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The Mystery of SS433

By piecing together evidence from many wavelengths -- from radio waves to gamma rays -- astronomers have deciphered one of the most interesting objects in the Milky Way

by Larry Krumenaker

StarDate magazine Nov/Dec 2004 coverAlmost three decades ago, an astronomer discovered a mystery object lurking inside our Milky Way galaxy. It has been called a microquasar because it spits out huge amounts of X-rays, gamma-rays, and radio waves, plus hydrogen gas flying at speeds of 25 percent of the speed of light. After decades of study, it appears to be a system of two objects orbiting each other. One is a fairly normal but overweight star that is shedding its excess gas to its companion -- a black hole or neutron star submerged in a dense, bright disk of this gas. The flow of gas is so overwhelming that the companion can't handle it all, so it ejects much of the gas in a pair of high-speed "jets." It acts like a small model of those extragalactic powerhouses, the quasars, but it is inside our galaxy, not near the edge of the universe. It is called SS433, and it's one of the most intensively studied objects in the galaxy.

Before it hit the big time, though, SS433 was found by an obscure young astronomer who called it K16. It was known only to readers of an obscure article in an astronomical journal. Little did I know then that the great writer Arthur C. Clarke would name this as one of the seven wonders of modern discovery.

A black hole steals gas from its companion star.
A black hole steals gas from its companion star.

Many years ago, my summer job was to spend hours peering through a microscope at photographs of the universe taken not with film but with glass plates. I wasn't looking at nice round images of stars. Instead, each glass plate contained hundreds, even thousands of long gray streaks. Each one was the negative image of a spectrum of a star. Spectrum is the astronomer's word for rainbow. They were made by placing a thin prism in front of my telescope to split the starlight into its component wavelengths, turning star images into gray rainbows.

Because they were negatives, where the star gave us light, the glass plate showed darkness. Where there was no light, there was clear glass. Anyway, I wasn't looking at the swaths of gray. Cutting across the narrow widths of these gray streaks were tiny lines, either white ones where some light of a particular color was missing (or "absorbed"), or black lines where there was some extra light (or "emission").

The lines in a spectrum are the fingerprints of the star's chemical elements. I was looking, in particular, for stars that had an extra emission -- a bright line across the gray rainbow -- indicating glowing hydrogen gas surrounding the star. This special feature is called the H-alpha line. Most often, H-alpha is a thin white or dark line on top of a star's regular spectrum. It takes a long time to check out hundreds of streaks for H-alpha lines. But I'd found a bunch already. I just wasn't prepared for the discovery on the next plate.

A radio image of SS433, with particles streaming away from the black hole in a corkscrew pattern.
A radio image of SS433, with particles streaming away from the black hole in a corkscrew pattern.

This particular star, which was southwest of Altair, the brightest star of Aquila, the eagle, was really dim -- much too faint to see with the average telescope. What set it apart from other stars was that its H-alpha line was three to four times wider than average. What could I be seeing? Several stars with H-alpha emission on top of each other? A flaw in the glass plate? Checking other plates showed the star and the funny line, always wide, always the same brightness. Unfortunately, glass plates are great survey tools, but not too good for details. A larger telescope was required to investigate further, but that wasn't an option at the time.

I gave the strange object a name, K16 -- "K" from my family name, Krumenaker.

I published the information in a 1975 article in Publications of the Astronomical Society of the Pacific, and kept this object and all the others in a project folder until I could get my hands on a bigger telescope.

In 1977, K16 became known as SS433 when Case Western Reserve University astronomers Bruce Stephenson and Nick Sanduleak listed it as the 433rd object in their catalog of objects with emission lines in their spectra.

Since then, many astro­nomers have studied SS433, in all regions of the spectrum, from long-wavelength radio waves to high-frequency gamma rays.

Each investigation has added a piece to this astronomical puzzle, and we now think we know what it is.

Early in 1978, P.G. Martin of the University of Cambridge, and P.G. Murdin and D.H. Clark of the Royal Greenwich Observatory, were conducting a survey at the Anglo-Australian Observatory. They were looking for stars with emission lines at the sites of newly found radio-wave emitting objects. They found an object with a strange spectrum that turned out to be (probably disappointingly) already recorded as SS433. Some of the lines on its spectrum could not be identified with any known chemical elements. This is one kind of mystery astronomers can not pass up! The game was under way to break open the mystery of SS433.

A series of images made with radio telescopes shows blobs of material shooting outward in the SS433 jets.  [NRAO]
A series of images made with radio telescopes shows blobs of material shooting outward in the SS433 jets. [NRAO]

That August, astronomer Bruce Margon, then at UCLA but now at the Space Telescope Science Institute, led a team that found that the unusual emission lines actually consisted of pairs of lines -- one shifted toward the red end of the spectrum and the other toward the blue. As they studied SS433 over time, these lines moved and crossed each other a period of approximately 160 days.

Such a shift in a spectrum's lines is not unusual. All stars do it. Car horns do it. It's called the Doppler shift. When an object is approaching the observer, wavelengths of sound or light are shortened; that makes the pitch of a car's horn go up, and in the spectrum of a star it makes the spectral features move a bit blueward. When the object is moving away, the light waves get redder. The faster things move, the more extreme the shift.

