Star light, star brightest:
the supernova of A.D. 1006

• by Francis Reddy

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Unusual afterglow

Only a few shreds of gas (upper right) suggest what happened here in 1006. The supernova remnant, which is about half a degree wide, occupies the entire field of view. Credit: Middlebury College/NOAA/AURA/NSF Click for full image. Click for a close-up.

ASCA x-ray spectra showed that most of the shell is filled with million-degree plasma. Intense emission in the bright cusps is nonthermal and probably synchrotron radiation. The scale is the same as the above image. Eric V. Gotthelf, USRA/GSFC/NASA Click for full image.

The ROSAT PSPC image shows that soft x-rays fill the shell, but more energetic radiation (blue) arises from limb features in the northeast and southwest. of Leicester, X-ray Astronomy Group Click for full image.

Astronomers were unable to locate the site of the explosion until 1965, when surveys at radio wavelengths turned up a source with all the makings of a young supernova remnant. The object is half a degree across, or about the apparent diameter of a full moon, and resembles a bright ring. This phenomenon, called limb brightening, occurs because our line of sight intersects more hot gas along the edge of the shell than it does through the middle. The ring is broken into two bright arcs of radio emission, a feature common to many shell-type supernova remnants. Some astronomers suspect that asymmetries in the interstellar environment — and even in the explosion itself — have something to do with it. " The orientation of the interstellar magnetic field probably plays a key role here, too" Winkler said, " but we're not quite sure."

Observations at x-ray wavelengths reveal that million-degree gas fills the entire shell. But, as with the radio emission, most of the x-rays come from two arcs in the northeast and southwest. In the mid-1990s, instruments on the Roentgen Satellite (ROSAT) and the Advanced Satellite for Cosmology and Astrophysics (ASCA) glimpsed some of the remarkable processes occurring there. Speeding electrons produce a characteristic radiation called synchrotron emission when they spiral through intense magnetic fields, such as those associated with a supernova shock wave. Satellite observations of the bright arcs in the 1006 remnant revealed that electrons were being accelerated to energies of more than 200 trillion electron volts. For comparison, that's about a billion times greater than the energy of medical x-rays. Such dramatic particle acceleration indirectly supports the long-held view that supernova shocks can produce the hyper fast ions known as cosmic rays.

The remnant looks very different at optical wavelengths. In fact, the first report of visible nebulosity followed the discovery of the radio source by no less than a decade. A few delicate wisps of gas, prominent only in the red light characteristic of hydrogen, occur along the shell's northwestern edge, where the radio emission is weakest. And that appeared to be it.

In the 1990s, Frank Winkler teamed up with Knox Long from the Space Telescope Science Institute to investigate the remnant's optical properties. " Young supernova remnants provide us with an opportunity to learn about an exploded star before its debris becomes hopelessly mixed up with the interstellar medium," Winkler said. Through a series of observations at the Cerro Tololo Inter-American Observatory in Chile, they discovered additional arcs of gas and a shell of faintly glowing hydrogen surrounding the site where the star exploded. The shell defined by the faintest optical emission forms an almost perfect circle, flattened slightly in the northwest where the brightest filaments have formed. This is probably where the shock wave is encountering interstellar gas of slightly greater density.

Even the spectrum of the remnant looks peculiar. The visible nebulosity of most supernova remnants displays a spectrum consisting of light emissions characteristic of a number of elements: oxygen, nitrogen, sulfur, as well as hydrogen. Yet the 1006 remnant glows almost exclusively in the so-called Balmer lines of hydrogen. In fact, it was only the second such remnant to be identified the first was the one associated with the new star studied by Tycho Brahe in 1572. So what's going on there?

The first element in understanding the 1006 remnant is the shock wave: a hot soup of electrons and ions and their associated magnetic field. This plasma is much thinner than the mean free path of a particle moving through it — that is, particles overrun by the shock only rarely collide with the particles that make it up. Since the shock wave is " collisionless" and cannot propagate by mechanical transmission, something else must sustain it: the collective electrical and magnetic properties of the plasma itself. Electrically charged gases in a magnetic field can move only in spiral paths, so the plasma's fast-moving magnetic field ensnares and energizes any ions and electrons it encounters. " The plasma is responsible for the magnetic field and it's the magnetic field that creates the shock wave," Winkler said.

Particles may be repeatedly forced across the shock, gaining energy with each passing. Astrophysicists suspect that this process, called Fermi acceleration, leads to the creation of high-energy cosmic rays in supernova remnants. Since the Fermi process energizes particles more efficiently where the shock wave moves roughly parallel to the interstellar magnetic field, astronomers think it may play a significant role in creating the remnant's broken-ring appearance.

The other aspect to figuring out the 1006 remnant is the nature of the interstellar gas outside the shock. It consists primarily of atomic hydrogen at a temperature of about 100 Kelvins. This cool neutral gas passes unaffected through the tangled electromagnetic environment at the shock, but once inside it encounters temperatures of tens of millions of degrees. Collisions with high-energy electrons and protons ionize the neutral gas and eventually dissociate it completely. The gas emits well-defined hydrogen Balmer lines when energized by electron strikes. Less frequently, racing protons collide with the atoms and the two exchange electrons, a process that manifests itself as faint " wings" added to the sharp spectral lines.

