CAMBRIDGE, Mass. — Like a frozen turkey that just won’t thaw, a strange star
near the center of the Milky Way is surprising MIT experts and colleagues
with its remarkably low temperature. The odd behavior is chilling current
theories of stellar physics.

A famously battered neutron star named KS 1731-260 appears no hotter than some
of its tranquil brethren, despite enduring the heat of constant thermonuclear
explosions with the force of billions of hydrogen bombs every second across a
region only a few miles wide for the past 12 years.

Dr. Rudi Wijnands, an astrophysicist at MIT’s Center for Space Research, used
the Chandra X-ray Observatory to measure the temperature of the neutron star
at a very opportune moment, only months after the nuclear war apparently
ended and the smoke cleared. He presented his team’s findings September 5 in
Washington, D.C. at a scientific conference entitled “Two Years of Science
with Chandra.”

“Twelve years of constant thermonuclear explosions: One would think that would
heat things up,” said Wijnands. “This leaves us wondering whether some neutron
stars are in the freezer for a much longer time than previously thought and
consequently take a long time to heat up, or whether they cool down incredibly
fast. Either explanation has profound implications for our field.”

Neutron stars are the dense, core remains of stars once many times more
massive than our Sun. They are created in dazzling supernovae, in which the
outer shell of the star explodes into space, and the core, containing about
as much mass as the Sun, implodes and collapses into a sphere no wider than
Cambridge, Massachusetts.

Despite their tiny size, neutron stars are visible in several ways. One is
through accretion. Neutron stars are a strong source of gravity. When they
exist in binary star systems, such as KS 1731-260, they can attract the gas
from what is often a “healthy” hydrogen-burning companion star (although
the nature of KS 1731-260’s companion is not clear.) Gas spirals around the
neutron star and comes crashing down onto its surface, leading to nuclear
explosions. The fury glows predominantly in X-ray radiation.

Neutron star binaries can experience varying periods of active accretion.
KS 1731-260’s period was particularly prolonged, lasting from mid-1988 to
the end of 2000. Other systems flare from week to week or year to year.
When the stars are in quiescence, they are harder to detect.

Chandra has the resolution and photon-collecting power and is in the
right energy range to detect neutron stars glowing dimly in quiescence.
KS 1731-260 is the first neutron star enduring such prolonged accretion to
enter into quiescence during Chandra’s reign. Other systems only dump matter
onto the neutron star for a short period at a time, so no one expects much
heating.

The Chandra observation provided the first chance to test the theory of
neutron star heating and cooling for a system with such prolonged episodes
of accretion. The team found KS 1731-260 to be at least 10 times cooler than
expected, a mere 3.5 million degrees. This is about the same temperature
as neutron stars that get only a week’s or month’s worth of dumping.
KS 1731-260 therefore appears to be much too cool, assuming that this and
the other neutron stars have accretion episodes at roughly similar time
intervals.

So what happened to the heat? There are complicated models of neutrino
cooling to explain rapid temperature decreases. But maybe KS 1731-260 was
in a relative deep freeze before 1988 and took 12 years just to get to the
temperature it is today.

According to the popular model of neutron star heating and cooling, Wijnands
calculated that KS 1731-260 would need to have been dormant (that is, no
accretion) for over a thousand years to get so cold that 12 years of fury
would only raise it to its current temperature.

“Neutron stars in such close binaries are not known to stop accreting for
periods longer that about 100 years,” said Walter Lewin, a professor of
physics also at MIT. “We may have identified a new type of neutron star
system that can lie dormant for thousands of years. If so, there could be
hundreds of these systems in our Galaxy.”

The next step is to use Chandra to take the temperature of scores of other
neutron stars experiencing various phases of accretion and quiescence. Other
members of the observation team include Jon Miller of MIT, Craig Markwardt
from NASA Goddard Space Flight Center, and Michiel van der Klis from the
University of Amsterdam.

The observation was made with Chandra’s Advanced CCD Imaging Spectrometer,
which was conceived and developed for NASA by Pennsylvania State University
and MIT. For images, refer to

http://universe.gsfc.nasa.gov/press/images/neutron/