Space Forum / Astronomy / April 2005

ESA News
http://www.esa.int

25 April 2005

XMM-Newton sees 'hot spots' on neutron stars

Thanks to data from ESA's XMM-Newton spacecraft, European astronomers
have observed for the first time rotating 'hot spots' on the surfaces of
three nearby neutron stars.

This result provides a breakthrough in understanding the 'thermal
geography' of neutron stars, and provides the first measurement of very
small-sized features on objects hundreds to thousands light-years away.
The spots vary in size from that of a football field to that of a golf
course.

Neutron stars are extremely dense and fast-rotating stars mainly
composed of neutrons. They are extremely hot when they are born, being
remnants of supernovae explosions. Their surface temperature is thought
to gradually cool down with time, decreasing to less than one million
degrees after 100 000 years.

However, astrophysicists had proposed the existence of physical
mechanisms by which the electromagnetic energy emitted by neutron stars
could be funnelled back into their surface in certain regions. Such
regions, or 'hot spots', would then be reheated and reach temperatures
much higher than the rest of the cooling surface. Such peculiar 'thermal
geography' of neutron stars, although speculated, could never be
observed directly before.

Using XMM-Newton data, a team of European astronomers have observed
rotating hot spots on three isolated neutron stars that are well-known
X-ray and gamma-ray emitters. The three observed neutron stars are 'PSR
B0656-14', 'PSR B1055-52', and 'Geminga', respectively at about 800,
2000 and 500 light-years away from us.

As for normal stars, the temperature of a neutron star is measured
through its colour that indicates the energy the star emits. The
astronomers have divided the neutron star surfaces into ten wedges and
have measured the temperature of each wedge. By doing so, they could
observe rise and fall of emission from the star's surface, as the hot
spots disappear and appear again while the star rotates. It is also the
first time that surface details ranging in size from less than 100
metres to about one kilometre are identified on the surface of objects
hundreds to thousands light-years away.

The team think that the hot spots are most probably linked to the polar
regions of the neutron stars. This is where the star's magnetic field
funnels charged particles back towards the surface, in a way somehow
similar to the 'Northern lights', or aurorae, seen at the poles of
planets which have magnetic fields, such as Earth, Jupiter and Saturn.

"This result is a first, and a key to understand the internal structure,
the dominant role of the magnetic field treading the star interior and
its magnetosphere, and the complex phenomenology of neutron stars," says
Patrizia Caraveo, of the Istituto Nazionale di Astrofisica (IASF),
Milan, Italy.

"It has been possible only thanks to the new capabilities provided by
the ESA XMM-Newton observatory. We look forward to applying our method
to many more magnetically isolated neutron stars," concludes Caraveo.

However, there is still a puzzle for the astronomers. If the three
'musketeers' are predicted to have polar caps of comparable dimensions,
why then are the hot spots observed in the three cases so different in
size, ranging from 60 metres to one kilometre? What mechanisms rule the
difference? Or does this mean some of the current predictions on neutron
stars magnetic fields need to be revised?

The result, by Andrea De Luca, Patrizia Caraveo, Sandro Mereghetti,
Matteo Negroni (IASF) and Giovanni Bignami of CESR, Toulouse and
University of Pavia, is published in the 20 April 05 issue of the
Astrophysical Journal (http://www.journals.uchicago.edu/ApJ, vol.
623:1051-1069).

For more information please contact:

Patrizia Caraveo, IASF, Milan, Italy
Tel: +39 02 2369 9326
E-mail: pat @ mi.iasf.cnr.it

Norbert Schartel, ESA XMM-Newton Project Scientist
Tel: +34 91 8131 184
E-mail: norbert.schartel @ sciops.esa.int

More information

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

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEMLY9NQS7E_index_1.html]
This is an X-ray image of the neutron star 'Geminga', as taken by
XMM-Newton on 5 April 2002. It lies about 500 light-years away from Earth.

Credits: ESA

[Image 2:
http://www.esa.int/esaCP/SEMLY9NQS7E_index_1.html#subhead1]
The left panel shows X-ray emitting regions from three neutron stars,
'PSR B0656-14', 'PSR B1055-52' and 'Geminga', nicknamed the 'three
musketeers', as measured by ESA's XMM-Newton, down to scale. While the
majority of the neutron stars surfaces emit at temperatures between 500
000 and 700 000 degrees Celsius, smaller spots have significantly higher
temperatures, well above one million degrees Celsius. The dimensions of
the hot spots are different for the three stars.

The right panel shows how the emitting surface of each star varies while
the star rotates (note that the hotter and the cooler surface are not
drawn to scale). From the animation, the hot spots for PSR 1055 and
Geminga disappear for a fraction of the star rotation, while for PSR
0656 the hot spot is always in sight. This offers a new clue to
understanding neutron star geometry.

Credits: IASF

[Image 3:
http://www.esa.int/esaCP/SEMLY9NQS7E_index_1.html#subhead2]
This is an X-ray image of the neutron star 'PSR B0656-14', as taken by
XMM-Newton on 23 October 2001. It lies about 800 light-years away from
Earth.

Credits: ESA

[Image 4:
http://www.esa.int/esaCP/SEMLY9NQS7E_index_1.html#subhead3]
This is an X-ray image of the neutron star 'PSR B1055-52', as taken by
XMM-Newton on 14 and 15 December 2000. It lies about 2000 light-years
away from Earth.

Credits: ESA