Afterlife of a Supernova

Chandra image of SN1970G. Image credit: NASA. Click to enlarge.
As astronomers look out over the Universe, one principle stands out in bas relief above the vast welter of data and information captured by their instruments – the Universe is a work in progress. From hydrogen atom to galaxy cluster, things undergo change in surprisingly similar ways. A principle of growth, maturation, death, and rebirth is at play in the Universe. Nowhere is that principle more fully embodied than in the primary sources of light we see through our instruments – the stars.

On June 1 2005, a pair of investigators (Stefan Immler of NASA’s Goddard Space Flight Center and K.D. Kuntz of John Hopkins University) published X-ray data collected from a variety of space-borne instruments. The data reveals how one massive star passing within a nearby galaxy (M101) can help us understand the relatively short period between a star’s death and the transformation of its luminous wreath of gas into a supernova remnant. That star – supernova SN 1970G – has now experienced some 35 years of a visible “afterlife” in the form of a rapidly spinning neutronic core within an expansive circumstellar aura of gas and dust (the CSM or circumstellar matter). Even now (from our perception) heavy metals race outward at a speed of thousands of kilometers per second – potentially planting seeds of organic matter within the Interstellar Medium (ISM) of a 27 million light year distant galaxy – one easily visible in the smallest of instruments within the spring constellation of Ursa Majoris. Only when the energy within that matter reaches the ISM, will 1970G have completed its cycle of birth and potential rebirth to take form in new stars and planets.

The destiny of a star is primarily determined by its mass. Surviving for as little as 50,000 years, the most massive stars (as great as 150 suns) condense out of vast concentrations of cold gas and dust to eventually live very fast lives. In youth, such stars exult as brilliant blue giants radiating near-ultraviolet light from a photosphere whose temperature may be five times greater than that of our own Sun. Within such stars nuclear furnaces rapidly accumulate giving off prodigious amounts of extremely intense radiation. Pressure from this radiation propels the star’s outer shroud outward many times over even as a howling gale of highly charged particles boils off its surface to become the stars CSM. Due to pressure exerted by its rapidly expanding core, such a star’s nuclear engine eventually becomes starved for fuel. The subsequent collapse is marked by a brilliant light show – one that can potentially outshine an entire galaxy. At magnitude 12.1, type II supernova 1970G never became bright enough to overcome its 8th magnitude host. But for some 30,000 years prior to its efflorescence, 1970G boiled off copious quantities of hydrogen and helium gas in the form of a powerful solar wind. Later, that same diaphanous aura of matter took the brunt of 1970G’s outburst shocking it into X-ray excitation. And it is that period of expanding shockwaves that has dominated the energy signature or “flux” of 1970G over the past 35 years of observation.

According to a paper entitled “Discovery of X-Ray Emission from Supernova 1970G with Chandra” Immler and Kuntz report that, “As the oldest SN detected in X-rays, SN 1970G allows, for the first time, direct observation of the transition from a SN to its supernova remnant (SNR) phase.”

Although the report cites X-ray data from a variety of X-ray satellites, the bulk of the information comes out of a series of five sessions using the NASA’s Chandra X-Ray Observatory during the period July 5-11, 2004. During those sessions a total of almost 40 hours of soft X-rays were collected. Chandra’s superior spatial resolution and the sensitivity gained from long-term observation allowed astronomers to fully resolve the supernova’s X-ray lightcurve from that of a nearby HII region within the galaxy – a region bright enough in visible light to have been included in J.L.E Dreyer’s New General Catalog compiled during the late 19th century – NGC 5455.

Results from this – and a handful of other observations of supernova afterglow using NASA’s Chandra and ESA’s XMM-Newton – have confirmed one of the leading theories of post-supernova X-ray lightcurves. From the paper: “high-quality X-ray spectra have confirmed the validity of the circumstellar interaction models which predict a hard spectral component for the forward shock emission during the early epoch (less than 100 days) and a soft thermal component for the reverse shock emission after the expanding shell has become optically thin.”

For tens of thousands of years before going supernova, the star that became SN 1970G quietly boiled away matter into space. This created an expansive extrastellar aura of hydrogen and helium in the form of a CSM. When it went supernova, a massive flux of hot matter shot into space as SN 1970G’s mantle rebounded after collapse onto its superheated core. For roughly 100 days, the density of this matter remained exceedingly high and – as it smacked into the CSM – hard X-rays dominated the output of the noval flux. These hard X-rays contain ten to twenty times as much energy as those to follow.

Later as this highly energized matter expanded enough to become optically transparent, a new period supervened – X-ray flux from the CSM itself caused a reverse flood of lower-energy “soft” X-rays. That period is expected to continue until the CSM expands to the point of fusion with Interstellar Matter (the ISM). At that time the supernova remnant will form and thermal energy within the CSM will ionize the ISM itself. Out of this will come the characteristically “blue-green” glow visible in such supernovae remnants as the Cygnus Loop when seen through even modest amateur instruments and appropriate filters.

Has SN 1970G evolved into a supernova remnant yet?

One important clue to solving this question is seen in the mass-loss rate of the supernova before eruption. According to Immler and Kuntz: “The measured mass-loss rate for SN 1970G is similar to those inferred for other Type II SNe, which typically range from 10^-5 to 10^-4 solar masses per year. This is indicative that the X-ray emission arises from shock-heated CSM deposited by the progenitor rather than shock-heated ISM, even at this late epoch after the outburst.”

According to Stefan Immler, “Supernovae usually fade away quickly in the near aftermath of their explosion as the shock wave reaches the outer boundaries of the stellar wind, which becomes thinner and thinner. A few hundred years later, however, the shock runs into the interstellar medium, and produces copious X-ray emission due to the high densities of the ISM. Measurements of the densities at the shock front of 1970G showed that they are characteristic of stellar winds, which are more than an order of magnitude smaller than the densities of the ISM.”

Because of the low levels of X-ray output, the authors have concluded that 1970G has yet to reach the supernova remnant phase – even at an age of 35 years after the explosion. Based on studies associated with supernova remnants such as the Cygnus Loop we know that once remnants are formed, they can persist for tens of thousands of years as superheated matter fuses with the ISM. Later, after the shock-heated ISM has finally cooled off, new stars and planets may form enriched by heavy atoms such as carbon, oxygen, and nitrogen along with even heavier elements (such as iron) produced during the brief moment of the actual supernova explosion – the stuff of life.

Clearly SN 1970G has a great deal more to teach us about the afterlife of massive stars and its march toward supernova remnant status will continue to be carefully monitored well into the future.

Written by Jeff Barbour