16 March 2006
Integral catches stellar 'corpses' by the tail
Tiny stellar 'corpses' have been caught blasting surprisingly powerful
X-rays and gamma rays across our galaxy by ESA's gamma-ray observatory
This discovery links these objects to the most magnetically active
bodies in the Universe and forces scientists to reconsider just how dead
such stellar corpses really are.
Known as anomalous X-ray pulsars (AXPs), the stellar corpses were first
spotted pulsing low-energy X-rays into space during the 1970s by the
Uhuru X-ray satellite. AXPs are extremely rare with only seven known to
exist. The X-rays were first thought to be produced by matter falling
from a companion star onto the AXP.
An alternative was that each AXP is the spinning core of a dead star,
known as a neutron star, sweeping beams of energy through space like a
cosmic lighthouse. When these beams cross Earth's line of sight, the AXP
blinks on and off.
However, this scenario required the AXP's magnetic field to be a
thousand million times stronger than the strongest steady magnetic field
achievable in a laboratory on Earth. Nevertheless, the Integral
observations show that the magnetic solution is correct.
The newly detected emission, known to astronomers as a 'hard tail', of
high-energy ('hard') X-rays and gamma rays also comes in the form of
regular pulses every 6–12 seconds depending upon which AXP is observed.
Discovered in three of the four AXPs studied, the hard tails have a
distinctive energy signature that forces astronomers to consider that
they are produced by super-strong magnetic fields.
"The amount of energy in the hard tail is ten to almost one thousand
times more than can be explained by a kind of magnetic friction between
the spinning AXP and surrounding space," said Wim Hermsen of SRON, the
Netherlands Institute for Space Research, Utrecht, who together with
SRON colleagues made the observations. This leaves so-called 'magnetic
field decay' as the only viable alternative.
Neutron stars with super-strong magnetic fields are dubbed 'magnetars'.
Created from the core of a gigantic star that has exploded at the end of
its life, each magnetar is only around 15 kilometres in diameter yet
contains more than one and a half times the mass of the Sun.
Magnetars are also responsible for the 'soft gamma-ray repeaters'
(SGRs), which explosively release massive quantities of energy when
catastrophic reorganisations of their magnetic fields spontaneously take
place. The big difference between an SGR and an AXP is that the process
is continuous rather than explosive in an AXP and less energetic.
"Somehow these objects are tapping the enormous magnetic energy
contained beneath their surfaces and funnelling it into space," said
Exactly how that happens is the focus of future work. It is possible
that SGRs, of which five are known, turn into AXPs once they have
exploded enough of their energy into space.
All known AXPs except one are clustered towards the plane of our galaxy,
the Milky Way, indicating that they are the result of recent stellar
explosions; some are even wreathed in the exploded gaseous remnants of
their former stars.
The other known AXP is in a satellite galaxy of the Milky Way. The hard
tails were discovered by Integral serendipitously, thanks to its unique
wide-field camera, the Imager on-Board Integral Satellite (IBIS).
"This is one of the things you hope for when you run an observatory like
Integral," said Christoph Winkler, ESA's Integral project scientist. As
the AXPs prove, the stellar afterlife is more alive than astronomers
Notes to editors:
The findings are accepted for publication in Astronomy and Astrophysics,
in a paper titled 'Integral survey of the Cassiopeia region in hard X
rays', by P.R. den Hartog and L. Kuiper (SRON, Netherlands Institute for
Space Research, Sorbonnelaan, Utrecht), W. Hermsen (SRON and
Astronomical Institute 'Anton Pannekoek', Univ. of Amsterdam, The
Netherlands), J. Vink and J.J.M. in 't Zand (Univ. of Utrecht and SRON),
and W. Collmar (Max-Planck-Institut fur extraterrestrische Physik,
More detailed findings are accepted for publication in the Astrophysical
Journal in the paper 'Discovery of luminous (pulsed) hard X-ray emission
from anomalous X-ray pulsars 1RXS J1708 - 4009, 4U 0142 + 61 and 1E 2259
+ 586 by Integral and RXTE', by L. Kuiper and P.R. den Hartog (SRON), W.
Hermsen (SRON and Astronomical Institute 'Anton Pannekoek', Univ. of
Amsterdam, The Netherlands), and W. Collmar (Max-Planck-Institut fur
extraterrestrische Physik, Garching, Germany).
In this paper, the results are also based on data from NASA's Rossi
X-Ray Timing Explorer.
For more information:
Wim Hermsen, SRON
Astronomical Institute 'Anton Pannekoek'
Univ. of Amsterdam, The Netherlands
E-mail: w.hermsen @ sron.nl
Chris Winkler, ESA Integral Project Scientist
E-mail: christoph.winkler @ esa.int
Arvind Parmar, ESA Integral Mission Manager
E-mail: arvind.parmar @ esa.int
* ESA's gamma-ray astronomy mission
* Integral factsheet
* Exceptional manoeuvres enable unique Integral science
* Integral identifies supernova rate for Milky Way
* Integral: three years of insight into the violent cosmos
* Star eats companion
* Three satellites needed to bring out 'shy star'
* Integral rolls back history of Milky Way's super-massive black hole
* ESA's Integral detects closest cosmic gamma-ray burst
* Observations: Seeing in the gamma-ray wavelengths
* Integral Science Operations Centre
* Max Planck Institut
This artist's impression shows an anomalous kind of X-ray pulsar as
observed by ESA's Integral gamma-ray observatory. Newly detected
emissions, known to astronomers as a 'hard tails' of high-energy
('hard') X-rays and gamma rays, have a distinctive energy signature that
forces astronomers to consider that they are produced by super-strong
The task of Integral, ESA's International Gamma-Ray Astrophysics
Laboratory, is to gather the most energetic radiation that comes from
space. The spacecraft was launched in October 2002 and it is helping to
solve some of the biggest mysteries in astronomy. Gamma rays are even
more powerful than the X-rays used in medical examinations. Fortunately,
Earth's atmosphere acts as a shield to protect us from this dangerous
cosmic radiation. However this means that gamma rays from space can only
be detected by satellites. Integral is the most sensitive gamma-ray
observatory ever launched. It detects radiation from the most violent
events far away and from processes that made the Universe habitable.
Credits: ESA (Medialab)