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Hunting the Last Galactic Supernova and Anticipating the Next

by Peter Edmonds

May 14, 2008 ::
A supernova is a spectacular event. It can outshine the optical light from an entire galaxy, and with the help of big ground-based telescopes, it can be seen across more than half the Universe. At least one of the supernovas in our galaxy was so bright it was visible to ancient astronomers during the day. All of this light comes with a tremendous amount of energy, which has a big impact on the supernova's home galaxy, by heating and pushing gas around, pumping out new elements and creating exotic objects in the form of neutron stars and black holes. Supernovas are vital not only for the life of a galaxy, but the life in a galaxy. They may have helped trigger the formation of the Sun and the planets, and they disperse many of the elements critical to life, like the iron in our blood and the oxygen that we breathe.

Light Travel Time

In this discussion the elapsed time since a supernova explosion - and equivalently the age of the corresponding remnant - is given in Earth's time frame. That is, we discuss when a supernova was observed or, in the case of an obscured supernova like G1.9+0.3, when the light could have been observed, given a big enough telescope. Because the speed of light is finite, it is impossible for us to know what is happening at this instant in a different part of the Galaxy, such as the Galactic center. However, the light travel time to even the other side of the Galaxy is still likely to be much less than the time it takes for significant changes in the Galactic supernova rate. Therefore, despite the finite speed of light we can still make useful comparisons of the supernova rate across the entire Galaxy.

How Often Do Supernovas Explode in Our Galaxy?

Given the incredible power of supernovas and their importance, you might think that astronomers have an excellent understanding of how often they explode in our cosmic backyard, the Milky Way galaxy. They don't. Mostly, they have given up on looking at our own Galaxy to estimate what its supernova rate is. Instead, they count the supernovas in a bunch of other, more distant, spiral galaxies that have about the same mass as the Milky Way. This is a bit like working out the average medical history of people with your size and assuming that it applies to yourself. A doctor would insist on a physical, but astronomers can't make demands of the Universe.

So, why is it so hard to directly estimate the supernova rate in our galaxy? A big problem is that most of the supernovas in our galaxy should be very faint in optical light - and some effectively invisible - because their light is blocked by dust and gas lying in the Galactic plane. For example, the most recent supernova in the Galaxy (as measured in Earth's time frame [see sidebar]) is estimated to be about a trillion times fainter, optically, than it would have been without the veil of dust and gas. This makes the difference between it being visible with the naked eye and only being visible using a deep observation with a very large telescope.

Many Recent Galactic Supernovas Have Been Missed

Another problem for estimating the Galactic supernova rate is that supernovas in galaxies like ours are thought to occur about once every 30 years. But, if you want to see the bright flash from a Galactic supernova explosion using an optical telescope, you're probably in for a much longer wait, since the supernova needs to be relatively close to the Earth to avoid being obscured by lots of dust and gas. The last one definitely seen by historical astronomers occurred over 400 years ago (Kepler's supernova remnant) well above the plane of the galaxy.

Astronomers have estimated the Galactic supernova rate by extrapolating from these historical ones easily seen in the local part of the Galaxy to the obscured population in the entire Galaxy, but the correction factor is large and uncertain. It's clear that we've missed a lot of supernovas in the Galaxy in the last 2000 years - what's unclear is how many we've missed.

Filling in the Gaps

One way to count how many have been missed is to use X-ray and radio observations to peer through the dust and gas, and observe the remains of supernovas. These supernova remnants emerge a few decades after a supernova and last for thousands of years, and are bright in X-rays and radio waves. (The most recent update of a catalog of Galactic supernova remnants lists 265 objects.)

Supernova Remnants Don't Come With a Driver's License

However, it is difficult to estimate the age of a supernova remnant, as with many objects in astronomy. The Galactic supernovas that occurred less than a few hundred years ago are most promising for estimating accurate ages, because expansion can be measured. Looking at how quickly something is expanding allows one to estimate how long it has been expanding.

Just the detection of one recent, obscured supernova in our Galaxy is exciting for starting to find these missing supernovas, since the previous youngest supernova remnant, Cas A, was known for over 50 years. Given the huge advances made in observational astronomy over the last ten or twenty years, that's a long time. Inspired by the successes of Cas A and G1.9+0.3, astronomers will be keen to continue finding evidence for recent supernovas in our galaxy by looking for expansion of remnants.

Another reason for excitement is that because no other remnant in the Galaxy is known to be as young, G1.9+0.3 offers a great opportunity to study the acceleration of cosmic rays, the dispersal of elements and the generation of X-ray and radio emission in unprecedented detail.

Could a Galactic Supernova Be Missed Now?

Since much work remains to be done, we cannot properly test yet whether a Galactic supernova does explode every 30 years or so. But, assuming it does, there's a decent chance one will go off in the career of a young astronomer and an obvious question arises: will such a supernova be detected today no matter where in the Galaxy it explodes? The answer depends on what sort of supernova it is. If it's a supernova generated by the collapse of a massive star, the flash of neutrinos caused by the explosion will definitely be seen by detectors on Earth no matter where it occurs in the Galaxy.

A neutrino flash was detected from Supernova 1987a, which occurred in a nearby galaxy. But, if it's a thermonuclear explosion of a white dwarf, a so-called Type Ia supernova, then it's much less clear whether it would definitely be detected. However, most supernovas are caused by core-collapse events, so there's still a decent chance that a Galactic supernova will be caught in action in the next few decades. When it does, it will be one of the biggest stories in modern astrophysics.

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