Supernova explodes in controversy
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 28, 2008

The interpretation of supernova explosion SN 2008D, which was captured by a serendipitous Swift telescope observation earlier this year, is causing controversy among different research groups who argue its origin and evolution as either a ‘normal’ supernova, or something more reminiscent of a gamma-ray burst.

While Swift was monitoring a supernova explosion in galaxy NGC 2770 in January this year, it captured a burst of X-rays coming from a different location within the same galaxy, making it the first observation of the earliest stages in the process of supernova explosions (you can read Astronomy Now's report of the discovery here). The supernova has since been well-monitored by a number of independent researchers who agree that a burst of X-rays dominated the initial observations. One group however, lead by Alicia Soderberg of Princeton University who first discovered the supernova, interpret this X-ray fingerprint as a ‘shock break out’ of light that is said to be expected at the onset of every supernova event, while a new paper which appeared online in Science Express last week, lead by Paolo Mazzali of the Italian National Institute for Astrophysics and the Max-Planck Institute for Astrophysics, proposes the event as more typical of the higher energy supernovae that host gamma-ray bursts.

Scientists studying supernova 2007uy in galaxy NGC2770, which was already several weeks old when seen in this visual, ultraviolet image (top left) and X-ray image (bottom left), also spotted a new supernova event, now known as SN 2008d (right hand images) exploding in front of their eyes. Image: NASA Swift team.

It is well understood that supernovae are the result of a massive stellar explosion brought about by the gravitational collapse of a star greater than about eight solar masses. Theory dictates that as the star’s core squeezes itself into higher and higher densities, a shock wave is formed that races outwards, destroying the star. The star’s last cry of agony is seen as a sudden pulse of X-rays or gammma-rays that is followed by the creation of either a neutron star or a black hole.

“This is the standard text book explanation for supernova explosions, and fully consistent with our data,” says Soderberg, who used radio observations to probe the very fastest material – moving at about 20 precent the speed of light – produced in the onset of the explosion of SN 2008D. “We clearly show that the X-ray outburst is naturally explained as shock break out light from the supernova, something that has long been expected from every ordinary supernova but only detectable if you are observing the star at the exact moment of explosion.”

But Mazzali and colleagues, who set up an observational campaign within 10 hours of the first alert of supernova activity, think that the supernova’s early behaviour is more energetic than Soderberg’s results imply, and question the ‘normality’ of the supernova. “A shock breakout interpretation is tempting,” Mazzali told Astronomy Now, “but a solution based on this assumption requires that the shock is seen as it emerges not from the star but from a blanket of material that surrounds the star, which may not be unreasonable, but would it be normal?”

‘Normal’ supernovae generate around 10 to the power of 51 ergs of energy (or 10 to the power of 44 Joules) while gamma-ray bursts produce 10 to 50 times that. The two research groups have used their own favoured models to estimate the energy output from SN 2008D and came up with subtly different results: Soderberg derive an energy of 3-4 times that of the ‘normal’ energy range, while Mazalli’s team get 7 times that amount. In the grand scheme of things, the results are actually quite similar, but at this high energy and so close to the lower limit of a gamma-ray burst event, Mazzali et al argue that the supernova was far too energetic to be considered as a normal supernova. Instead, they suggest that the X-ray flash was the signature of a mildly relativistic jet brought about by the collapse of the star into a black hole.

Artist impression of a dying, massive star. Twin beams of matter and energy have broken out of the star's collapsing shell, which could be in the form of energetic gamma-ray bursts. Image: NASA E/PO, Sonoma State University, Aurore Simonnet.

Mazalli's theoretical models also suggest 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, which would likely result in the formation of a black hole. Moreover, the group observed that the original star had shed much of its outer layers of hydrogen before exploding, but that the helium layers lingered in the aftermath, indicating that the progenitor star was not stripped as deeply as supernovae associated with typical gamma-ray bursts.

“SN 2008D is a type of supernova where the star’s hydrogen envelope has been lost, but a typical core-collapse supernova has the hydrogen envelope in place,” comments Mazzali.

But Soderberg’s observations suggest that the supernova is evolving just like other core-collapse supernova, and specifically very similar to other similarly classified supernova events. “It shows no indication of being associated with anything unusual or 'special' like a gamma-ray burst jet,” she tells Astronomy Now. “While at first blush our conclusion may appear less exciting, in fact our conclusion is the most exciting since it implies that every supernova can be detected in X-rays thanks to their shock break out light and not just the 0.1 percent associated with a powerful gamma-ray burst.”

While Mazzali et al agree that the energy imparted by the supernova explosion is much less than typical gamma-ray bursts, they suggest that gamma-ray burst-like ‘inner engine activity’ exists in all black hole forming supernova, and

SN 2008D should be treated as a borderline event that displays only weak X-ray jets because the collapsing mass is relatively small and the star's lingering helium layer dampens the jet.

“Reality is that there is probably a continuum of event properties,” says Mazzali. “We have been uncovering a very diversified zoo of supernova, ranging from very low to very high energies, with different masses generating known different outcomes like a neutron star or a black hole. In a nutshell, nature does not come in black and white.”

No doubt the debate into what constitutes a ‘normal’ supernova will continue, but despite the controversy, the discovery and continued assessment of SN 2008D is extremely important, allowing astronomers to study the end point of a star’s life and its subsequent metamorphosis in unprecedented detail.