XMM-Newton reveals a tumbling neutron star

Spinning neutron stars, also known as pulsars, are generally known to be highly stable rotators. Their periodic signals, either in the radio or X-ray bands, can serve as very accurate clocks. Using ESA's XMM-Newton X-ray observatory, an international group of astrophysicists discovered that one such spinning neutron star, named RX J0720.4-3125, appears not as stable as expected. They found that over the past 4.5 years the temperature of this enigmatic object kept rising. Very recent observations have, however, shown that this trend reversed and the temperature is now decreasing. According to their recent publication, this effect is not due to a real variation in temperature, but instead to a changing viewing geometry. RX J0720.4-3125 is most probably precessing, i.e. there is a slow tumbling of the star, which exposes different areas of the surface over time. The X-ray observations promise to give new insights into the thermal evolution and finally the interior structure of neutron stars.
Artist's impression of XMM-Newton
XMM-Newton X-ray image of the sky around RX J0720.4-3125 which is the central bright object appearing in red
Neutron stars are one of the endpoints of stellar evolution. With a mass comparable to that of our Sun confined into a sphere of 20-40 km 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 their temperature is of the order of 1.000.000 degrees and the bulk of their thermal emission falls in the X-ray band of the electromagnetic spectrum. Young isolated neutron stars are slowly cooling down and it takes a Million years before they become too cold to be observable in X-rays.

Neutron stars are known to possess very strong magnetic fields, typically several trillion times stronger than that of the Earth. The field can be so strong that it influences the heat transport from the stellar interior through the crust leading to hot spots around the magnetic poles on the star surface. It is the emission from these hotter polar caps which dominates the X-ray spectrum. There are only a few isolated neutron stars known from which we can directly observe the thermal emission from the surface of the star. One of them is RX J0720.4-3125, rotating with a period of about 8.4 s. "Given the long cooling time scale it was therefore highly unexpected to see its X-ray spectrum changing over a couple of years," said Frank Haberl from the Max-Planck-Institut for extraterrestrial physics in Garching, Germany, who led the research group.

It is very unlikely that the global temperature of the neutron star changes that quickly, we rather watch 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, i.e. their contribution to the total emission changes. A similar effect on a much longer time scale can be observed when the neutron star precesses (similar to a spinning top). In that case 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.
Animation of a rotating and precessing neutron star with two hot polar caps
During the first XMM-Newton observation of RX J0720.4-3125 (May 2000), the observed temperature was at minimum and the cooler, larger spot was predominantly visible, while four years later (May 2004), precession brought into view mostly the second, hotter and smaller spot, increasing the observed temperature. This likely explains the observed temperature/emitting area variations and their anti-correlation.
Temperature variations observed from RX J0720.4-3125
In their work Haberl et al. developped a model for RX J0720.4-3125 which can explain many of the peculiar characteristics which have been a challenge so far. In their scenario the long-term change in temperature is produced by the different fractions of the two hot polar caps which enter into view as the star precesses with a period of about 7-8 years. In order for such a model to work, the two emitting regions need to have different temperatures and sizes, as it has been recently proposed in the case of another member of the same class of isolated neutron stars. Learning more about the temperature distribution on the surface of a highly magnetized cooling neutron star will provide insights into the magnetic field geometry and its influence on the heat transport through the neutron star crust.

According to the team "RX J0720.4-3125 is probably the best case to study precession of a neutron star via its X-ray emission which we see directly from the stellar surface. Precession may 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. Additional XMM-Newton observations are planned to further monitor this intriguing object. We are continuing the theoretical modelling 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."

Notes to editors:

These results will appear in an article in the scientific journal Astronomy & Astrophysics. The article, ‘Evidence for precession of the isolated neutron star RX J0720.4-3125’, is by Frank Haberl (Max-Planck-Institut fur extraterrestrische Physik, Garching, Germany, email: fwh @ mpe.mpg.de), Roberto Turolla (University of Padua, Italy, roberto.turolla @ pd.infn.it), Cor P. De Vries (SRON, Netherlands Institute for Space Research, Utrecht, The Netherlands, C.P.de.Vries @ sron.nl), Silvia Zane (Mullard Space Science Laboratory, University College London, UK, sz @ mssl.ucl.ac.uk), Jacco Vink (University Utrecht, The Netherlands, j.vink @ astro.uu.nl), Mariano Méndez (SRON, mariano @ sron.nl) and Frank Verbunt (University Utrecht, F.W.M.Verbunt @ astro.uu.nl).

The paper is available at Astronomy & Astrophysics, forthcoming papers
or at Astrophysics abstracts, astro-ph/0603724