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
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.
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.
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 J17480–2446, 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. As only the
second bright X-ray source to be found in the cluster, Linares and his
colleagues shortened its moniker to T5X2.
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.
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.
the T5X2 system, matter streams from the sun-like star to the neutron
star, a process called accretion. 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 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.
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
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.
at the highest rates, the strong spikes disappeared and the pattern
transformed into gentle waves of emission. Linares and his colleagues
interpret this as a sign of marginally stable nuclear fusion, where the
reactions take place evenly throughout the fuel layer, just as theory
see T5X2 as a ‘model burster,’ the one that’s doing everything expected
of it,” said Diego Altamirano, an astrophysicist at the University of
Amsterdam in The Netherlands and a co-author on the paper describing the
question now before the team is why this system is so different from
all others studied in previous decades. Linares suspects that T5X2’s
slow rotation may hold the key. Faster rotation would introduce friction
between the neutron star’s surface and its fuel layers, and this
frictional heat may be sufficient to alter the rate of nuclear burning
in all other bursting neutron stars previously studied.