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XMM-Newton reveals giant flare from a tiny star

(20 February 2020 - ESA) 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.

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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 (courtesy: ESA)

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.”

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.”

Publication

“EXTraS discovery of an X-ray superflare from an L dwarf” by A. De Luca et al. 2020 is published in Astronomy & Astrophysics.