Spinning neutron stars, also known as pulsars, generally rotate on highly stable axes. Thanks to their periodic signals, emitted either in radio or X-ray wavelengths, they can serve as very accurate astronomical clocks.

Regarding pulsar RX J0720.4-3125, however, the team found that over the past four and a half years, its temperature has been rising - until very recently, when the trend reversed and the temperature began decreasing.

This effect cannot be due to a real variation in temperature, however, the scientists wrote - in an article appearing in an upcoming issue of Astronomy & Astrophysics - but instead it must be caused by a changing viewing geometry. RX J0720.4-3125 most probably is undergoing a phenomenon called precessing, in which it slowly tumbles on its axis and, over time, exposes different areas of its surface.

Neutron stars are one of the endpoints of stellar evolution. With a mass comparable to the Sun's confined into a sphere no more than 20 kilometers to 40 kilometers (12 miles to 25 miles) in diameter, their density is even somewhat higher than that of an atomic nucleus - a billion tons per cubic centimeter.

Soon after their birth in a supernova explosion, they reach temperatures in the range of 1 million degrees Celsius and begin generating huge amounts of X-ray energy. Then they cool slowly, taking over a million years before their X-ray output falls below XMM-Newton's observable range.

Neutron stars also can generate very strong magnetic fields - several trillion times stronger than Earth's. Their magnetic field can be so strong it influences the heat transport from the stellar interior through the crust, leading to hot spots around the magnetic poles on the star's surface - the presumed source of the X-rays.

Only a few isolated neutron stars are directly visible to Earth-based X-ray instruments, and one of them is RX J0720.4-3125, which rotates at the relatively lengthy period of about eight and a half seconds.

"Given the long cooling time scale it was therefore highly unexpected to see its X-ray spectrum changing over a couple of years," said team leader Frank Haberl, of the Max-Planck-Institute for Extraterrestrial Physics.

"It is very unlikely that the global temperature of the neutron star changes that quickly," Haberl said. "We are rather seeing different areas of the stellar surface at different times. This is also observed during the rotation period of the neutron star when the hot spots are moving in and out of our line of sight, and so their contribution to the total emission changes."

A similar effect on a much longer time scale can be observed when the neutron star precesses, similarly to a spinning top, in which the rotation axis itself moves around a cone leading to a slow change of the viewing geometry over the years. Free precession can be caused by a slight deformation of the star from a perfect sphere, which may have its origin in the very strong magnetic field.

During XMM-Newton's first observation of RX J0720.4-3125 in May 2000, its observed temperature was at minimum and the cooler, larger spot was predominantly visible. On the other hand, four years later, in May 2004, the precession brought into view mostly the second, hotter and smaller spot, which made the observed temperature increase. Haberl said this probably explains the observed variation in temperature and emitting areas, and their anti-correlation.

Haberl and colleagues have developed a model for RX J0720.4-3125, which they said can explain many of the star's peculiar characteristics. The model produces the long-term change in temperature via the different fractions of the two hot polar caps, which enter into view as the star precesses with a period of about seven to eight years.

In order for such a model to work, the two emitting polar regions must exhibit different temperatures and sizes, as it has been recently proposed in the case of another member of the same class of isolated neutron stars.

RX J0720.4-3125 could be the best case to study precession of a neutron star via its X-ray emission directly visible from the stellar surface. So, precession might be a powerful tool to probe the neutron star interior and learn about the state of matter under conditions which we can not produce in the laboratory.

The team plans more XMM-Newton observations to monitor the pulsar further. "We are continuing the theoretical modeling, from which we hope to learn more about the thermal evolution, the magnetic field geometry of this particular star and the interior structure of neutron stars in general," Haberl said.

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