Weekend Feature: New Discovery About the Supernova of 1572

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In this weekend's image, the red circle visible in the upper left is SN 1572, often called “Tycho’s Supernova”. In the center, is a star forming nebula of dust and gas about 3,500 light years away and 35 light-years across. It is being heated by radiation from young hot stars within it, and the dust within the cloud radiates infrared light.


The sudden appearance in the Milky Way of Tycho's Supernova -a type Ia explosion in which a white dwarf star has accreted matter from a companion until it reaches the Chandrasekhar limit and explodes- is thought to perhaps one of the two or three most important events in the history of astronomy, helping to revise ancient models of the cosmos and to inaugurate a revolution in astronomy that began with the appearance of better astrometric star catalogues.

Now, X-ray stripes (shonw below) , which have never been seen before in any supernova remnant, have been spotted lurking in X-ray observations of its high-energy blast wave. Expanding debris from the explosion shown in image below shines in low-energy (red) X-rays, while the blast wave, a shell of energetic electrons, shines in high-energy (blue) X-rays.

The stripes are believed  to be regions where magnetic fields in the blast wave are more tangled than in surrounding areas as electrons spiral around the magnetic field lines that corresponds in size to particles at energies about 100 times as high as those produced at the Large Hadron Collider, according to Kristoffer Eriksen of Rutgers University, and colleagues, whose study suggests that supernova remnants can account for some of the high-energy particles called cosmic rays that bombard Earth from space.

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"The question of what causes a Type Ia supernova is one of the great unsolved mysteries in astronomy," according to Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA).

A massive supernova variety – Type Ia – brightens and dims so predictably that astronomers use them to measure the universe's expansion -called a "standard candle.". The resulting discovery of dark energy and the accelerating universe rewrote our understanding of the cosmos. Yet theorigin of these supernovae, which have proved so useful, remains unknown.

Astronomers have very strong evidence that Type Ia supernovae come from exploding stellar remnants called white dwarfs. To detonate, the white dwarf must gain mass until it reaches a tipping point and can no longer support itself.

There are two leading scenarios for the intermediate step from stable white dwarf to supernova, both of which require a companion star. In the first possibility, a white dwarf swallows gas blowing from a neighboring giant star. In the second possibility, two white dwarfs collide and merge. To establish which option is correct (or at least more common), astronomerslook for evidence of these binary systems.

Given the average rate of supernovae, scientists can estimate how many pre-supernova white dwarfs should exist in a galaxy. But the search forthese progenitors has turned up mostly empty-handed.

To hunt for accreting white dwarfs, astronomers looked for X-rays of a particular energy, produced when gas hitting the star's surface undergoes nuclear fusion. A typical galaxy should contain hundreds of such "super-soft" X-ray sources. Instead we see only a handful. As a result, a recent paper suggested that the alternative, merger scenario was the source of Type Ia supernovae, at least in many galaxies.

That conclusion relies on the assumption that accreting white dwarfs will appear as super-soft X-ray sources when the incoming matter experiences nuclear fusion. Di Stefano and her colleagues have argued that the data do not support this hypothesis.

Di Stefano points out that a merger-induced supernova would also be preceded by an epoch during which a white dwarf accretes matter that should undergo nuclear fusion. White dwarfs are produced when stars age, and different stars age at different rates. Any close double white-dwarf system will pass through a phase in which the first-formed white dwarf gains and burns matter from its slower-aging companion. If these white dwarfs produce X-rays, then we should find roughly a hundred times as many super-soft X-ray sources as we do.

Since both scenarios – an accretion-driven explosion and a merger-driven explosion – involve accretion and fusion at some point, the lack of super-soft X-ray sources would seem to rule out both types of progenitor. The alternative proposed by Di Stefano is that the white dwarfs are not luminous at X-ray wavelengths for long stretches of time. Perhaps material surrounding a white dwarf can absorb X-rays, or accreting white dwarfs might emit most of their energy at other wavelengths.

If this is the correct explanation, says Di Stefano, "we must devise new methods to search for the elusive progenitors of Type Ia supernovae."

The Daily Galaxy via The Astrophysical Journal Letters and newscientist.com

Image credit: X-ray: NASA/CXC/Rutgers/K.Eriksen et al.; Optical: DSS and NASA's Wide-field Infrared Survey Explorer (image top of page)

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