A joint Fermilab/SLAC publication
Image: Roger Romani
Matt Beardsley, SLAC

Stellar black widows entrap companion stars


Of the hundreds of objects in the universe emitting gamma rays, two look to be "black widows," ancient stars extending their lives by sucking in material from companion stars. Stanford physicist Roger Romani is hot on the trail of these extreme stars.

In its four years in orbit, the Fermi Gamma-ray Space Telescope has found a cosmos teeming with points of gamma-ray light. Newly discovered gamma-ray sources run the gamut from the expected, like supernova remnants and active galactic nuclei, to the surprising, like gamma rays from the sun or Earth-bound lightning strikes.

But a considerable percentage of the gamma-ray sources discovered by Fermi can’t be matched up with any type of object, expected or not. Of the more than 1800 sources found by Fermi’s main instrument, the Large Area Telescope, in its first two years of operation, almost a third fell into this category.

These "unassociated objects," as they're called, are the ones Stanford physics professor Roger Romani likes to study. Romani, a member of the Kavli Institute for Particle Astrophysics and Cosmology, an institute run jointly by Stanford and SLAC National Accelerator Laboratory, has spent the past few years identifying these sources. He's found most of them to be common astronomical objects that, for one reason or another, were just a bit more difficult to recognize. Two of them, however, appear to be "black widows," ancient stars extending their lives by sucking in material from companion stars. And there may be more.

"I've been interested in gamma-ray sources for years," Romani says, and what could be more tantalizing than an anomalous gamma-ray source?

Romani and his students studied the brightest 250 unassociated objects on the list. The great majority of them proved to be fast-spinning pulsars—stellar corpses made of neutrons that spin off extra energy left over from their collapse—or active galactic nuclei—supermassive black holes lurking at the centers of galaxies.

But a half-dozen objects remained mysterious. They weren’t associated with galaxies, so they couldn’t be active galactic nuclei. And yet the objects didn’t seem to be any known type of pulsar, either.

“That’s why they were so exciting,” Romani says. “They could be something completely new.”

Romani had a suspicion, though. He began to wonder if the unknowns were black widows.

Had black widow pulsars been named today they might well be called vampire pulsars. They’re very old neutron stars that have already spun down to a stop but have gained a new life from their companion stars. What makes a black widow especially vicious is that once it starts spinning again, the beams of energy it emits blast across the companion star hundreds of times a second. What a black widow doesn’t eat, it blows away.

But there was one problem stopping Romani from making the diagnosis: These potential black widows were not visible in radio waves. The speed at which black widows rotate make them members of a class of pulsars called millisecond pulsars. “To date, every millisecond pulsar can be seen in radio waves,” Romani says.

Except these.

One possible explanation for these six would-be pulsars’ radio silence came from the destructive nature of black widows. Radio waves can’t penetrate thick clouds of plasma, but gamma rays are powerful enough to punch right through. If the objects really were black widows shredding their companion stars, they might be close enough to the companions to shroud themselves in gas and dust as well.

To test his suspicions, Romani went on the hunt for companion stars using good old-fashioned visible light. Even if the companion stars were almost gone, Romani says, “They’d be so hot because of all the energy being dumped on them by the pulsars that we could see them.”

Romani and his team combed through archived data and pointed optical telescopes like the Wisconsin-Indiana-Yale-NOAO telescope in Arizona, the Hobby-Eberly Telescope in Texas, and even telescopes at Stanford’s own student observatory in California toward the potential black widows. They found that two of the six gamma-ray sources had a nearby star that alternated between bright blue and faint red on a regular schedule. Each star, Romani concluded, appeared bright blue when its black widow companion was blasting away at the face visible from Earth, and faint red when the black widow had orbited around the companion to blast at the side facing away from the telescopes.

Using information gathered from these observations, Romani set the orbital periods for his two black widows around their hapless companions at a speedy 4.6 hours for the first and a scorching 94 minutes for the second—the shortest orbit of any known millisecond pulsar. This second black widow blasts its companion star with so much energy that it raises the star’s heated face from 2000 to up to 14,000 kelvin. “This is an extreme black widow system,” Romani says.

Using his information, one team of scientists was able to pinpoint the spin period of the second pulsar, while another found stray radio signals that managed to slip through the plasma blown off the companion star. Work continues to pinpoint the second black widow, but as Romani says, “When something looks like a duck and quacks like a duck, it’s probably a duck.” An extreme duck.

Romani and his team continue to investigate the other four unidentified sources; they may also turn out to be extreme stars of the black widow variety.

Romani likes extremes, and black widows make him very happy. “Not only are these pulsars really fast neutron stars, they’re really fat neutron stars,” he says. So fat in the case of the second pulsar that it borders on black hole-dom.

Measuring just how fat these black widows are, Romani says, could offer insight into the behavior of quark-gluon matter at very high densities.

“I find Fermi to be a wonderful signpost to extreme physics,” he says.

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