Supernovae, the massive explosions that mark the death of a star, are a headline event in astronomy, so for one of these to play an encore is particularly exciting. That's just what scientists have witnessed from a superluminous supernova (SLSN) that appears to have exploded twice in quick succession. The discovery is shedding new light on the mysterious SLSN phenomenon.

Most supernovae are the result of stars exhausting their supply of fuel, which causes their centers to collapse and expel the outer material into space. Type Ia supernovae occur in binary systems, when a white dwarf absorbs mass from its twin star and grows unstable, triggering a thermonuclear explosion. The intense burst of light given off can be seen in the sky for weeks afterwards, before it gradually burns out.

Superluminous supernovae, as their name suggests, are far bigger, and their aftermath can be seen for up to six months. However, since only a dozen or so have been discovered, they're still quite mysterious. Using the Gran Telescopio CANARIAS (GTC) in Spain, researchers have recently been able to learn more by observing an SLSN in action, and their observations have been surprising: the supernova appears to have exploded twice, several weeks apart. Plotted out, the brightness shows a clear initial spike, before dipping and then brightening again during the main sequence of the supernova.

"What we have managed to observe, which is completely new, is that before the major explosion there is a shorter, less luminous outburst, which we can pick out because it is followed by a dip in the light curve, and which lasts just a few days," says Mathew Smith, the lead researcher on the study. "From our data we have tried to determine if this is a characteristic unique to this object, or whether it is a common feature of all superluminous supernovas, but has not been observed before, which is perfectly possible given their unpredictable nature."

Using data gathered by the GTC and other telescopes, the team was able to plot out the timeline of the brightness of the object, which has since been given the name "DES14X3taz", and determine that it lies about 6.4 billion light years from Earth. The researchers conclude that the most likely explanation is that they witnessed the birth of a magnetar, a neutron star that has a particularly strong magnetic field and rapid rotation.

"We think that a very massive star, some 200 times the mass of the Sun, collapses to form a magnetar," explains Smith. "In the process the first explosion occurs, which expels into space a quantity of matter equivalent to the mass of our Sun, and this gives rise to the first peak of the graph. The second peak occurs when the star collapses to form the magnetar, which is a very dense object rotating rapidly on its axis, and which heats up the matter expelled from the first explosion. This heating is what generates the second peak in the luminosity."

The team at the GTC was pointed in the right direction by the Dark Energy Survey, which identified the object as a potential point of interest in December 2014. The GTC then made its observations over two nights, one in January and one in February, the following year.

As has been done with type Ia supernovae, the scientists hope that studying the origin and nature of SLSN will lead to a "standardization" of their properties, which would allow them to serve as a reference for measuring distances on larger scales and with greater accuracy than is possible currently.

The research was published in The Astrophysical Journal Letters.

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