美宇航局观测到中子星的热核现象

A neutron¹ star is the closest thing to a black hole that astronomers² can observe directly, crushing half a million times more mass than Earth into a sphere（范围，球体） no larger than a city. In October 2010, a neutron star near the center of our galaxy³ erupted with hundreds of X-ray bursts that were powered by a barrage⁴（弹幕） of thermonuclear explosions on the star's surface. NASA's Rossi X-ray Timing⁵ 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. "In a single month from this unique system, we have identified behavior not seen in observations of nearly 100 bursting neutron stars during the past 30 years," said Manuel Linares, a postdoctoral researcher at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology in Cambridge. He led a study of the RXTE data that will be published in the March 20 issue of The Astrophysical Journal.

On Oct. 10, 2010, the European Space Agency's INTEGRAL satellite detected a transient（短暂的） X-ray source in the direction of Terzan 5, a globular star cluster about 25,000 light-years away toward the constellation⁶ Sagittarius. The object, dubbed⁷ IGR J17480-2446, is classed as a low-mass X-ray binary⁸ 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 emission⁹, indicating that the object was a pulsar -- a type of neutron star that emits electromagnetic energy at periodic intervals¹⁰. 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 sedate¹¹（安静的） -- 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 revolve¹² around each other every 21 hours.

That same day, RXTE observed its first burst from the system: an intense spike¹³ in X-rays lasting¹⁴ 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 observatories¹⁵. NASA decommissioned（退役的） RXTE on Jan. 5, 2012.

In the T5X2 system, matter streams from the sun-like star to the neutron star, a process called accretion¹⁶. Because a neutron star packs more than the sun's mass into a sphere between 10 and 15 miles across -- about the size of Manhattan or the District of Columbia -- its surface gravity is extremely high. The gas rains onto the pulsar's surface with incredible force and ultimately coats the neutron star in a layer of hydrogen and helium fuel. When the layer builds to a certain depth, the fuel undergoes a runaway¹⁷ thermonuclear reaction and explodes, creating intense X-ray spikes¹⁸ detected by RXTE and other spacecraft. The bigger the blast, the more intense its X-ray emission.