The Mysterious Ring Of A Soft Gamma Repeater – Magnetar SGR 1900+14
universe is a big place, and weird things can happen. I was flipping
through archived Spitzer data of the object, and that’s when I noticed
it was surrounded by a ring we’d never seen before, "said Stephanie
Wachter of NASA’s Spitzer Science Center at the California Institute of
Technology about a mysterious infrared ring a dead star that displays a
magnetic field trillions of times more intense than Earth’s.
NASA’s Spitzer Space Telescope detected the ring around magnetar SGR
1900+14 at two narrow infrared frequencies in 2005 and 2007. The ringed
magnetar is of a type called a soft gamma repeater (SGR) because it
repeatedly emits bursts of gamma rays.
This image shows a ghostly ring extending seven light-years
across around the corpse of a massive star. Magnetar SGR 1900 14, is
located at the exact center of this image. The magnetar itself is not
visible in this image, as it has not been detected at infrared
wavelengths (it has been seen in X-ray light). Credit: NASA/JPL-Caltech
Wachter enlisted Vikram Dwarkadas, a Senior Research Associate in
Astronomy & Astrophysics at the University of Chicago, to help
determine how the ring formed. Wachter, Dwarkadas and five other
co-authors present the results of their investigation in the May 29
issue of the journal Nature.
Magnetars interest astrophysicists because of their mysterious and
unusual characteristics. When massive stars collapse, they usually form
compact objects called neutron stars or black holes. "We have no idea
why some neutron stars are magnetars and some are not," Dwarkadas said.
SGR 1900+14 seems to belong to a nearby cluster of massive stars
that resides along the plane of the Milky Way. Since the most massive
stars live the shortest lives, the object hints that perhaps only the
most massive stars become magnetars.
When Wachter’s team began pondering the origin of the ring, "We
thought initially of all the standard explanations," Dwarkadas said.
But the team considered and eliminated several possibilities before
concluding that a powerful flare that burst from the magnetar formed
the ring, which measures seven light-years across.
"It’s as if the magnetar became a huge flaming torch and obliterated
the dust around it, creating a massive cavity," said co-author Chryssa
Kouveliotou, senior astrophysicist at NASA’s Marshall Space Flight
Center in Alabama. "Then the stars nearby lit up a ring of fire around
the dead star, marking it for eternity."
A theoretical astrophysicist supported by the National Science
Foundation and NASA, Dwarkadas specializes in various phenomena related
to supernova remnants and stellar winds. He helped Wachter’s team
systematically eliminate several potential causes for the ring.
Was the ring an infrared echo, a mass of dust lit up by a flare
moving out from the magnetar" The 2007 Spitzer image showed no
discernable change in the ring after two years. "If it hasn’t moved, it
hasn’t changed, it can’t be an infrared echo," Dwarkadas said. "It’s a
Could the ring be a bubble blown by solar winds emitted from the
star before it exploded" Shock waves of a supernova travel at
approximately 10,000 miles a second. If the ring was a wind-blown
bubble, the supernova shock wave would overtake it somewhere between a
few decades to a century or two, at most.
"It would mean that the supernova should have actually gone through
and destroyed the ring unless it was very, very recent," Dwarkadas
said. If the ring was a wind-blown bubble that somehow survived the
supernova shock wave, "then you’d need a massive bubble," he said. "We
did some calculations and we ran some simulations, and it just didn’t
Wachter’s team next considered whether the ring could be related to
the supernova. That possibility also failed to pan out. "If there is a
supernova, there would be shocks. You would see X-ray, radio and
optical emission. We looked at archival data, and there was no emission
at any wavelength except in the Spitzer images," Dwarkadas said.
The paper’s other co-authors are Jonathan Granot of the University
of Hertfordshire, England; Enrico Ramirez-Ruiz of the University of
California, Santa Cruz; Sandy Patel of the Optical Sciences
Corporation, Huntsville, Ala.; and Don Figer at the Rochester Institute
of Technology in New York.