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Surprising Flashes from a possible Magnetar
Observations of optical flares reveal limits of established theories on
By means of the high-speed photometer OPTIMA of the Max Planck Institute for
Extraterrestrial Physics (MPE), a team of MPE scientists might have detected
an unexpected new sub-category of astronomical objects. It appears to be a
magnetar with bursts in the visible part of the spectrum, in contrast to the
X-ray and gamma flashes, which are considered to be characteristic for
magnetars (Nature, September 2008).
A notice was received by the scientists around Alexander Stefanescu in real
time, pertaining to a brief outburst of high-energy radiation detected by
the NASA Swift satellite. After observing the location of this outburst,
they quickly discovered they had not observed a normal gamma-ray burst (GRB):
Instead of the slow decay in brightness usually expected along with occasional
short episodes of re-brightening, the scientists observed sudden bright
flashes. The observations grew stranger still, when the activity hadn't ceased
the next night, but had in fact become stronger, not ceasing completely for
Artist's impression of the observed object
Illustration: A. Stefanescu, MPE
Examining the radiation emitted during an X-ray outburst, they found out
that a part of it had been absorbed by hydrogen gas on the way from the
object to Earth. After mapping the gas masses along the line of sight it
became clear that the object was most probably situated within our own Galaxy.
This meant it could not have been a normal GRB, because those usually do not
happen so close in our "next neighbourhood", but in distant galaxies.
The breakthrough was achieved, when the scientists put the special
characteristics of OPTIMA to good use. This high-speed photometer built by MPE
and mounted at the 1.3m teleskope of the Skinakas observatory on Crete (a
joint project of the University Crete, the Foundation for Research
and Technology -- Hellas, and the MPE)
is the only instrument worldwide combining high time resolution with triggering
on unexpected events. In the detectors of their system, the arrival time of each
individual photon is recorded - down to an accuracy of four millionths of a
second. This enables the scientists to reconstruct in detail how the brightness
of an object changes.
Detection of individual photons is common practice in the high-energy regime, but
OPTIMA is one of the few devices capable of doing this in the optical spectrum.
The fast and strong variability of the brightness of the object, observable only
with high time resolution, was instrumental in ruling out the initial hypothesis
that this was a GRB.
The unknown object was determined to be about a tenth the size of the Sun - but
at the same time almost a hundred times as bright. Assuming normal thermal radiation
as it is emitted e.g. by the Sun, extraordinarily high temperatures would be
necessary to explain this kind of luminosity. "So high, in fact, that it's hard to
see how an object of this size can heat up and then immediately cool down so quickly",
explains Stefanescu, member of the OPTIMA team and first author of the Nature
paper. "So the only possible conclusion was that we had observed a non-thermal
process: light that is not produced by heat as in a light-bulb or in a candle,
but e.g. by particles in a magnetic field."
The observation of short, bright flashes, continuing over several days, reminded
the scientists of non-thermal high-energy outbursts of so-called Soft Gamma Repeaters
(SGRs). Not only the shape, but also the statistical distribution of the brightness
of individual flashes, as well as a slight indication of periodic emission were
quite similar to what is observed in SGRs. Therefore the scientists conjectured that
the same type of object is involved as in SGRs: a magnetar. This hypothesis is
reinforced by a second Nature paper on multiwavelength observations of this source
by Alberto Castro-Tirado (Consejo Superior de Investigaciones Cientificas, IAA-CSIC,
Granada). Magnetars are a special type of neutron stars with an extraordinarily
powerful magnetic field.
Neutron stars form in a supernova, when a massive star collapses. If a newborn
neutron star rotates very quickly, its strong magnetic field can be amplified further
by a factor of 1000, the resulting field reaching 100 Gigatesla - more than a billion
times stronger than the strongest fields generated in labs on Earth. The field is so
strong that atoms in its vicinity are distorted into thin needles, and credit cards
would be erased even from the distance of the moon.
Changes in the configuration of the magnetic field during the first 10,000 years of
its existence exert forces of such strength on the crust of the magnetar that the
crust is heated up and can occasionally crack. The resulting star-quakes produce
those outbursts of high-energy radiation so similar to the optical outbursts observed
But what makes the presumable magnetar emit in the optical instead of in gamma-rays?
One possible theory is that highly charged ions are ripped out of the surface of the
magnetar and gyrate along the field lines. Since ions are much heavier than electrons,
they gyrate a lot slower, emitting electromagnetic radiation of much lower energy.
Most observations of magnetars have so far taken place in the high-energy range. "We
know 15 other magnetars, but up to now, no optical flashes of these have ever been
seen", says Stefanescu. "Accordingly the main efforts of theoreticians were made in
the high-energy regime. That's why we don't have an adequate theory against which
to compare the observations with OPTIMA." The next step of the scientists therefore
must be to study the consequences established magnetar theories predict for optical
Dr. Mona Clerico
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone +49 89 30000-3980
Max-Planck-Institut für extraterrestrische Physik
Phone: +49 89 30000-3853
Nature 455, 503-505 (25 September 2008)
"Editor's Summary" of the Nature issue that contains the paper
OPTIMA web pages at MPE
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