Tuesday, July 22, 2008

The no-view nova

X-ray image of the nova nobody saw
Image Credit: Contours: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.), Background: SSS

Most of the time, the stars and galaxies that we astronomers look do not change much, if at all, over a human lifetime. So, our only hurry in looking at a star is to do it before somebody else does. If the weather is bad or the telescope breaks, we can come back another night, or even another year, and there is little lost.

For some astronomical objects, though, time is critical. Supernova explosions, for example, are only visible for a few months or so before fading away from sight. Another, more common explosion, called a nova, only lasts a few nights. Glows from gamma-ray bursts last just a few hours. If one of these events occurs, we need to hop on it fast, or lose it forever.

The problem is, you've got to be looking in the right place at the right time to see one of these. At present, there are only a few small telescopes that take pictures of the entire sky on a regular basis. Such a search produces tremendous amounts of data, and on big telescopes, the biggest cameras can only image about one quarter of one ten thousandth of the entire sky in a single picture. So, much of the sky is not searched by professional astronomers for these time-critical events. Those who do search for these events tend to focus on tiny patches of the sky. Though they'll miss most explosions, they'll still see enough for their science. (The one exception are gamma ray bursts, because the gamma ray detectors in space actually can look at most of the sky in a single picture.)

Typically, this is where amateur astronomers step in. These men and women are often out conducting searches of their own, often using their own eyes and star charts to try and spot something out of place. It may be a comet, or it may be an explosion, or it could be some other event. Amateurs are pretty good at this, and are discovering comets, supernovae and novae all the time. They get a little bit of glory, and a lot of personal pride, out of beating us professionals. And they deserve it.

One of the big prizes is discovering something that will become bright enough to see with the naked eye (i.e., without a telescope or binoculars). Then people around the world will be able to go out and see your discovery, sometimes with your name attached (like Comet Hale-Bopp).

Still, even a small army of amateurs can't catch everything. This was proven in a press release last Friday from the European Space Agency's XMM-Newton X-ray telescope.

The X-ray telescope, like an optical telescope, points at interesting targets and takes pictures. When it moves from one target to another (which takes a long time in space), the cameras are usually turned off. But a group of scientists including Andy Read of the University of Leicester and Richard Saxton of the European Space Agency are running a project where, sometimes, the cameras are kept running as the telescope moves, allowing random objects to drift into the field of view.

Last October, a bright X-ray source popped into the XMM-Newton camera during one of these moves, but, according to catalogs, nothing should have been there. After some quick legwork and a few phone calls to big telescopes, it was determined that the X-rays were coming from a previously unknown nova.

A nova is a distant cousin of a supernova. In a supernova, an entire star explodes during a runaway nuclear explosion. In a nova, the outer layers of a white dwarf star explode like a hydrogen bomb, but the explosion is too weak to blow the entire star apart. As you might guess from the names, a supernova is many times brighter than a nova. But novae are actually more common, because there are a lot of white dwarfs in our galaxy. Several novae are found every year, and every few years, one is bright enough to see with the naked eye. As with bright comets, most novae are found by amateur astronomers, and not by professionals.

The odd thing about the XMM-Newton's discovery, though, is that novae don't make a lot of X-rays early on. So, the nova that XMM-Newton found was actually a few months old, but it had never been reported. So, the XMM-Newton team called up the operators of a robotic all-sky survey called ASAS. They combed through old data, and found that the nova had indeed been picked up by their optical telescopes on June 5, 2007. Not only that, but the nova had gotten bright enough that it would have been easily visible to the naked eye, the brightest nova in over a decade. And yet, not one human knowingly saw it!

How did everyone miss it? Well, the nova was in the constellation Puppis, which is not visible in most of the northern hemisphere (where most amateur astronomers live). And Puppis lies near the Milky Way, so it is full of stars -- only a trained eye would have been able to pick out the new one. But novae are found in Puppis by professional and amateur astronomers quite a bit. We just got unlucky with this one.

Discoveries like this make us wonder how many interesting things happen in the sky on time scales so short that nobody has a chance to see them. For that reason, astronomers are starting to build telescopes that will image the entire sky to very faint limits every few days. The ultimate data will come from the Large Synoptic Survey Telescope or LSST, which will soon be built in Chile. The mirror for this telescope is huge -- 8 meters across, making it one of the largest telescopes in the world. The telescope, in a single picture, can image an area of sky about 50 times the area of the full moon. A single 30 second exposure will be able to see objects about 2 million times fainter than what your eye can see.

Amazingly, the hard part of this project is not the telescope (though it will be one of the most complex mirrors and cameras ever built). The hard part will be the data volume: 30 terabytes of data every night. That's 30,000 gigabytes, or, if you were to put it on a normal DVD, about 6000 DVDs worth of information every single night. For five years. And we want to be able to analyze that data on the fly, so that interesting objects (like novae) can be observed with other instruments at other telescopes as soon as possible. To help with this, money and assistance from Google and the Bill and Melinda Gates Foundation (along with other technologically-oriented companies) are pouring into the project.

The LSST mirror is under construction in Tucson, Arizona, and the construction will soon start in Chile, with hopes of opening this new eye on the Universe in 2014. Hopefully no more novae will slip through the cracks!

1 comment:

  1. This is the best reporting of this story I have read. Poor reporting of this discovery annoyed me so much I had to write a blog about it! Keep up the excellent work. I love your blog.