Magnetars, The Most Magnetic Stars Known, More Common Than Previously Thought

"We only know of about ten magnetars in the Milky Way galaxy," said the investigation's leader, Dr. Peter Woods of NASA's Marshall Space Flight Center in Huntsville, Ala., based at the National Space Science and Technology in Huntsville. "If the antics of the magnetar we are studying now are typical, turning on and off but never getting exceptionally bright, then there very well could be hundreds more out there."

Wood's colleagues are: Dr. Vicky Kaspi and Mr. Fotis Gavriil of McGill University in Montreal; Dr. Christopher Thompson of the Canadian Institute for Theoretical Astrophysics; Drs. Herman Marshall, Deepto Chakrabarty and Kathy Flanagan at the Massachusetts Institute of Technology; and Drs. Jeremy Heyl and Lars Hernquist at the Harvard-Smithsonian Center for Astrophysics.

The source in question is a magnetar "candidate" named 1E 2259+586 in the constellation Cassiopeia, approximately 18,000 light years from Earth. A magnetar is a special neutron star. A neutron star is a compact sphere approximately 15 kilometers (10 miles) in diameter, the core remains of a collapsed star roughly ten times more massive than the Sun. Magnetars, for reasons poorly understood, have magnetic fields a thousand times stronger than ordinary neutron stars, measuring 1014 to 1015 Gauss (or about a hundred-trillion refrigerator magnets; the Sun's magnetic field is about 5 Gauss.)

Not all scientists are convinced that neutron stars can be so magnetic. As such, magnetar candidates are often referred to in the scientific literature as either Soft Gamma-ray Repeaters (SRGs) or Anomalous X-ray Pulsars (AXPs), depending on their bursting characteristics. Members of this observation team helped established the connection between SRGs and AXPs in 2002. The source 1E 2259 is sometimes called an AXP.

For all their power, magnetars are not always majestic beacons. The opportunity to study them comes when they erupt for hours to months, without warning, emitting visible light and other wavelengths before growing dim once more. Magnetar 1E 2259 suddenly began bursting in June 2002. Scientists collected data on over 80 bursts recorded within a 4-hour window. No other bursts have been detected since.

These same changes in emissions happened 12 years ago and remained a mystery until this study. "Knowing what we know now, we realize that the earlier burst activity was too dim to observe," said Woods.

The cumulative properties of the outburst in 1E 2259+586 led the team to make several conclusions: First, the star suffered some major event lasting several days with two distinct components, one on the surface of the star (perhaps a fracture in the crust) and the other beneath the surface.

According to Kaspi, "The changes in persistent emission properties suggest that the star underwent a plastic deformation of the crust that simultaneously impacted the superfluid interior and the magnetosphere." (A neutron star's interior is thought to be a superfluid of neutrons. The magnetosphere refers to the region in which the neutron star's magnetic field controls the behavior of the charged particles.)

The emission after the bursting was similar to that of an SGR, further blurring the distinction between these two exotic species, Kaspi said. Also, from the changes in emission, the scientists could infer previous burst active episodes from this and other magnetar candidates.

"This sort of behavior could be happening all the time in other sources like it throughout the Galaxy and we would never know it because our gamma-ray 'eyes' are not sensitive enough," said Woods.

Thus, the non-detection of such outbursts by telescopes scanning the entire sky for X-ray and gamma-ray sources suggests that the number of magnetar candidates in our Galaxy is larger than previously thought but that they are in a prolonged dim phase. The team plans to calculate this number. Helping them will be the NASA Swift Gamma-Ray Burst Explorer, scheduled for launch in mid-2004. Swift will be about 20 times more sensitive to magnetar bursts than anything that has flown before. "If there is a big population of these objects out there, Swift should find them," Woods said.

"Magnetars are not just the most magnetic stars known but they are stars not powered by a conventional mechanism such as nuclear fusion, rotation or accretion," Kaspi said. "Magnetars represent a new way for a star to shine, which makes this a fascinating field."

ESA's XMM-Newton was launched in December 1999. NASA helped fund mission development and supports guest observer time. The Rossi Explorer was launched in December 1995. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the day-to-day operation of the satellite and maintains its data archive.

Peter Woods joins the National Space Science and Technology Center through the Universities Space Research Association. Fotis Gavriil is a graduate student in the Physics Department of McGill University.