What makes the Balmer-dominated supernova remnants such uninteresting objects at optical wavelengths is the paucity of interstellar matter around them. The region beyond the 1006 remnant may hold as few as two atoms of neutral hydrogen in a five-cubic-centimeter volume of space. In the vicinity of bright remnants like IC 443, Puppis A, or the familiar Cygnus Loop, the gas density is at least a thousand times higher. When a supernova shock wave encounters dense clouds of interstellar gas, it engulfs them and triggers a secondary shock that works its way through the cloud. This heats and ionizes the gases, which cool by radiating the energy we see.

" Balmer-line filaments can only occur right at the shock front," Winkler notes, " because once the shock wave passes through the gas quickly dissociates and no longer emits light." In fact, most of the supernova remnants astronomers know about lack any substantial visible component. Some of that is due to absorption of light by the interstellar medium, but it's clear that the majority of supernovas leave little in the way of a lasting visual imprint on the sky. In a curious turn, the 1006 remnant is more typical of supernova remnants than the shreds of bright nebulosity we've come to expect from images of the Vela or Cygnus Loops. Faint Balmer emission has been detected around the Cygnus Loop and the remnant of Kepler's supernova, but it always encloses the visible nebulosity.

A question of distance

Winkler and Long compared their optical images of the 1006 remnant to radio maps and ROSAT x-ray data. Structural details correspond well in both wavelengths, which lends support to the idea that the strongest emission stems from shock-accelerated electrons. The peak of x-ray emission occurs just eight arcseconds inside the brightest of the visible filaments in the northwest. Recent spectral observations of these filaments indicate that the shock is moving at about 2,900 kilometers a second — nearly one percent the speed of light — so the distance between the filaments and the x-ray peak indicates that neutral gas starts emitting x-rays about a century after crossing the shock.

But the key to finding out the brightness of the 1006 supernova lies in pinning down its distance. Since visible filaments mark where the shock front is moving into sheets of atomic hydrogen, Winkler, Long, and Middlebury College undergraduate Gaurav Gupta used CCD images spanning a period of eleven years to measure how fast the brightest filaments in the shell — and thus the shock wave itself — are expanding. In the March 1, 2003, issue of The Astrophysical Journal, the team reported a shift of 280 milliarcseconds per year along the entire length where the filaments are well defined.

CCD images of the brightest filaments in the remnant of the 1006 supernova were combined as an anaglyph to illustrate their motion across the sky. The green image was taken in 1991, the red image in 2002. The offset of the filaments shows how much they have moved over eleven years. Viewed with red-blue " 3D" glasses, the motion of the filament makes it stand out from background stars. A black-and-white GIF animation is available here. Middlebury College/NOAA/AURA/NSF anaglyph by Francis Reddy

The brightness and duration of the 1006 supernova suggest that it was a Type Ia event. It's generally believed that these supernovas originate in binary systems, where matter flows from a normal star and accumulates onto a white dwarf companion. The added mass ultimately makes the dwarf unstable, triggering a wave of explosive nuclear burning that completely incinerates the star. The absolute magnitude for Type Ia events is about –19.5. Since astronomers know their true luminosity, these explosions serve as standard candles — Winkler likes to call them " standard bombs" — that have become an increasingly important tool in establishing extragalactic distances.

Knowing both the rate at which the distant shell appears to be expanding and its corresponding true velocity, Winkler's group used simple geometry to calculate a precise distance from Earth to the shell. The result: 2.18 kiloparsecs, or 7,100 light-years, must also be the distance to the exploded star. " By knowing this distance and the standard luminosity of Type Ia supernovae, we can calculate, in retrospect, just how bright the star must have appeared to eleventh century observers," Winkler said. On the modern astronomical magnitude scale, the supernova falls at –7.5, or a little less than halfway between Venus and the full moon.

Ali ibn Ridwan compared the new star to both objects. " He's talking about using Venus and the full moon as benchmarks on a scale and is trying to describe how bright the star appears relative to them," Winkler said. The early evening sky of mid-May 1006 was completely dominated by these three objects. " It's taken a long time to interpret what he meant, but now I think we've finally got it right."

All of that light would have been concentrated in a single point. The star cast definite shadows and must have twinkled madly when observed near the horizon. " There's no doubt that it would have been a truly dazzling sight," Winkler notes. " In the spring of 1006, people could probably have read manuscripts at midnight by its light."

Ali ibn Ridwan noted that the star vanished three months after it appeared, at a time when it was above the horizon only during the day. A look at the light curve of Type Ia supernovas shows that after three months the new star would have faded to about magnitude –4.0, well below the brightness at which Venus can be seen during the day. And what of the Chinese astrologers who claimed to have monitored the new star for at least two years? Type Ia supernovas fade by about eight magnitudes over a year, but even with such a loss the 1006 event still ranked among the brightest stars in the sky. After two years, it would have faded to about the limit of human vision, around magnitude +6.5, so the Chinese account must be considered plausible.

Most of the elements heavier than helium form within the thermonuclear cauldrons of stars. Supernovas effectively disperse this material throughout the galaxy. For example, more than half a solar mass of iron may lie within the remnant of the 1006 supernova alone. Eons ago, exploding stars blasted into space many of the elements comprising our planet, and even our own bodies. Supernovas not only mix and heat the interstellar medium, they may provide the hammer blows that initiate the collapse of molecular clouds and trigger the formation of new stars.

Astrophysicists relish the opportunity to study these powerful events. Yet the closest and best studied supernova since the invention of the telescope, the one that appeared in the Large Magellanic Cloud in February 1987, was more than twenty-three times the distance of the 1006 blast. Sooner or later, the next visual supernova will blaze forth in our own neighborhood, delighting stargazers and scientists alike. In fact, it's a safe bet its light is already headed our way.…

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