For the Love of Physics (38 page)

Shklovsky turned out to be right. But here’s the funny thing. He was only talking about Sco X-1 at the time, and most of us didn’t take his idea too seriously. But that’s often the case with theory. I don’t think I would be offending any of my theoretician colleagues by saying that the great majority of theory in astrophysics turns out to be wrong. So of course many of us in observational astrophysics don’t pay much attention to most of it.

It turns out that accreting neutron stars are in fact the perfect environments to produce X-rays. How did we find out that Shklovsky was right?

It took until the early seventies for astronomers to nail down the binary nature of some X-ray sources—but that didn’t necessarily mean that they were accreting neutron stars. The very first source to reveal its secrets was Cyg X-1, and it turned out to be one of the most important in all of X-ray astronomy. Discovered during a rocket flight in 1964, it is a very bright and powerful source of X-rays, so it has attracted the attention of X-ray astronomers ever since.

Radio astronomers then discovered radio waves from Cyg X-1 in 1971. Their radio telescopes pinpointed Cyg X-1’s position to a region (an error box) in the sky of about 350 square arc seconds, about twenty times smaller than had been possible by tracking its X-rays. They went looking for its optical counterpart. In other words, they wanted to
see
, in visible light, the star that was generating the mysterious X-rays.

There was a very bright blue supergiant known as HDE 226868 in the radio error box. Given the kind of star it was, astronomers could make comparisons with other very similar stars to make a pretty good estimate of its mass. Five astronomers, including the world-famous Allan Sandage, concluded that HDE 226868 was just a “normal B0 supergiant,
with no peculiarities,” and they dismissed the fact that it was the optical counterpart of Cyg X-1. Other (at the time less famous) optical astronomers examined the star more closely and made some earthshaking discoveries.

They discovered that the star was a member of a binary system with an orbital period of 5.6 days. They argued correctly that the strong X-ray flux from this binary system was due to the accretion of gas from the optical star (the donor) to a very small—compact—object. Only a gas flow onto a massive but very small object could explain the copious X-ray flux.

They made Doppler-shift measurements of absorption lines in the spectrum of the donor star as it moved around in its orbit (remember, as it moved toward Earth, the spectra would shift toward the blue end, and as it moved away, it would shift toward the red) and concluded that the X-ray-generating companion star was too massive to be either a neutron star or a white dwarf (another compact, very dense star, like Sirius B). Well, if it couldn’t be either of those, and it was even more massive than a neutron star, what else could it be? Of course—a black hole! And that’s what they proposed.

As observational scientists, however, they stated their conclusions more circumspectly. Louise Webster and Paul Murdin, whose discovery ran in
Nature
on January 7, 1972, put it this way: “The mass of the companion being probably larger than 2 solar masses, it is inevitable that we should also speculate that it might be a black hole.” Here’s what Tom Bolton wrote a month later in
Nature
: “This raises the distinct possibility that the secondary [the accretor] is a black hole.” A picture of an artistic impression of Cyg X-1 can be seen in the insert.

So these wonderful astronomers, Webster and Murdin in England and Bolton in Toronto, shared the discovery of X-ray binaries
and
finding the first black hole in our galaxy. (Bolton was so proud, he had the license plate Cyg X-1 for a number of years.)

I’ve always thought it was odd that they never received a major prize for their absolutely phenomenal discovery. After all, they hit the field at
its heart, and they were
first
! They nailed the first X-ray binary system. And they said that the accretor was probably a black hole. What a piece of work!

In 1975 none other than Stephen Hawking bet his friend, fellow theoretical physicist Kip Thorne, that Cyg X-1 wasn’t a black hole at all—even though most astronomers thought it was by then. He eventually conceded the bet, fifteen years later, I think to his own delight, since so much of his work has revolved around black holes. The most recent (soon to be published) and most accurate measurement of the mass of the black hole in Cyg X-1 is about 15 solar masses (private communication from Jerry Orosz and my former student Jeff McClintock).

