Universe Today

All-sky map of the best fitting ‘halo+disk’ model of 511 keV gamma-ray line emission. Image credit: INTEGRAL. Click to enlarge.
The positron, the anti-matter counterpart to the electron, was predicted by Paul Dirac’s – at the time revolutionary – quantum wave equation for the electron. A few years later, in 1932, Carl Anderson discovered the positron in cosmic rays, and Dirac got the Nobel Prize in 1933 and Anderson in 1936.

When a positron meets an electron, they annihilate, producing two gamma rays. Sometimes however, the annihilation is preceded by the formation of positronium, which is like a hydrogen atom with the proton replaced by a positron (positronium has its own symbol, Ps). Positronium comes in two forms, is unstable, and decays into either two gammas (within about 0.1 nanoseconds) or three (within about 100 nanoseconds).

Astronomers have known since the 1970s that there must be a lot of positrons in the universe. Why? Because when a positron and electron annihilate to give two gammas, both have the same wavelength, about 0.024 Å, or 0.0024 nm (astronomers, like particle physicists, don’t talk about the wavelengths of gamma rays, they talk about their energy; 511 keV in this case). So, if you look at the sky with gamma-ray vision – from above the atmosphere of course! – you know there was lots of positrons because you can see lots of gammas of a single ‘colour’, 511 keV (it’s similar to concluding there’s lots of hydrogen in the universe by noticing lots of the red (1.9 eV) H alpha in the night sky).

From the spectrum of the three-gamma decay of positronium, compared with the 511 keV line intensity, astronomers four years ago worked out that about 93% of positrons whose annihilation we see form positronium before they decay.

How much positronium? In the Milky Way bulge, about 15 billion (thousand million) tons of positrons are annihilated every second. That’s as much mass as the electrons in tens of trillions of tons of stuff we’re used to, like rocks or water; about as much as in a mid-sized asteroid, 40 km across.

By analyzing the publicly released INTEGRAL data (about one year’s worth), J?rgen Kn?dlseder and his colleagues found that:

• the positrons which are being annihilated in the Milky Way disk most likely come from the beta+ (i.e. positron) decay of the isotopes Aluminium-26 and Titanium-44, which themselves were produced in recent supernovae (remember, astronomers call even 10 million years ago ‘recent’)
• however, there are more positrons being annihilated in the Milky Way bulge than in the disk, by a factor of five
• there don’t seem to be any ‘point’ sources.

Of course, to an INTEGRAL scientist, a ‘point’ source doesn’t have quite the same meaning as it does to an amateur astronomer! Gamma-ray vision in the positronium line is incredibly blurry, an object six Moons across (3?) would look like a ‘point’! Nonetheless, Kn?dlseder and his team of astrophysics sleuths are able to say that “none of the sources we searched for showed a significant 511 keV flux”; these 40 ‘usual suspects’ include pulsars, quasars, black holes, supernovae remnants, star-forming regions, rich galaxy clusters, satellite galaxies, and blazars. But, they’re still looking, “We have indeed [planned,] dedicated INTEGRAL observations of the usual suspects, such as Type Ia supernovae (SN1006, Tycho), and LMXB (Cen X-4) which might help to solve this problem.”

So, where do the 15 billion tons of positrons being annihilated every second in the bulge come from? “For me the most important thing about the positron annihilation is that the principal source is still a mystery,” says Kn?dlseder. “We can explain the faint emission from the disk by Aluminium-26 decay, but the bulk of positrons are situated in the bulge region of the Galaxy, and we have no source that can easily explain all observational characteristics. In particular, if you compare the 511 keV sky to the sky observed at other wavelengths you recognise that the 511 keV sky is unique! There is no other sky that resembles to what we observe.”

The INTEGRAL team feel they can rule out massive stars, collapsars, pulsars, or cosmic ray interactions, for if these were the source of the bulge positrons, then the disk would be much brighter in 511 keV light.

The bulge positrons may come from low-mass X-ray binaries, classical novae, or Type 1a supernovae, through a variety of processes. The challenge in each case is to understand how sufficient positrons created by these could survive long enough afterwards and diffuse far enough from their birthplaces.

What about cosmic strings? While the recent Tanmay Vachaspati paper proposing these as a possible source of the bulge positrons came out too recently for Kn?dlseder et al. to consider for their paper, “Yet for me it is not obvious that we have enough observational constraints to state that cosmic strings make the 511 keV; we don’t even know if cosmic strings exist. One would need a unique characteristic of cosmic strings that exclude all other sources, and today I think we are far from this.”

Perhaps most excitingly, the positrons may come from the annihilation of a low-mass dark matter particle and its anti-particle, or as Kn?dlseder et al. put it “Light dark matter (1-100 MeV) annihilation, as suggested recently by Boehm et al. (2004), is probably the most exotic but also the most exciting candidate source of galactic positrons.” Dark matter is even more exotic than positronium; dark matter is not anti-matter, and no one has been able to capture it, let alone study it in a lab. Astronomers accept that it is ubiquitous and tracking down its nature is one of the hottest topics in both astrophysics and particle physics. If the billions of tons per second of positrons that are annihilated in the Milky Way bulge cannot have come from classical novae or thermonuclear supernovae, then perhaps good old dark matter is to blame.

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