Something's Out There
In 1975, we reported on the discovery of some mysterious messages from outer space. Alerted by an orbiting satellite to an extraordinarily powerful and sudden burst of x-ray emissions coming from a portion of the constellation Orion, Professor Forrest Boley and a graduate student, working at the Dartmouth-M.I.T.- Michigan observatory at Kitt Peak, Arizona, were able to train their 52-inch reflecting telescope on the area and obtain the first photographs of what soon became an exciting puzzle. We recently asked Boley for a progress report on the find. His reply was a broad smile and a four-inch-thick sheaf of follow-up research data from other institutions.
"We received some beautiful analysis on this x-ray emitter," says Boley. "People all over the world grabbed on to the problem." At the time it was discovered, scientists suggested the phenomenon might be a huge mass of matter collapsing into a black hole or possibly the remnant of a stellar explosion, but recent evidence indicates it is what Boley suspected all along - a recurrent nova: "It appears we have two stars in a binary system, each member with a highly eccentric orbit. Every 60 years or so [the last such flare-up was in 1917] they approach close enough together for one to exchange matter with the other, erupting into a huge thermonuclear explosion."
When such an event occurs, radiation is released all along the electromagnetic spectrum, from very short gamma rays through x-rays, ultraviolet, visible light, infrared, and down to very long radio waves. Just part of the energy released was in the x-ray range - "formed by the stream of matter colliding with the opposing star's atmosphere, something like what happens in the machine in a dentist's office.'" But the extraordinary thing was the enormous emissions of x-ray radiation. Also significant was the accompanying outburst in the visible range, which was especially satisfying to Boley: "To be able to identify an optical nova with an x-ray nova really made the latter much less mysterious. When I first made the suggestion that the two occur together, it wasn't received well at all. My goodness, the shouting was intense. But now two similar pairings have been found."
Since that time, nothing quite as spectacular has appeared in front of Dartmouth lenses, but the Kitt Peak observatory continues to study interesting x-ray emitters. As recently as January, a Dartmouth group led by Boley and including nine undergraduates photographed another peculiar binary star system, again first picked up by an x-ray-sensitive satellite. In this case the two stars - both about the mass of our sun, though one has collapsed into a white dwarf - are revolving about each other in a tight orbit, the larger gradually giving up its mass to the smaller.
The period of revolution is a startling 81 minutes. "Now, that's really moving," Boley says.
Though astronomers like Boley can use orbiting observatories to identify x-ray sources of interest, the instruments are not yet sensitive enough to be of much help in more detailed analysis. For instance, though these rapidly revolving stars were discovered through their unusual x-ray emissions, optical spectroscopy was needed to record the blue and red Doppler shifts as one star alternately approached us and receded as it rotated about the other. In a similar manner, radio astronomy also began by playing a subordinate role to optical research, though today radio telescopes reach vastly farther into the universe. The future is changing for x-ray astronomy: Within the last two months, a new x-ray satellite was launched that, unlike the old one, can now send back x-ray photographs Boley describes as "fantastic, just fantastic."
As such new equipment becomes available, Boley is training his eye out even farther, to certain extra-galactic x-ray emitters known as BL Lac objects, named for the first one found in the constellation Lacertae. These objects appear to be the nuclei of distant galaxies where the amount of radiation emitted changes inside a period of only a few days. "Now if you can tell me," says Boley, "how the entire center of a galaxy can vary like that within a few days...." His voice trails off. Responding to a question about the possible number of BL Lac objects, Boley offers an appropriately astronomical answer: "They could be relatively rare, and still might number in the thousands. I mean, after all, there are even more galaxies out there than there are stars in our own galaxy."
