Astronomers report the brightest flash of light ever recorded from beyond the solar system. In just 0.2 second on Dec. 27, 2004, spacecraft detected a stellar outburst that radiated as much energy as the sun emits in 250,000 years.
But breaking a record wasn’t the only reason that astronomers were beaming at a NASA press briefing last week. They may also have solved a mystery. By tracing the outburst to a magnetar, the rare type of star with the strongest magnetic field known, the scientists may be able to account for a puzzling group of extremely short-lived gamma-ray bursts from distant galaxies.
These bursts pack a greater intensity of the highest-energy gamma rays than do others, lasting more than 2 seconds, that have been identified as the likely birth cries of black holes.
A slew of telescopes, many of which were temporarily blinded by the outburst, traced the explosion to the strongly magnetic neutron star SGR 1806-20, which lies in the Milky Way an estimated 50,000 light-years from Earth. The dense cinder of a massive star, a neutron star packs as much mass as the sun into a ball only 20 kilometers in diameter.
If Earth had as strong a magnetic field as SGR 1806-20 does, it would wipe out the information on a credit card located halfway to the moon. Theory suggests that when a magnetar’s twisted field snaps, the star’s crust rips apart, unleashing gamma rays.
Weaker blasts of gamma-ray emissions have been seen from only two other magnetars (SN: 9/12/98, p. 164: https://www.sciencenews.org/pages/sn_arc98/9_12_98/fob1.htm). Because the most recent event was 100 times as energetic as those earlier bursts, astronomers could observe the outburst and its aftermath in unprecedented detail.
“This might be a once-in-a-lifetime event for astronomers as well as for the neutron star,” notes David Palmer of Los Alamos (N.M.) National Laboratory.
Immediately after the Dec. 27 gamma-ray spike, an X-ray afterglow lasted for about 6 minutes. Recorded by NASA’s orbiting Reuven Ramaty High Energy Solar Spectroscopic Imager, the X rays pulsed in sync with the star’s 7.56-second rotation.
The X rays come from hot, charged gas imprisoned in a section of the star’s magnetic field, says Robert Duncan of the University of Texas at Austin. This finding confirms the extraordinary strength of the star’s magnetic field.
The X-ray findings and the number of magnetars in our galaxy also suggest that more-distant magnetars could be the source of some of the short-lived gamma-ray bursts that spacecraft have detected in galaxies far beyond the Milky Way, argue Duncan and his colleagues. Those galaxies are so distant that an X-ray glow from a magnetar eruption would be too faint to detect.
Radio wave telescopes are still recording the aftermath of the Dec. 27 event, and some of those observations, now posted online (http://xxx.lanl.gov/abs/astro-ph/0502428), suggest that the magnetar SGR 1806-20 might lie just 30,000 light-years from Earth.
According to Shri Kulkarni of the California Institute of Technology in Pasadena and his colleagues, if the closer distance is correct, then the outburst, though still a record breaker, might not be as powerful as calculated. In that case, most magnetars in distant galaxies might not have enough oomph to produce short-lived gamma-ray bursts, so the source of most such bursts would still be undetermined.
Duncan says that he disagrees with the distance calculations by Kulkarni and his colleagues. “Magnetar flares constitute at least a substantial fraction of observed short-duration bursts,” he asserts.
