A star of about eight percent the Sun’s mass has been caught emitting an
enormous ‘super flare’ of X-rays – a dramatic high-energy eruption that poses a
fundamental problem for astronomers, who did not think it possible on stars
that small.
The culprit,
known by its catalogue number J0331-27, is a kind of star called an
L dwarf. This is a star with so little mass that it is only just above the
boundary of actually being a star. If it had any less mass, it would not
possess the internal conditions necessary to generate its own energy.
Astronomers
spotted the enormous X-ray flare in data recorded on 5 July 2008 by the
European Photon Imaging Camera (EPIC) onboard ESA’s XMM-Newton X-ray
observatory. In a matter of minutes, the tiny star released more than ten times
more energy of even the most intense flares suffered by the Sun.
Flares are
released when the magnetic field in a star’s atmosphere becomes unstable and
collapses into a simpler configuration. In the process, it releases a large
proportion of the energy that has been stored in it.
This explosive
release of energy creates a sudden brightening – the flare – and this is where
the new observations present their biggest puzzle.
“This is the
most interesting scientific part of the discovery, because we did not expect
L-dwarf stars to store enough energy in their magnetic fields to give rise to
such outbursts,” says Beate Stelzer, Institut für Astronomie und Astrophysik
Tübingen, Germany, and INAF – Osservatorio Astronomico di Palermo, Italy, who
was part of the study team.
Energy can only
be placed in a star’s magnetic field by charged particles, which are also known
as ionised material and created in high-temperature environments. As an
L dwarf, however, J0331-27 has a low surface temperature for a star – just
2100K compared to the roughly 6000K on the Sun. Astronomers did not think such
a low temperature would be capable of generating enough charged particles to
feed so much energy into the magnetic field. So the conundrum is: how a super
flare is even possible on such a star?
“That’s a good
question,” says Beate. “We just don’t know – nobody knows.”
Artist’s
impression of an L dwarf star, a star with so little mass that it is only
just above the boundary of actually being a star, caught in the act of emitting
an enormous ‘super flare’ of X-rays, as detected by ESA’s XMM-Newton X-ray
space observatory.
The super flare was discovered in
the XMM-Newton data archive as part of a large research project led by Andrea
De Luca of INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica in Milan,
Italy. The project studied the temporal variability of around 400 000
sources detected by XMM-Newton over 13 years
Andrea and collaborators were
particularly looking for peculiar phenomena and in J0331-27 they certainly got
that. A number of similar stars had been seen to emit super flares in the
optical part of the spectrum, but this is the first unambiguous detection of
such an eruption at X-ray wavelengths.
The wavelength is significant
because it signals which part of the atmosphere the super flare is coming from:
optical light comes from deeper in the star’s atmosphere, near its visible
surface, whereas X-rays come from higher up in the atmosphere.
Understanding the similarities and
differences between this new – and so far unique – super flare on the
L dwarf and previously observed flares, detected at all wavelengths on
stars of higher mass is now a priority for the team. But to do that, they need
to find more examples.
“There is still much to be
discovered in the XMM-Newton archive,” says Andrea. “In a sense, I think this
is only the tip of the iceberg.”
One clue they do have is that there
is only one flare from J0331-27 in the data, despite XMM-Newton having observed
the star for a total of 3.5 million seconds – about 40 days. This is peculiar
because other flaring stars tend to suffer from numerous smaller flares too.
“The data seem to imply that it
takes an L dwarf longer to build up the energy, and then there is one sudden
big release,” says Beate.
Stars that flare more frequently
release less energy each time, while this L dwarf seems to release energy
very rarely but then in a really big event. Why this might be the case is still
an open question that needs further investigation.
“The discovery of this L dwarf
super flare is a great example of research based on the XMM-Newton archive,
demonstrating the mission’s enormous scientific potential,” says Norbert
Schartel, XMM-Newton project scientist for ESA. “I look forward to the next
surprise.”
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