If you’re sharp, I know you’re already thinking, “Hold it! You just said black holes don’t emit anything, that nothing can escape their gravitational field—how can they emit X-rays?” Terrific question, which I promise to answer eventually, but here’s a preview: the X-rays emitted by a black hole do not come from inside the event horizon—they’re emitted by matter on the way
into
the black hole. While a black hole explained our observations of Cyg X-1, it could not explain what was seen in terms of X-ray emission from other binary stars. For that we needed neutron star binaries, which were discovered with the wonderful satellite Uhuru.

The field of X-ray astronomy dramatically changed in December 1970, when the first satellite totally dedicated to X-ray astronomy went into orbit. Launched from Kenya on the seventh anniversary of Kenyan independence, the satellite was named Uhuru, Swahili for “freedom.”

Uhuru began a revolution that hasn’t stopped to this day. Think about what a satellite could do. Observations 365 days a year, twenty-four hours a day, with no atmosphere at all! Uhuru was able to observe in ways we could only have dreamed about a half dozen years earlier. In just a little over two years, Uhuru mapped the X-ray sky with counters that could pick up sources five hundred times fainter than the Crab Nebula, ten thousand times fainter than Sco X-1. It found 339 of them (we’d only found several dozen before that) and provided the first X-ray map of the entire sky.

Freeing us at last from atmospheric shackles, satellite observatories have reshaped our view of the universe, as we learned to see deep space—and the astonishing objects it contains—through every area of the electromagnetic spectrum. The Hubble Space Telescope expanded our view of the optical universe, while a series of X-ray observatories did the same for the X-ray universe. Gamma-ray observatories are now observing the universe at even higher energies.

In 1971 Uhuru discovered 4.84-second pulsations from Cen X-3 (in the constellation Centaurus). During a one-day interval Uhuru observed a change in the X-ray flux by a factor of ten in about one hour. The period of the pulsations first decreased and then increased by about 0.02 and 0.04 percent, each change of period occurring in about an hour. All this was very exciting but also very puzzling. The pulsations couldn’t be the result of a spinning neutron star; their rotation periods were known to be steady like a rock. None of the known pulsars could possibly change their period by 0.04 percent in an hour.

The entire picture came together beautifully when the Uhuru group later discovered that Cen X-3 was a binary system with an orbital period of 2.09 days. The 4.84-second pulsations were due to the rotation of the accreting neutron star. The evidence was overwhelming. First, they clearly saw repetitive eclipses (every 2.09 days) when the neutron star hides behind the donor star, blocking the X-rays emissions. And second, they were able to measure the Doppler shift in the periods of the pulsations. When the neutron star is moving toward us, the pulsation period is a little shorter, a little longer when moving away. These earthshaking results were published in March 1972. All this naturally explained the phenomena that seemed so puzzling in the 1971 paper. It was just as Shklovsky had predicted for Sco X-1: a binary system with a donor star and an accreting neutron star.

Later that very same year, Giacconi’s group found yet another source, Hercules X-1 (or Her X-1, as we like to say), with pulsations and eclipses. Another neutron star X-ray binary!

These were absolutely stunning discoveries that transformed X-ray
astronomy, dominating the field for decades to come. X-ray binaries are very rare; perhaps only one in a hundred million binary stars in our galaxy is an X-ray binary. Even so, we now know that there are several hundred X-ray binaries in our galaxy. In most cases the compact object, the accretor, is a white dwarf or a neutron star, but there are at least two dozen known systems in which the accretor is a black hole.

Remember the 2.3-minute periodicity that my group discovered in 1970 (before the launch of Uhuru)? At the time we had no clue what these periodic changes meant. Well, we now know that GX 1+4 is an X-ray binary system with an orbital period of about 304 days, and the accreting neutron star spins around in about 2.3 minutes.

X-ray Binaries: How They Work

When a neutron star pairs up with the right-size donor star at the right distance, it can create some amazing fireworks. There, in the reaches of space, stars Isaac Newton could never even have imagined perform a beautiful dance, all the while utterly bound by the laws of classical mechanics any undergraduate science major can grasp.