But pairs of lines crisscrossing each other was most unusual. Furthermore, nobody knew what chemical elements were producing the emission lines. And with no reference to compare the lines to, astronomers couldn't measure how fast SS433 was moving, either.

In 1979, following announcements of this unusual object, astronomers in Oregon found that SS433 varied in brightness. Harvard College Observatory's archive of photographic plates showed that SS433 had showed the same 160-day variation in brightness over the previous 50 years.

About the same time, astronomers in Arizona found that the pairs of lines were actually triplets. Astronomers realized that these were normal hydrogen lines -- that is, lines produced by the "glow" of hydrogen in SS433 -- but with a difference. One line stayed in place, and didn't shift back and forth. But this line was flanked by grotesquely shifted and shifting lines on either side of it! No one had ever seen such spectral lines before. The shift meant that whatever was producing the outer lines -- the ones that moved back and forth -- must sometimes move at one-fourth of lightspeed, and at other times slow to zero! The only good interpretation was that hydrogen atoms were being ejected from a central object in "jets" -- like geysers of hot gas spewing in opposite directions. The gas in the jets must move at tremendous speeds, and must rotate around the center over about a five-month period. The object became known as "the star that was coming and going," and even entered popular culture: It was mentioned on "Saturday Night Live." Next, David Crampton and others of Canada's Dominion Astronomical Observatory announced that the central line of SS433's hydrogen triplet also moved, in a 13-day cycle. That means that SS433 was not just a single object with jets -- it had to be two objects with jets. It was a binary ...

something. At this point, astronomers had learned that SS433 consists of an object of considerable mass, with incredibly high-speed gases that change speeds in a periodic manner. Some aspect of the system causes brightness and color variations. The basic picture of SS433 was now complete, but the details were still hazy. We had learned all we were about to learn about SS433 from visible light -- for a while.

Now it was time for astronomers who study energy from other parts of the spectrum to get into the act. Ken Johnston of the Naval Research Laboratory located a radio-wave-emitting object in the sky at the exact position of SS433. The radio waves varied with a period of 6.6 days. Later detections indicated there were flares of radio waves. Radio telescopes produced the first detailed studies of SS433's physical arrangement. SS433 showed itself to be a radio-bright, compact object with "bumps" on its sides.

Others noted that SS433 lies near a dim supernova remnant called W50. A supernova remnant is all we see of a star that blew itself to bits thousands of years ago. Some features in SS433 are aligned with those of W50, showing the winds from SS433 are affecting the remnant, which is at least 15,000 light-years away. SS433 and W50 probably are all that's left of a once-normal binary-star system.

X-rays from SS433 also showed flares. When the X-ray flares occurred, SS433 produced little radio energy, and vice versa. The X-ray flares had a period of 13 days, the same as the orbital period of the central objects. This meant the flares must be coming from one of these objects -- presumably the compact one. In 1979, the satellite HEAO-3 was observing that part of the sky when SS433 produced a gamma-ray flare. The energy was large and indicated that the gamma rays were born from oxygen that formed during the process that powers old stars. This gives evidence that SS433 is a product of stellar aging, not youth.

Today we know that SS433 consists of a pair of jets on opposite sides of a central system. This system consists of a compact object -- a huge black hole, a regular black hole, or a neutron star -- orbiting every 13 days around a bloated star that dumps gas toward the compact object. The gas temporarily resides in a thick, slightly wobbly accretion disk that causes a 6- to 6.5-day variation in radiation and is the source of the majority of the visible light seen as a "star" on photographs.

Like flowing water that smashes into a wall and flies upwards, the excess infalling gas is ejected at a significant fraction of the speed of light.

These jets flow more or less at right angles to the accretion disk. The jets and disk are not perfectly aligned with the orbit of the two central objects, so they wobble like a spinning top, looking for all the world like a berserk two-nozzle lawn sprinkler. The jets maintain their orientations for some distance, even affecting the surrounding material of W50.

The general picture has been fairly clear for some time, but there was still a lot of uncertainty about what the objects in SS433 are! Todd Hillwig and Douglas Gies, two Georgia State University astronomers, recently led a team of observers to an answer.

As the two objects orbit each other, they produce an eclipse every 13 days, causing the system's overall brightness to dip a little. Since the disk surrounding the compact object provides most of the system's visible light, the dimmer object must be covering up only part of the disk. Even so, the stellar companion's light is still usually overwhelmed by that of the disk.

However, during times when the jets' own 162-day wobble puts the disk more below the star than usual as seen from Earth, the faint star might just send enough light toward us for instruments to record. This clockwork arrangement occurs only about twice a year. By observing in a part of the spectrum in which the disk and jets produce fairly little energy, the majority of the light must be from the companion star.

The newly uncovered spectrum of the bloated star appears to be that of an old A-class supergiant -- a large, hot star about 11 times as massive as the Sun. Since the disk and compact object combined are three times as massive as the Sun, Hillwig and Gies believe the compact object is a low-mass black hole. Thus, the mystery of SS433 -- a mystery presented by a young astronomer almost three decades ago -- may finally be solved.

Larry Krumenaker is an astronomer turned science writer.