Young pulsar has a split personality

There’s a new addition to the pulsar family of celestial objects, astronomers announced last week.

An artist’s representation of a pulsar. Curved lines represent magnetic fields. NASA/Honeywell Max Q Digital Grp., D. Berry

At approximately 700 years of age, PSR J1846-0258, in the Kes 75 supernova remnant, is a mere babe among pulsars. Astronomers are now asking whether the youngster is a regular pulsar or a more exotic magnetar.

Pulsars are offspring of the events surrounding a star’s demise. In the death throes of a supernova, some stars release massive amounts of energy in a half-second of collapse, ejecting most of their mass.

The core left behind—smaller than many earthly icebergs—has a density of a billion tons per teaspoonful. Highly magnetized, the collapsed star spins rapidly amidst the glowing nebula formed by its own detritus. Each rotation shows up on Earth as a regular pulse. Astronomers have studied pulsars since 1967.

They confirmed the presence of magnetars, pulsars with a relatively slow rotation and an especially strong magnetic field—more than 100 trillion times that of Earth’s—only 2 years ago (SN: 9/12/98, p. 164: http://www.sciencenews.org/sn_arc98/9_12_98/Fob1.htm).

Researchers detected the Kes 75 pulsar using NASA’s Rossi X-ray Timing Explorer, a satellite that monitors cosmic X-ray emissions. The young pulsar is puzzling because it has properties of both regular pulsars and magnetars, says Gautam Vasisht of California Institute of Technology in Pasadena, an author of the Aug. 6 Internet report of the discovery at http://xxx.lanl.gov/abs/astro-ph/0008097.

Regular pulsars are like the one in the Crab nebula, whose spectacular fireworks of birth on July 4, 1054, were noted the world over. These pulsars have a magnetic field strength of 1012 or 1013 gauss and rotate completely every few milliseconds. Magnetars take 5 to 10 seconds to rotate and have a magnetic field around 1015 gauss, Vasisht says.

Above a critical field strength of 4.4 x 1013 gauss, processes come into play that can’t emerge in pulsars with weaker magnetic fields, says Cole Miller, an astronomer at the University of Maryland at College Park. The Kes 75 pulsar has a magnetic field strength of 4.8 x 1013 gauss, says Miller, “just over the hairy edge.”

In regular pulsars, Vasisht explains, magnetism strips and accelerates electrons from the core’s surface, making high-energy particles called gamma ray photons. These photons then transform into pairs of electrons and positrons. The energy shed from these excited pairs streams away and is received on Earth as broadband radiation.

In magnetars, the strong field also produces photons, Vasisht says. However, rather than decaying into positron-electron pairs, the photons split into two new photons of lesser energy.  They emit only X-ray radiation, which makes them harder to spot. “[The Kes 75 pulsar] is a transitional object,” says Vasisht. “Its field is right there at the [critical] point, but yet it shows all the classic behavior of the Crab pulsar.”

Like the Crab, says Frederick Lamb, an astronomer at the University of Illinois in Urbana-Champaign, the Kes 75 pulsar powers its emissions from the speed of its spin. Also like the Crab, its surface appears quiescent, free from the starquakes that rumble through magnetars.

David Helfand, a Columbia University astronomer whose research team discovered the Kes 75 supernova  remnant in 1984, is delighted by the new find. Kes 75 “was sort of our posterboy supernova,” he says. More than the other 224 supernova remnants in the galaxy, Kes 75 matched theoretical predictions—except for 16 years, no one could find its pulsar. “Now it has a pulsar,” he says. “It’s complete.”

Helfand isn’t troubled by the Kes 75 pulsar’s magnetic field because he places pulsars and magnetars on a continuum rather than in separate categories. Helfand says it’s premature to characterize the young pulsar’s field strength as unusual simply because it differs from the Crab’s. “We don’t have enough pulsars to say what’s typical,” he says.

Vasisht says astronomers now hope to determine whether the new pulsar has pulsed radio emissions like the Crab does. “If there is radio emission, it looks very much like the Crab,” he says. “If there isn’t any, then you start getting suspicious about what’s really going on in this object.”

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