To understand this better, let’s start close to home. The Earth and the Moon are a binary system. If you draw a line from the center of the Earth to the center of the Moon, there is a point on that line where the gravitational force toward the Moon is equal but opposite to the gravitational force toward Earth. If you were there, the net force on you would be zero. If you were on one side of that point you would fall to Earth; if you were on the other side you would fall toward the Moon. That point has a name; we call it the inner Lagrangian point. Of course, it lies very close to the moon, because the Moon’s mass is about eighty times smaller than that of the Earth.

Let’s now return to X-ray binaries consisting of an accreting neutron star and a much larger donor star. If the two stars are very close to each other, the inner Lagrangian point can lie below the surface of the donor star. If that is the case, some of the matter of the donor star will experience
a gravitational force toward the neutron star that is larger than the gravitational force toward the center of the donor star. Consequently matter—hot hydrogen gas—will flow from the donor star to the neutron star.

Since the stars are orbiting their common center of mass, the matter cannot fall directly toward the neutron star. Before it reaches the surface, the matter falls into an orbit around the neutron star, creating a spinning disk of hot gas that we call an accretion disk. Some of the gas on the inner ring of the disk ultimately finds its way down to the surface of the neutron star.

Now an interesting piece of physics gets involved that you are already familiar with in another context. Since the gas is very hot, it is ionized, consisting of positively charged protons and negatively charged electrons. But since the neutron stars have very strong magnetic fields, these charged particles are forced to follow the star’s magnetic field lines, so most of this plasma ends up at the magnetic poles of the neutron star, like the aurora borealis on Earth. The neutron star’s magnetic poles (where matter crashes onto the neutron star) become hot spots with temperatures of millions of degrees kelvin, emitting X-rays. And as magnetic poles generally do not coincide with the poles of the axis of rotation (see
chapter 12
), we on Earth will only receive a high X-ray flux when a hot spot is facing us. Since the neutron star rotates, it appears to pulsate.

Every X-ray binary has an accretion disk orbiting the accretor, be it a neutron star, a white dwarf or, as in Cyg X-1, a black hole. Accretion disks are among the most extraordinary objects in the universe, and almost no one except professional astronomers has ever even heard of them.

There are accretion disks around all black hole X-ray binaries. There are even accretion disks orbiting supermassive black holes at the center of many galaxies, though probably not, as it turns out, around the supermassive black hole at the center of our own galaxy.

The study of accretion disks is now an entire field of astrophysics. You can see some wonderful images of them here:
www.google.com/images?hl=en&q=xray+binaries&um=1&ie=UTF
. There is still lots about accretion
disks that we don’t know. One of the most embarrassing problems is that we still don’t understand well how the matter in the accretion disks finds its way to the compact object. Another remaining problem is our lack of understanding of instabilities in the accretion disks, which give rise to variability in the matter flow onto the compact object, and the variability in X-ray luminosity. Our understanding of radio jets present in several X-ray binaries is also very poor.

A donor star can transfer up to about 10
¹⁸
grams per second to the accreting neutron star. It sounds like a lot, but even at that rate it would take two hundred years to transfer an amount of matter equal to the Earth’s mass. Matter from the disk flows toward the accretor in the grip of its intense gravitational field, which accelerates the gas to an extremely high speed: about one third to one half the speed of light. Gravitational potential energy released by this matter is converted into kinetic energy (roughly 5 × 10
³⁰
watts) and heats the racing hydrogen gas to a temperature of millions of degrees.

You know that when matter is heated it gives off blackbody radiation (see
chapter 14
). The higher the temperature, the more energetic the radiation, making shorter wavelengths and higher frequencies. When matter reaches 10 to 100 million kelvin, the radiation it generates is mostly in X-rays. Almost all 5 × 10
³⁰
watts are released in the form of X-rays; compare that with the total luminosity of our Sun (4 × 10
²⁶
watts) of which only about 10
²⁰
watts is in the form of X-rays. Our Sun’s surface temperature is a veritable ice cube in comparison.