Exploding star not as normal as initially thought

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Munich, July 25 : Astronomers have indicated that a recent supernova may not be normal as initially thought, with the star that exploded now understood to have collapsed into a black hole, producing a weak jet, typical of much more violent events, the so-called gamma-ray bursts.

The object, SN 2008D, is thus probably among the weakest explosions that produce very fast moving jets.

This discovery represents a crucial milestone in the understanding of the most violent phenomena observed in the Universe.

Stars that were at birth more massive than about 8 times the mass of our Sun end their relatively short life in a cosmic, cataclysmic firework lighting up the Universe.

The outcome is the formation of the densest objects that exist, neutron stars and black holes.

When exploding, some of the most massive stars emit a short cry of agony, in the form of a burst of very energetic light, X- or gamma-rays.

On 9th January 2008, the NASA/STFC/ASI Swift telescope discovered serendipitously a 5-minute long burst of X-rays coming from within the spiral galaxy NGC 2770, located 90 million light-years away towards the Lynx constellation.

"What made this event very interesting is that the X-ray signal was very weak and 'soft', very different from a gamma-ray burst and more in line with what is expected from a normal supernova," said Paolo Mazzali of INAF's Padova Observatory and MPA.

But, Mazzali and his team think otherwise.

"Our observations and modeling show this to be a rather unusual event, to be better understood in terms of an object lying at the boundary between normal supernovae and gamma-ray bursts," he said.

"These are supernovae produced by stars that have lost their hydrogen and helium-rich outermost layers before exploding, and are the only type of supernovae which are associated with (long) gamma-ray bursts," explained Mazzali.

Over the years, Mazzali and his group have developed theoretical models to analyse the properties of supernovae.

When applied to SN2008D, their models indicated that the progenitor star was at birth as massive as 30 times the Sun, but had lost so much mass that at the time of the explosion the star had a mass of only 8-10 solar masses.

The likely result of the collapse of such a massive star is a black hole.

According to Massimo Della Valle, co-author, "Since the masses and energies involved are smaller than in every known gamma-ray burst related supernova, we think that the collapse of the star gave rise to a weak jet, and that the presence of the Helium layer made it even more difficult for the jet to remain collimated, so that when it emerged from the stellar surface the signal was weak."


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