For the Love of Physics (39 page)

The neutron stars themselves are much too small to be seen optically—but we can see the much larger donor stars and the accretion disks with optical telescopes. The disks themselves can radiate quite a bit of light partly due to a process called X-ray heating. When the matter from the disk crashes onto the surface of the neutron star, the resultant X-rays go off in all directions and thus also slam into the disk itself, heating it to even higher temperatures. I will tell you more about that in the next chapter, on X-ray bursts.

The discovery of X-ray binaries solved the first mystery of extrasolar X-rays. We now understand why the X-ray luminosity of a source like Sco X-1 is ten thousand times greater than its optical luminosity. The X-rays come from the very hot neutron star (with temperatures of tens of millions kelvin), and the optical light comes from the much cooler donor star and the accretion disk.

When we thought that we had a fair understanding of how X-ray binaries work, nature had another surprise in store for us. The X-ray astronomers began making observational discoveries that were outstripping the theoretical models.

In 1975, the discovery of something truly bizarre led to a high point of my scientific career. I became completely immersed in the effort to observe, study, and explain these remarkable and mysterious phenomena: X-ray bursts.

Part of the story about X-ray bursts includes a battle I had with Russian scientists who completely misinterpreted their data and also with some of my colleagues at Harvard who believed that X-ray bursts were produced by very massive black holes (poor black holes, they have been unjustly blamed for so much). Believe it or not, I was even called (more than once) to not publish some data on bursts for reasons of national security.

CHAPTER 14

X-ray Bursters!

N
ature is always full of surprises, and in 1975 it rocked the X-ray community. Things became so intense that emotions at times got out of control, and I was in the middle of it all. For years I was arguing with a colleague of mine at Harvard (who would not listen), but I had more luck with my Russian colleagues (who did listen). Because of my central role in all of this it may be very difficult for me to be objective, but I’ll try!

The new thing was X-ray bursts. They were discovered independently in 1975 by Grindlay and Heise using data from the Astronomical Netherlands Satellite (ANS) and by Belian, Conner, and Evans, using data from the United States’ two Vela-5 spy satellites designed to detect nuclear tests. X-ray bursts were a completely different animal from the variability we discovered from Sco X-1, which had a flare-up by a factor of four over a ten-minute period that lasted tens of minutes. X-ray bursts were much faster, much brighter, and they lasted only tens of seconds.

At MIT we had our own satellite (launched in May 1975) known as the Third Small Astronomy Satellite, or SAS-3. Its name wasn’t as romantic as “Uhuru,” but the work was the most absorbing of my entire life. We
had heard about bursters and began looking for them in January 1976; by March we’d found five of our own. By the end of the year we’d found a total of ten. Because of the sensitivity of SAS-3, and the way it was configured, it turned out to be the ideal instrument to discover burst sources and to study them. Of course, it wasn’t specially designed to detect X-ray bursts; so in a way it was a bit of luck. You see what a leading role Lady Luck has played in my life! We were getting amazing data—a bit of gold pouring out of the sky every day, twenty-four hours a day—and I worked around the clock. I was dedicated, but also obsessed. It was a once in a lifetime opportunity to have an X-ray observatory you can point in any direction you want to and get data of high quality.

The truth is that we all caught “burst fever”—undergraduates and graduate students, support staff and postdocs and faculty—and I can still remember the feeling, like a glow. We ended up in different observing groups, which meant that we got competitive, even with one another. Some of us didn’t like that, but I have to say that I think it pushed us to do more and better, and the scientific results were just fantastic.

That level of obsession was not good for my marriage, and not good for my family life either. My scientific life was immeasurably enhanced, but my first marriage dissolved. Of course it was my fault. For years I’d been going away for months at a time to fly balloons halfway around the globe. Now that we had our own satellite, I might as well still have been in Australia.

The burst sources became a kind of substitute family. After all, we lived with them and slept with them and learned them inside out. Like friends, each one was unique, with its own idiosyncrasies. Even now, I recognize many of these telltale burst profiles.

Most of these sources were about 25,000 light-years away, which allowed us to calculate that the total X-ray energy in a burst (emitted in less than a minute) was about 10
³²
joules, a number that’s almost impossible to grasp. So look at it this way: it takes our Sun about three days to emit 10
³²
joules of energy in all wavelengths.

Some of these bursts came with nearly clocklike regularity, such as the
bursts from MXB 1659-29, which produced bursts at 2.4-hour intervals, while others changed their burst intervals from hours to days, and some showed no bursts at all for several months. The M in MXB stands for MIT, the X for X-rays, and the B for burster. The numbers indicate the source’s celestial coordinates in what’s known as the equatorial coordinate system. For the amateur astronomers among you, this will be familiar.

The key question, of course, was what caused these bursts? Two of my colleagues at Harvard (including Josh Grindlay, who was one of the codiscoverers of X-ray bursts) got carried away and proposed in 1976 that the bursts were produced by black holes with a mass greater than several hundred times the mass of the Sun.

We soon discovered that the spectra during X-ray bursts resemble the spectra from a cooling black body. A black body is not a black hole. It’s an ideal construct to stand in for an object that absorbs all the radiation that strikes it, rather than reflecting any of it. (As you know, black absorbs radiation, while white reflects it—which is why in summer in Miami a black car left in a beach parking lot will always be hotter inside than a white one.) The other thing about an ideal black body is that since it reflects nothing, the only radiation it can emit is the result of its own temperature. Think about a heating element in an electric stove. When it reaches a cooking temperature, it begins to glow red, emitting low-frequency red light. As it gets hotter it reaches orange, then yellow, and usually not much more. When you turn off the electricity, the element cools, and the radiation it emits has a profile more or less like the tail end of bursts. The spectra of black bodies are so well known that if you measure the spectrum over time, you can calculate the temperature as it cools.

