Subaru sees new light from Tycho’s 16th century supernova
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 04 December, 2008

Astronomers using the Subaru Telescope have observed light echoes from Tycho’s Supernova Remnant that provide new insight into the exploding star’s origin and type.

A new star was spotted in the 16th century night sky by Tycho Brahe, or so he thought, for the ‘star’, which outshined even Venus, was actually a rare supernova event, whereby a star explodes at the end of its life, emitting an extremely bright outburst of energy. Tycho studied the brightness and colour of the ‘star’ for several months until it faded from view and the remains of this milestone event can still be observed today, and is known as Tycho's Supernova Remnant.

Wide Field Image of Tycho's Supernova Remnant. It is a colour composite of Mid-Infrared by Spitzer Space Telescope (red), and X-ray (blue: high-energy X-ray, green: middle energy, yellow: low-energy) by Chandra X-Ray Observatory. The remnant is approximately 25 ly in diameter. Image: Subaru Telescope.

Using the Subaru Telescope, an international team of 21st century astronomers picked up the supernova’s light echoes. That is, light from the original supernova event that was bouncing off dust particles in surrounding interstellar clouds many years after the direct light passed us by, revealing the supernova’s origin and exact type. The same team used similar methods to uncover the origin of supernova remnant Cassiopeia A in 2007.

"Using light echoes in supernova remnants is time-travelling in a way, in that it allows us to go back hundreds of years to observe the first light from a supernova event,” says Dr Tomonori Usuda, Associate Director for the Subaru Telescope. “We got to relive a significant historical moment and see it as famed astronomer Tycho Brahe did hundreds of years ago. More importantly, we get to see how a supernova in our own Galaxy behaves from its origin."

Such a light echo is typically visible for a couple of weeks, similar to the length of the brightness maximum of the original supernova. “However, as the light flash propagates through interstellar clouds, new echoes are generated in the vicinity of the older, fading ones,” says Dr Oliver Krause of the Max Planck Institute for Astronomy in Berlin. “This happens because the interstellar matter is organised in denser condensations (and those stand out as the brightest echoes) embedded in more extended and diffuse clouds.”

The view of the light echoes from Tycho’s supernova. Optical light arrived at Earth in 1572 (blue arrow). Light was scattered by dust clouds around the supernova in 2008 (yellow arrows). Since the emitting regions were apparently shifted from 23 August 2008 to September 24, the optical lights were confirmed as light echoes. Image: Subaru Telescope.

Using the Faint Object Camera and Spectrograph (FOCAS) instrument the team broke the light echoes down into spectra – the signatures of the atoms present when the supernova exploded, which bear all the information about the nature of the original blast. The results showed clear absorption of once-ionized silicon and an absence of hydrogen H-alpha emission, findings typical of a Type Ia supernova observed at maximum brightness of its outburst.

The typical source of a Type Ia supernova is a white dwarf star in a close binary system. As the gas of the companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst. However, Type Ia supernovae with luminosity both brighter and fainter than ‘normal’ have recently been reported, resulting in the understanding of supernova outburst mechanisms coming under debate.

In order to explain the diversity of the Type Ia supernovae, the Subaru team studied the outburst mechanisms in more detail, and discovered that the Tycho Supernova Remnant is asymmetric. “An asymmetric explosion could be the effect of the detonation itself (how the flame propagates) or it could be already triggered by the asymmetric mass transfer from a companion star in a Ia explosion,” Krause tells Astronomy Now. The characteristics of this explosion resembled closely that of the known aspherical explosion of SN 2001e1, which lead the team to the conclusion that Tycho’s supernova also exploded asymmetrically. “The jury is still out about the exact explosion mechanism of type Ia Supernovae,” adds Krause. “It could be triggered by mass accretion onto a white dwarf or the merging of two white dwarfs. The two possibilities might be reflected in a luminosity difference.”

Follow-up comparisons with template spectra of Type Ia supernovae found outside our Galaxy confirm Tycho's classification as a Normal Type Ia, and as such, is now the first confirmed and precisely classified supernova in our Galaxy. Type Ia supernovae play an important role as cosmological distance indicators, serving as 'standard candles' because the level of the luminosity is always the same for this type of supernova.

This observational study at Subaru established how light echoes can be used in a spectroscopic manner to study supernovae outburst that occurred hundreds of years ago. The light echoes, when observed at different position angles from the source, enabled the team to look at the supernova in a three dimensional view. For the future, this 3D aspect will accelerate the study of the outburst mechanism of supernova based on their spatial structure, which, to date, has been impossible with distant supernovae in galaxies outside the Milky Way.

“Extragalactic supernovae are so distant that details of the progenitor stars are hidden,” says Krause. “That's the reason why observations of historic Galactic supernovae with the help of light echoes become so important: Supernovae in our own Galaxy are so close that details about their progenitors can in principle still be revealed. And using light echo spectroscopy these details on the progenitors can now be compared with signatures of the explosion itself - although it happened long ago.”