GW/NASA RESEARCHERS FIND DIRECT EVIDENCE OF
NEW TYPE OF STAR

GW Physics Doctoral Student Confirms Magnetar with Extreme Magnetic Field

WASHINGTON - Astrophysicists, in an international collaboration led by scientists at NASA Goddard Space Flight Center and The George Washington University, are now confident that a new type of star exists.  For the first time, these scientists were able to spot protons switching gears while revolving in a record-high magnetic field near a special kind of neutron star called a magnetar.  This star now holds the title for having the most powerful large-scale magnetic field yet detected in the Universe.

With NASA's Rossi X-ray Timing Explorer satellite, physics doctoral candidate Alaa Ibrahim of GW (under the direction of Dr. Jean Swank at NASA and Professor William Parke, Chair of the GW Physics Department) observed bright flashes of gamma-rays from a cosmic object known as SGR 1806-20.  Ibrahim led the team in identifying a proton imprint at five thousand electron volts.  The results are published in two articles in the Astrophysical Journal Letters.

The proton signature allowed the team to make an unprecedented direct measurement of the magnetar’s magnetic field, revealing a record-high million-billion Gauss.  (In physics, a Gauss is the unit of measurement for magnetic field strength).  By contrast, the Earth’s magnetic field is only about half a Gauss. 

“If such an object with that field were located at the moon’s distance from the Earth, it could erase our credit cards,” said Ibrahim.  “Luckily, SGR 1806-20 is a relatively safe 50,000 light-years from Earth. [A light-year is the distance light goes traveling for a whole year, approximately six trillion miles.]  We now have methods of probing such bodies from afar to learn about the physics of matter under extreme gravitational and magnetic forces.” 

Neutron stars are born as remnants of supernova explosions.  "When stars bigger in mass than our Sun run out of fuel, their interior can collapse, perhaps leaving a compact sphere about the size of Washington, D.C., but the mass of the Sun," said Parke.  "Using the instruments placed by NASA into Earth orbit, we are now able to see signatures of materials near a neutron star’s surface, and thereby understand the behavior of matter under very exotic conditions not found on Earth."

SGR 1806-20 is one of a small group of magnetars whose eruptions are powered by magnetic disturbances.  Like earthquakes, a magnetic-driven starquake causes the magnetar's solid crust to crack and eject protons and other plasma particles that become trapped in the intense magnetic field.  The trapped plasma can radiate the gamma-ray flashes producing the "bursts" seen on Earth.  The radiation from these distant bursts can be so intense that it affects even the Earth's atmosphere, causing momentary disruption in radio transmission and communication.

Ibrahim identified an energy feature consistent with a proton signature in many of the bursts from SGR 1806-20.  The result meets theoretical predictions made by a number of astrophysicists, including Silvia Zane of the Mullard Space Science Laboratory, United Kingdom and Roberto Turolla of the University of Podova, Italy.

"A proton spectral-feature provides a robust, direct measurement of the magnetic field strength, because in an intense magnetic field, charged particles absorb and emit light in a quantized fashion," said Ibrahim. "For more typical neutron stars, electron signatures have already provided key information within the pulses powered by the rotation of the star’s magnetic fields.  Now protons and ions are revealing their presence on magnetars, giving us the most clear evidence for the existence of these exotic bodies since their prediction by Robert Duncan at the University of Texas and Chris Thompson at University of North Carolina a decade ago."

Previous observations using star spin periods and spin-down rates only gave indirect evidence of large-scale magnetic fields near magnetars.  The new finding allowed the team to refine estimates of the mass and radius of magnetars.  The results do not restrict SGR 1806-20 to have neutron cores; rather, they could even be stranger by having quark interiors.  These so called quark stars have been theorized to be a possible state of matter at extreme densities and at early hot stages of our universe.  Scientists continue to look for conclusive evidence for quark matter in neutron stars.

Co-authors with Alaa Ibrahim on the Astrophysical Journal Letters reports are Jean Swank of NASA Goddard (Rossi Explorer Project Scientist); William Parke of The George Washington University; Samar Safi-Harb of the University of Manitoba, Canada; Silvia Zane of the Mullard Space Science Laboratory in the United Kingdom; and Roberto Turolla of the University of Podova, Italy.