A Swift glance at red dwarfs

November 20, 2014 is a huge day for NASA’s Swift spacecraft, as it marks the tenth anniversary of its launch. Currently orbiting our planet, Swift is scanning the skies for potential sources of events known as Gamma-ray bursts (GRB’s). Each burst is a huge, but relatively brief, flash of very high energy radiation coming from interstellar space. Astronomers believe they happen fairly frequently (we detect around one per day according to NASA), and last from a few milliseconds up to a few minutes. Swift’s role is to detect these events using the Burst Alert Telescope (BAT), in order to find out what exactly is causing them. This telescope has a very wide angle lens, which can see about a sixth of the sky at any one time – about the same as one of our eyes can see - and scans around 88% of the entire sky each day^[1].

Gamma-ray bursts can happen at any moment, so Dr Kim Page and her team at the UK Swift Science data centre (UKSSDC) based in the University of Leicester take turns to be ‘on-call’ for when BAT is triggered. There are two ways BAT can be provoked into action; by a quick rapid spike in high-energy waves or by a cumulative increase in a particular part of the sky over a longer period of time. These two mechanisms are known as the “rate trigger” and “image trigger” respectively, named for the specific way in which they pick up radiation. If the telescope is triggered it sends a message to the various cameras onboard Swift to “switch on”, which then turn and face the area of sky from where the original signal was first detected. At the same time, it sends an automated SMS to the on-call team members at the UKSSDC, whilst also writing each team member an email containing the relevant data about the location, time and intensity of the trigger source. The X-ray, UV and optical cameras on Swift can then investigate the radiation burst further. The Swift team have found this to be extremely effective and usually the phenomenal increase in energy has a Gamma-ray burst at the centre. But, as we will see, this is not always the case.

The Swift spacecraft has been orbiting the Earth since 2004, collecting data about the locations of over 800 gamma-ray bursts; extremely high energy events which occur deep in space. From understanding more about the causes and nature of GRB’s, astronomers hope to understand the early universe better. Image credit: NASA E/PO, Sonoma State University/Aurore Simonnet

Occasionally, an event which isn’t a GRB will cause the detectors to swing round and peer into the depths of space. On April 23, 2014, BAT picked up a huge influx of energy coming from a small constellation known as Canes Venatici (part of the Ursa Major group). The image trigger alerted Dr Page with a text message at around 10pm, telling her and her team that there was a potential GRB in this constellation. Within two minutes the Swift cameras had collected as much data about the position of the source as they could. This data was then cross-referenced with catalogues of stars and galaxies, to see if this patch of sky had produced GRB candidates on previous occasions - making it more likely to be a “false positive”. As a matter of fact it had – and the team found that the patch of sky they were looking at contained a binary red dwarf system, about 60 light years away. So, by then we knew what was setting BAT off, says Dr Page, Red dwarf stars are well known for their highly energetic flares. But the fact that this system could be expected to produce powerful flares didn’t prepare Dr Page and her team for the sheer enormity of the flares they were seeing this time.

Red Dwarf stars have a lower mass than our Sun, and tend to rotate much faster. This means they have greater stellar activity because the higher rotational speeds cause the magnetic field lines to get tangled and twisted. The charged particles which make up stars follows these magnetic field lines and so hot plasma material can be thrown out into space when a flare event takes place. Image credit: NASA/Walt Feimer

The first flare which originally caught the spacecraft’s attention was incredibly powerful and lasted almost three weeks, shattering all preconceptions about flares being able to last no longer than a couple of days at most. The high energy radiation thrown out by this first event was similar to the kind of energy we would expect to see from gamma-ray burst events. So at first glance it could be easy to mistake one for the other, despite them being entirely different events caused by completely different things. While the mechanism by which a flare occurs is quite well understood (or so we believe), flares of this size had never before been catalogued, so there could be more going on here than we think.

Dr Page added, When something’s generally not understood in space, we tend to blame magnetism! But in this case, we’re pretty sure the magnetic field [of the red dwarf binary system] is the cause.

Stars are huge balls of ionized gas which gets heated and burned in a series of complicated chemical reactions. This ball of gas rotates on its axis, just like the Earth does, however because each particle of gas has a charge there is an intense magnetic field strength. Due to differing speeds from the poles to the equator, the magnetic field gets bunched up in places and stretched out in others. The tangled magnetic field lines twist together, capturing huge amounts of energy. Eventually they snap back into their normal position, releasing the stored energy and radiation which has been pent up in the twisting of the magnetic field lines. The overall velocity of the star’s rotation would also have an effect. Red dwarf stars are smaller than stars like our Sun, which means that in general they tend to rotate a lot faster.

This image taken by the Solar Dynamics Observatory shows an impressive and beautiful solar flare which was caused by the twisting of the Sun’s magnetic field. This one is actually a prominence; both ends are anchored to the surface of the Sun. Were this prominence to snap, material would be ejected out with a powerful flare. This magnificent event is tame compared to the flares spotted by Swift earlier this year. Image credit: NASA/SDO/AIA

There could be some clues about exactly how flares are caused by observing the pattern of ‘shockwave’ flares observed. Possibly the magnetic field bouncing back into place, but perhaps our original picture of what exactly causes these energetic outbursts is missing a piece. Can our star, the Sun, produce the same kind of energetic flare? What would happen if it did? Luckily for us, according to Dr Page the chances of this kind of event happening to the Sun remain very low: Our Sun is simply not rotating fast enough to cause this kind of event. But there is research to be done on how that red dwarf system can. Our Sun was once a red dwarf star before settling into the middle-aged yellow star we see today. Further research into this area could open up a whole world of understanding about the history of the Sun, and the current situation of many red dwarf systems further out in space.

Swift currently detects around 90 Gamma-ray bursts each year, but could it be utilised to discover more about other high-energy triggers? Who knows what kind of phenomena BAT will pick up. The mission has already been extended more than once, and it is hoped to continue further as the data this mission has produced has been invaluable to research.

This article was written by our Physics Editor, Grace Mason-Jarrett. With thanks to Dr Kim Page, for her time, support and answers to our questions.

References
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[1] H. A. Krimm et al. "THE SWIFT/BAT HARD X-RAY TRANSIENT MONITOR" ApJS 209 14. DOI: 10.1088/0067-0049/209/1/14