Not the bang they were looking for

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A satellite detects a bright burst of gamma-rays. Within minutes telescopes swing in to action expecting to see a massive star being torn apart by a cataclysmic explosion in a far-flung corner of the universe. But that wasn’t what they found……..

Gamma Ray Bursts (GRBs) are thought to be explosions of some of the most massive stars viewed at a fortuitous angle where we are blasted with a cone of high energy photons as the star itself implodes into a black hole. These happen in distant galaxies, billions of light years away.

There are huge observing campaigns to observe these blasts using quick reactions from a series of different telescopes. The sequence begins when a dedicated satellite detects a burst of gamma rays. It then locates the area where the burst is coming from and transmits the coordinates. These then trigger overrides where other telescopes stop what they are doing and swing to the position they are given, hoping to catch burst in other wavelengths of light before it fades away to nothing.

At 5:13 GMT on the 25th of April 2008 the Burst Alert Telescope on the Swift satellite detected a flood of gamma rays coming from the constellation of Lacertae. Within three minutes the other two instruments on Swift were taking data on the sources in X-rays and the ultraviolet. Twelve minutes after this the Liverpool Telescope on La Palma in the Canary Islands swung towards the target and took observations in visible light.

But quickly it became clear that this was no blast from a distant dying star, but something closer to home. The position of the source was one of a know nearby star EV Lac. Measurements showed it was six times brighter than its catalogued magnitude, so why was this star causing such an interstellar fuss?

A flare in the Solar atmosphere. Credit: milazinkova

EV Lac is a cool, red star 16 light years away, it’s also known to flare up every so often. Its a low mass star and has a very strong magnetic field. Higher mass stars like the Sun have weaker magnetic fields. The Sun can also have smaller outbursts of X-rays and gamma rays called solar flares (right). These are caused by the magnetic fields near the Sun’s surface rearranging themselves. This same thing happened in EV Lac, but because it has a much stronger magnetic field than the Sun the flare was much stronger. Even then this was a massive flare even for a star with a strong magnetic field. This flare was so bright that at one point 63% of the star’s energy output was in highly energetic X-rays. These factors combined with its relative proximity to the Earth lead to this stellar flare appearing as bright as the massive, distant bang the satellites were actually looking for.

Rachel A. Osten, Olivier Godet, Stephen Drake, Jack Tueller, Jay Cummings, Hans Krimm, John Pye, Valentin Pal’shin, Sergei Golenetskii, Fabio Reale, Samantha R. Oates, Mat J. Page, & Andrea Melandri (2010). The Mouse that Roared: A Superflare from the dMe Flare Star EV Lac
detected by Swift and Konus-Wind Accepted for publication in the Astrophysical Journal arXiv: 1007.5300v1

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