XMM-Newton reveals a tumbling neutron star (Forwarded)

ESA News
http://www.esa.int

19 April 2006

XMM-Newton reveals a tumbling neutron star

Using data from ESA's XMM-Newton X-ray observatory, an international
group of astrophysicists discovered that one spinning neutron star
doesn’t appear to be the stable rotator scientists would expect. These
X-ray observations promise to give new insights into the thermal
evolution and finally the interior structure of neutron stars.

Spinning neutron stars, also known as pulsars, are generally known to be
highly stable rotators. Thanks to their periodic signals, emitted either
in the radio or in the X-ray wavelength, they can serve as very accurate
astronomical 'clocks’.

The scientists found that over the past four and a half years the
temperature of one enigmatic object, named RX J0720.4-3125, kept rising.
However, very recent observations have shown that this trend reversed
and the temperature is now decreasing.

According to the scientists 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’, that is it is slowly
tumbling and therefore, over time, it exposes to the observers different
areas of the surface.

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 tonnes per cubic centimetre. Soon after their birth
in a supernova explosion their temperature is of the order of 1 000 000
C 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
magnetic 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 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 Frank Haberl from the Max-Planck-Institute 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 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," Haberl continued.

A similar effect on a much longer time scale can be observed when the
neutron star precesses (similarly 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.

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

In their work Haberl and colleagues developed a model for RX
J0720.4-3125 which can explain many of the peculiar characteristics
which have been a challenge to explain so far. In this model 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 seven to eight years.

In order for such a model to work, the two emitting polar 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.

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 directly
visible 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," Haberl concluded.

Notes to editors:

These results will appear in an article in the scientific journal
Astronomy & Astrophysics (astro-ph/0603724). The article, "Evidence for
precession of the isolated neutron star RX J0720.4-3125", is by Frank
Haberl (Max-Planck-Institut für extraterrestrische Physik, Garching,
Germany), Roberto Turolla (University of Padua, Italy), Cor P. De Vries
(SRON, Utrecht, The Netherlands), Silvia Zane (Mullard Space Science
Laboratory, University College London, UK), Jacco Vink (University
Utrecht, The Netherlands), Mariano Méndez (SRON, Utrecht, The
Netherlands) and Frank Verbunt (University Utrecht, The Netherlands).

For more information:

Frank Haberl, Max-Planck-Institut Für Extraterrestrische Physik,
Garching, Germany
E-mail: fwh @ mpe.mpg.de

Norbert Schartel, ESA XMM-Newton Project Scientist
E-mail: norbert.schartel @ sciops.esa.int

More about...

* XMM-Newton overview
http://www.esa.int/esaSC/120385_index_0_m.html

Related articles

* Cannibal stars like their food hot, XMM-Newton reveals
http://www.esa.int/esaCP/SEM7T6OVGJE_index_0.html
* 'Deep impact’ of pulsar around companion star
http://www.esa.int/esaCP/SEMK6HMVGJE_index_0.html
* XMM-Newton scores 1000 top-class science results
http://www.esa.int/esaCP/SEMAB0NZCIE_index_0.html
* ESA’s Integral and XMM-Newton missions extended
http://www.esa.int/esaCP/SEM31GVLWFE_index_0.html
* XMM-Newton sees 'hot spots' on neutron stars
http://www.esa.int/esaCP/SEMLY9NQS7E_index_0.html
* ESA is hot on the trail of Geminga
http://www.esa.int/esaCP/SEMQB4YO4HD_index_0.html
* XMM-Newton probes formation of galaxy clusters
http://www.esa.int/esaCP/SEMDW5A5QCE_index_0.html
* XMM-Newton's fifth anniversary in orbit
http://www.esa.int/esaSC/SEMZ5CXJD1E_index_0.html

[NOTE: Images supporting this release are available at
http://www.esa.int/esaCP/SEMSIWNFGLE_index_1.html ]