Since black bodies are very well understood, we can deduce a great deal about bursts based on elementary physics, which is quite amazing. Here we were, analyzing X-ray emission spectra of unknown sources 25,000 light-years away, and we made breakthroughs using the same physics that first-year college students learn at MIT!

We know that the total luminosity of a black body (how much energy per second it radiates) is proportional to the fourth power of its temperature
(this is by no means intuitive), and it is proportional to its surface area (that’s intuitive—the larger the area, the more energy can get out). So, if we have two spheres a meter in diameter, and one is twice as hot as the other, the hotter one will emit sixteen times (2
⁴
) more energy per second. Since the surface area of a sphere is proportional to the square of its radius, we also know that if an object’s temperature stays the same but triples in size, it will emit nine times more energy per second.

The X-ray spectrum at any moment in time of the burst tells us the blackbody temperature of the emitting object. During a burst, the temperature quickly rises to about 30 million kelvin and decreases slowly thereafter. But since we knew the approximate distance to these bursters, we could also calculate the luminosity of the source at any moment during the burst. But once you know both the blackbody temperature and the luminosity, you can calculate the radius of the emitting object, and that too can be done for any moment in time during the burst. The person who did this first was Jean Swank of NASA’s Goddard Space Flight Center; we at MIT followed quickly and concluded that the bursts came from a cooling object with a radius of about 10 kilometers. This was strong evidence that the burst sources were neutron stars, not very massive black holes. And if they were neutron stars, they were probably X-ray binaries.

The Italian astronomer Laura Maraschi was visiting MIT in 1976, and one day in February she walked into my office and suggested that the bursts were the result of thermonuclear flashes, huge thermonuclear explosions on the surface of accreting neutron stars. When hydrogen accretes onto a neutron star, gravitational potential energy is converted to such tremendous heat that X-rays are emitted (see previous chapter). But as this accreted matter accumulates on the surface of the neutron star, Maraschi suggested, it might undergo nuclear fusion in a runaway process (like in a hydrogen bomb) and that might cause an X-ray burst. The next explosion might go off a few hours later when enough new nuclear fuel had been accreted to ignite. Maraschi demonstrated with a simple calculation on my blackboard that matter racing at roughly half the speed of light to the surface of a neutron star releases much more
energy than what is released during the thermonuclear explosions, and that is what the data showed.

I was impressed—this explanation made sense to me. Thermonuclear explosions fit the bill. The cooling pattern we’d observed during the bursts also made sense if what we were seeing was a massive explosion on a neutron star. And her model explained the interval between bursts well since the amount of matter required for an explosion had to build up over time. At the normal rate of accretion, it should take a few hours to build up a critical mass, which was the kind of interval we found in many burst sources.

I keep a funny kind of radio in my office that always unsettles visitors. It’s got a solar-powered battery inside, and it works only when the battery has enough juice. As the radio sits there soaking up sunlight, it slowly fills up with juice (a lot more slowly in the winter), then every ten minutes or so—sometimes longer if the weather’s rotten—it suddenly starts playing, but only for a couple of seconds, as it quickly exhausts its supply of electricity. You see? The buildup in its battery is just like the buildup of accreted matter on the neutron star: when it gets to the right amount, the explosion goes off, and then fades away.

Then, several weeks after Maraschi’s visit, on March 2, 1976, in the middle of burst fever, we discovered an X-ray source that I named MXB 1730-335 that was producing
a few thousand bursts per day.
The bursts came like machine-gun fire—many were only 6 seconds apart! I don’t know if I can completely convey just how bizarre this seemed to us. This source (now called the Rapid Burster) was a complete outlier, and it immediately killed Maraschi’s idea. First, there is no way that a sufficient amount of nuclear fuel could build up in six seconds on the surface of a neutron star to produce a thermonuclear explosion. Not only that, but if the bursts were a by-product of accretion, we should see a strong X-ray flux due to accretion alone (release of gravitational potential energy), far exceeding the energy present in the bursts, but that was not the case. So it seemed in early March 1976 that Maraschi’s wonderful thermonuclear model for the bursts was as dead as the proverbial doornail. In our publication
on MXB 1730-335, we suggested that the bursts are caused by “spasmodic accretion” onto a neutron star. In other words, what in most X-ray binaries is a steady flow of hot matter from the accretion disk onto the neutron star is very irregular in the case of the Rapid Burster.

When we measured the bursts over time, we found that the bigger the burst, the longer the wait before the next one. The waiting time to the next burst could be as short as six seconds and as long as eight minutes. Lightning does something similar. When there’s a particularly large lightning bolt, the large discharge means that the wait needs to be longer for the electric field to build up its potential to the point that it can discharge again.

Later that year a translation of a 1975 Russian paper about X-ray bursts surfaced out of nowhere; it had been reporting burst detections made in 1971 with the Kosmos 428 satellite. We were stunned; the Russians had discovered X-ray bursts, and they had beaten the West! However, as I heard more and more about these bursts, I became very skeptical. Their bursts behaved so very, very differently from the many bursts that I had detected with SAS-3 that I began to seriously doubt whether the Russian bursts were real. I suspected that they were either man-made or produced near Earth in some odd, bizarre way. The iron curtain made it difficult to pursue this; there was no way to find out. However, I had the good fortune to be invited to attend a high-level conference in the Soviet Union in the summer of 1977. Only twelve Russians and twelve U.S. astrophysicists had been invited. That’s where I met for the first time the world famous scientists Joseph Shklovsky, Roald Sagdeev, Yakov Zel’dovich, and Rashid Sunyaev.