RXCaptures Thermonuclear Behavior of Unique Neutron Star

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 October 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 J17480–2446, 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. 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 showed that T5X2 spins at a sedate, for neutron stars, rate of 11 times a second. In the T5X2 system, matter streams from the sun-like star to the neutron star, a process called accretion. Neutron star 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 runaway thermonuclear reaction and explodes, creating intense X-ray spikes detected by RXTE and other spacecraft. The bigger the blast, the more intense its X-ray emission. Models designed to explain these processes made one prediction that had never been confirmed by observation. At the highest rates of accretion, they said, the flow of fuel onto the neutron star can support continuous and stable thermonuclear reactions without building up and triggering episodic explosions. At low rates of accretion, T5X2 displays the familiar X-ray pattern of fuel build-up and explosion, a strong spike of emission followed by a long lull as the fuel layer reforms. At higher accretion rates, where a greater volume of gas is falling onto the star, the character of the pattern changes: the emission spikes are smaller and occur more often. But at the highest rates, the strong spikes disappeared and the pattern transformed into gentle waves of emission. Scientists interpret this as a sign of marginally stable nuclear fusion, where the reactions take place evenly throughout the fuel layer, just as theory predicted. The question now before the team is why this system is so different from all others studied in previous decades.