Scraps of brightest exploding stars stretch over time

Some of the biggest and brightest exploding stars don’t keep a spherical shape, new observations show, but may periodically stretch into a hot dog bun shape.

Cosimo Inserra from Queen’s University Belfast in the UK and colleagues measured polarised light, which gives information about asymmetries of the source, emanating from the superluminous supernova 2015bn. They found it changed shape over the course of a couple of months, pulling from a ball into an ellipsoid after peak brightness.

The work, published in The Astrophysical Journal, provides another insight into the lifecycle of these strange cosmic objects.

Supernovae are produced when a star in its death throes and collapses on itself, blasting a shell of material away from a black hole or a dense, spinning object with an immense magnetic field called a magnetar left in the centre.

Superluminous supernovae, as their name suggests, are particularly bright – but they’re mysterious.

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While they explode with billions of times the energy of the sun – and last longer than a typical supernova, stretching months instead of weeks – astronomers have only known of their existence for the past six years or so. 

One of the closest superluminous supernovae – SN 2015bn – is fading in visible light, but undulating in the ultraviolet part of the spectrum. This, astronomers think, is the result of a magnetar reheating material around it, which results in a burst of ejecta every 30 to 50 days.

But while it was ramping up to peak brightness, Cosimo and his colleagues trained a spectrograph on Chile’s Very Large Telescope on SN 2015bn to detect polarised light.

Where unpolarised light waves move in, say, horizontal and vertical planes, polarised light moves in a single plane. Measuring polarised light – called polarimetry – and analysing it with come nifty calculations can give astronomers the rough shape of an object, such as the layers of supernova ejecta.

The best fitting model comprised two layers of ejecta. Some 24 days before peak brightness, SN 2015bn’s outside ejecta layer was the same shape as the inner – like a soccer ball inside a basketball.

But 28 days after the brightness started waning, more polarised light intimated that the inner ejecta had morphed into an ellipsoid while the outer later stayed roughly spherical – like a small rounded Australian football in a basketball.

So what does this mean?

The axisymmetric shape, the researchers write, is in line with a core-collapse explosion. A central inner engine of a magnetar or black hole pumps energy into the layers, causing the asymmetry over time.

As to whether the shape is typical for a superluminous supernova or not is unknown. More observations and detailed modelling of other superluminous supernovae – and time – will tell.