the width of the NaD line to distance. This gives a result of ~1.4 kpc (kiloparsecs), which returns some typical values for low mass X-ray binaries and justifies confidence in its validity. Using this distance and the spectral features of the binary, the companion star seems to be a late main sequence star, which is in agreement with current theories of low-mass X-ray binaries. What this all boils down to is that the binary X-ray nova Muscae 1991 behaves very similarly to other black hole candidates in the galaxy, and gives a picture of the nova as a burst of gravitational potential energy released as matter from the disk accreted onto the compact object. The large amounts of energy released in the nova as X-rays indicates the companion is at least a neutron star and possibly a black hole, but no obvious conclusions can be made as to Muscae 1991’s containing a black hole. Cygnus X-1 is accepted as a black hole by most astronomers, there is still nothing about it that demands unequivocally to be accepted as such.
Cygnus X-1 is the best X-ray astronomy can give us. But X-rays and visible light are not the only ways of probing the sky. Radio astronomy was also discovered accidentally. In the 1930’s, a technician trying to clear up intercontinental phone calls discovered radio waves coming from the Milky Way. Curiously enough, nobody really seemed to care very much; an amateur built the world’s first radio telescope.
A modest 9 meters in size, it had extremely poor resolution, and the larger dishes that were to slowly follow did not fare much better. As in X-ray astronomy, the astronomers couldn’t do anything really useful with cosmic radio waves until they could identify an optical counterpart. Since radio waves are on the order of meters long, diffraction effects would require unreasonably large dishes to acquire any decent resolution. To counter this, astronomers came up with radio interferometry. At first the bodies that shone most brightly in the sky could not be associated with an optical counterpart.
As radio telescopes improved, the error boxes for these sources shrank until, in 1953, a team at Cambridge had a sufficiently accurate estimate that other astronomers at the Palomar 5-meter optical telescope could identify the radio source Cyngus A with an optical source. This source turned out to be a galaxy, and once it’s redshift, and hence distance, were measured, it was found that this galaxy’s radio luminosity was millions of times brighter than that of an ordinary galaxy. The first radio galaxy had been found. Now that the technology was in place, more and more of these galaxies were discovered and they began to be studied in great detail. The results troubled astronomers; radio galaxies had two lobes of radio emissions with the dim optical galaxy in the center. These lobes stretched out millions of light-years, indicating a stable source of emission, and conservative estimates of the energy involved in their production was on the order of 10^61 ergs, as much energy as would be released in ten billion supernovas.
Radio galaxies were among the first in what are today classified as AGN – active galactic nuclei. Other types of AGN include Seyfert galaxies, N galaxies, BL Lacertae objects, and quasars. They all demonstrate violent behavior that can’t be associated with the ordinary behavior of stars and interstellar dust, whether it be matter and energy ejected from the nucleus to luminosities of truly astronomical proportions. While all these objects were regarded as puzzles, it was really the quasars that could not be explained by any astronomical processes at all. Of course they do exist, and astronomers rushed to find explanations for them.
It was in this storm of hypotheses that the idea of a super-massive black hole lost it’s exotic nature and became the most reasonable explanation. In fact, many of the other realistic explanations also support this idea, for they could evolve into a super-massive black hole . If there are a lot of star-star collisions occurring, the stars will lose enough energy such that they become bound in a binary which fairly rapidly decays, if they do not coalesce directly with each other. Such models of AGN could have two natural results without invoking black holes: supernova explosions, or clusters of pulsars. The supernova explosions are only as efficient as regular nuclear burning in stars, and must occur at a rate of about 5 to 10 a year.
Furthermore, these supernovas cannot be ordinary stellar supernovas but rather a sort of ‘hypernova’ , wherein neutron stars must pass through the cores of super-massive stars, due to calculations of the energies released. If the cluster evolves into a cluster of pulsars, it is the rotational energy of the pulsars that powers the quasars. Through horrendously complicated interactions of particles and strong electromagnetic fields, this energy could be released into the universe, but both this and the supernova model have another serious flaw; there is no directionality of the radiation that could result in the observed jets of quasars and other AGN. To correct this would require a flattened cloud of gas that would either hasten the death of the cluster and it would collapse into a black hole, or the luminosity would be so great that the resulting wind of radiation would drive the gas into space, thereby destroying the model entirely. Other models involve the rotational energies of massive uncollapsed bodies. Known as super-massive stars, magnetoids, or spinars, they are all basically the same; a massive, spinning flattened disk (a super-massive rotating star will evolve into a disk).
One way these spinars could liberate energy is by gravitational contraction, releasing up to a few percent of their rest mass as energy. However, to remain stable against collapse, a very large ultraviolet radiation pressure must be present, and such radiation is not found in radio galaxies, though they might be in high-redshift quasars. A pulsar is a rotating neutron star with skewed magnetic poles . Radiation is emitted in the direction of the magnetic poles, and if this beam passes earth, it has the same effect as a lighthouse. The incredible angular momentum of a pulsar makes its pulses extremely regular, to a degree of accuracy elsewhere found only in atomic clocks.
As such, the orbit of a binary pulsar can be scrutinized in extreme detail, and has been. The results are amazing; the period of the stars is declining and their orbit is slowly decaying to exactly the degree predicted by general relativity. A better proof of gravitational radiation could hardly be imagined. The first person to attempt to detect this radiation was Joseph Weber. He eventually came up with the first bar gravity-wave detector. This was a long aluminum cylinder, 2 m by 1/2 m, that should be compressed with an incoming gravity wave.
To detect this compression he wired piezoelectric crystals, which respond to pressure by generating an electric current, to the outside surface of the bar. Although it didn’t work, other bar detectors were built that used a device called a stroboscopic sensor to filter out random vibrations. This was an ingenious device, but it too proved to be a non-contributor in the advancement of learning more of the galaxy. Just as X-ray astronomy went from simple detectors in the noses of rockets to full fledged X-ray telescopes housed in orbiting satellites, and radio astronomy went from crude dishes to continent spanning arrays, gravity wave detectors may show a completely new spectrum. And, just as X-rays brought a completely new universe into focus, one can hardly imagine what a gravitational view of the universe will reveal.
At the very least, we will have definitive proof or denial of black holes, but we may find that black holes are some of the more subtle features of the universe. Physics.