A neutron star is the closest thing to a black hole that astronomers can observe directly, crushing half a million times more mass than Earth into a sphere no larger than a city. In October 2010, a neutron star near the center of our galaxy erupted with hundreds of X-ray bursts that were powered by a barrage of thermonuclear explosions on the star's surface. NASA's Rossi X-ray Timing Explorer (RXTE) captured the month-long fusillade in extreme detail. Using this data, an international team of astronomers has been able to bridge a long-standing gap between theory and observation.
On Oct. 10, 2010, the European Space Agency's INTEGRAL satellite detected a transient X-ray source in the direction of Terzan 5, a globular star cluster about 25,000 light-years away toward the constellation Sagittarius. The object, dubbed IGR J17480, is classed as a low-mass X-ray binary system, in which the neutron star orbits a star much like the sun and draws a stream of matter from it. As only the second bright X-ray source to be found in the cluster, Linares and his colleagues shortened its moniker to T5X2.
Three days after the source's discovery, RXTE targeted T5X2 and detected regular pulses in its emission, indicating that the object was a pulsar -- a type of neutron star that emits electromagnetic energy at periodic intervals. The object's powerful magnetic field directs infalling gas onto the star's magnetic poles, producing hot spots that rotate with the neutron star and give rise to X-ray pulses. At NASA's Goddard Space Flight Center in Greenbelt, Md., RXTE scientists Tod Strohmayer and Craig Markwardt showed that T5X2 spins at a sedate -- for neutron stars -- rate of 11 times a second. And because the pulsar's orbital motion imparts small but regular changes in the pulse frequency, they showed that the pulsar and its sun-like companion revolve around each other every 21 hours.
This illustration compares the size of a neutron star to Manhattan. The crushed core of a star that has exploded as a supernova, a neutron star packs more mass than the sun into a sphere just 10 to 15 miles wide. Credit: NASA/Goddard Space Flight Center
That same day, RXTE observed its first burst from the system: an intense spike in X-rays lasting nearly 3 minutes and caused by a thermonuclear explosion on the neutron star's surface. Ultimately, RXTE cataloged some 400 events like this between Oct. 13 and Nov. 19, with additional bursts observed by INTEGRAL and NASA's Swift and Chandra observatories. NASA decommissioned RXTE on Jan. 5, 2012.
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