ASKAP discovers repeating outbursts from nearby star

The ASKAP Survey for Variables and Slow Transients (VAST) project team observed the flare star UV Ceti and discovered a repeating pattern in its activity, distinguishing these flares form the ones observed in our own Sun.  

We used ASKAP to observe the flare star UV Ceti, as test observations for the upcoming VAST Survey. For decades, this red dwarf has been well-known to produce extremely powerful bursts and flares across the whole electromagnetic spectrum -- to the extent that flare stars are also called "UV Ceti stars". Generally, the way that we have interpreted these flares has been informed by the behaviour of our Sun.

However, there have been recent hints that at low radio frequencies, the bursts from UV Ceti might not occur at random intervals like we'd expect from the Sun, but recur periodically with the rotation of the star. This sort of behaviour is seen on magnetised planets like Jupiter, and is known as auroral radio emission. Other objects known to produce this type of emission are Earth, and some of the lowest-mass stars called brown dwarfs. Despite these recent hints, auroral radio emission has never been confirmed to operate in an active flare star like UV Ceti.

At 10 and 10.5 hours long, our two observations were the longest continuous stares at UV Ceti with a radio interferometer to date. What we found was that rather than the radio bursts occurring at random intervals (like we would expect if it behaved like the Sun), the bursts repeat themselves every 5.44 hours, the rotational period of the star. This occurs because the emission is highly beamed, and these beams of radiation flash past the Earth like a lighthouse, causing the pulsed emission. This confirms recent suggestions that the radio bursts from UV Ceti are periodic, and shows that UV Ceti also operates in the auroral activity paradigm.

Animation showing 50 minutes of data around a bright pulse from UV Ceti, shown at image centre in crosshairs.

Some bursts showed a very uncommon type of polarisation called elliptical polarisation. This feature implies that the region of the star's atmosphere where the emission originates from is extremely rarefied and under-dense. The expected density of electrons at this region is about 100 million electrons per cubic centimetre, but the elliptical polarisation tells us that the density at the emission region must be less than 40 electrons per cubic centimetre! This implies that the emission originates from an extreme cavity inside the magnetosphere of UV Ceti. The most well-known analogue of this phenomenon is actually high above Earth's polar regions, where auroral radio emission originates from "auroral cavities". This is the first time that we have realised that this phenomenon can occur in the corona of a star.

Light-curves showing polarised intensity versus time for the ASKAP observations. The different types of polarisation are indicated with I, Q, U, and V, representing the four Stokes parameters. These light-curves show that the pulses repeat every 5.44 hours, which is the rotational period of the star. The relative longitude of UV Ceti is shown on the top axis of the light-curves.

All of these features confirm that auroral activity can operate in active flare stars, and suggests that these types of stars mark the transition from Solar-like behaviour toward planetary, auroral behaviour. This might make it easier to interpret the low-frequency radio bursts from active M-dwarfs, which have often been difficult to interpret in terms of Solar activity.

This work really highlights some of the key strengths of ASKAP. Our observations were taken at a frequency range usually completely compromised by mobile phones and other radio-frequency interference. ASKAP is at one of the most radio-quiet locations in the world, meaning that our observations were almost completely uncontaminated by interference. Detecting the elliptical polarisation was made much easier by the unique "roll axis" of ASKAP, which kept the receivers at a constant orientation with respect to the star. The good sensitivity, and broad bandwidth of ASKAP also greatly helped to obtain these results.

A paper describing this research is available  here.

Andrew Zic on behalf of the VAST Team.

Other
Access: 
Public