A rupture in the fabric of physics
Orbiting high above the Earth, the Neil Gehrels Swift Observatory monitors the most violent events in the cosmos. Launched on November 20, 2004, its primary mission is to detect gamma-ray bursts (GRBs)—colossal explosions from the far reaches of the universe. Just over a month after it became operational, Swift and numerous other satellites were struck by a wave of energy from an object that had erupted 50,000 light-years away.
The source was SGR 1806-20, a magnetar. Magnetars are a rare type of neutron star, the dense remnants of massive stellar explosions. A tablespoon of neutron star material has a mass of over 100 million tons. Magnetars are unique because their magnetic field, the most powerful known in the universe, measuring up to a thousand trillion times stronger than Earth's. This immense field is so powerful it would distort atoms into needle-thin cylinders.
On December 27, 2004, the crust of SGR 1806-20 ruptured. The resulting "starquake" caused a catastrophic rearrangement of its magnetic field, releasing a blast of gamma rays and X-rays. In the first tenth of a second, the flare unleashed more energy than our Sun will produce in 250,000 years. The blast was so intense it saturated the detectors of most gamma-ray satellites, including those on RHESSI, an observatory designed to study the Sun's most powerful flares. Even from 50,000 light-years away, the radiation was strong enough to hit Earth's upper atmosphere, causing a measurable disturbance and a brief expansion of the ionosphere.
Swift's rapid response
The Neil Gehrels Swift Observatory is uniquely equipped to study events like this. It carries three co-aligned instruments: the Burst Alert Telescope (BAT), the X-ray Telescope (XRT), and the Ultraviolet/Optical Telescope (UVOT). The BAT has a wide field of view and is designed to detect the initial flash of a gamma-ray burst.
Once the BAT identifies a burst, the spacecraft can autonomously slew—or reorient itself—with incredible speed. Within 20 to 70 seconds, it can point its narrow-field XRT and UVOT instruments directly at the source of the blast. This rapid response allows scientists to capture the fading "afterglow" of the explosion across multiple wavelengths. The XRT can pinpoint the source's location with a precision of about 3 arcseconds.
While the 2004 magnetar flare was an unexpected event for the newly launched Swift, the observatory has since become an important tool for observing all types of high-energy transients. It detects around 90 gamma-ray bursts per year and its flexible scheduling allows it to respond to thousands of "Target of Opportunity" requests from astronomers worldwide. By studying the seismic vibrations from starquakes, scientists can study the interior of neutron stars, where matter exists at densities up to ten times that of an atomic nucleus.