Original URL: http://www.theregister.co.uk/2011/08/26/type_1a_ptf_11kly_supernova_discovered/

Supercomputer and superboffins spot rare baby supernova

Dark matter secrets probed

By Rik Myslewski

Posted in Science, 26th August 2011 23:16 GMT

A newborn, nearby supernova with the potential to significantly improve our knowledge of the universe has been discovered by a supercomputer, two telescopes on opposite sides of the world, a sharp-eyed astronomer, and his helpful Oxford colleague.

The Reg spoke with Peter Nugent, the California astronomer who first spotted the supernova, and who is a senior staff scientist at Lawrence Berkeley Lab and an adjunct professor of astronomy at the University of California, Berkeley.

Nugent explained that images of the exploding star were captured by the Palomar Transient Factory (PTF) survey, using the 48-inch Samuel Oschin Telescope in Palomar Mountain, California. PTF scans the sky nightly, and sends its data to the National Energy Research Scientific Computing Center (NERSC) in Berkeley for analysis.

Sifting through that data to uncover such nuggets as the new supernova now known rather prosaically as PTF 11kly requires the combined efforts of man and beast – the beast in this case being NERSC's Carver IBM iDataPlex system.

Carver is the junior partner in NERSC's supercomputer team, with 400 compute nodes, each with two Nehalem quad-cores. When all 3,200 cores are up and running, Carver has a theoretical peak performance of 34 teraflops per second. NERSC also houses Franklin, a massively parallel Cray XT4 with 38,128 Opteron compute cores that, at 352 Tflop/sec, puts out over ten times Carver's peak flops.

The big girl on NERSC's team, Hopper, is far and away more powerful still: it's a Cray XE6 with 153,216 compute cores that came in fifth in November 2010's Top500 List, with a sustained performance of 1.05 petaflops/sec.

The Carver IBM iDataPlex system at the US National Energy Research Scientific Computing Center

The Carver IBM iDataPlex at the US National Energy Research Scientific Computing Center (click to enlarge)

Although Nugent and his team also use Hopper when the need arises, Carver was the machine of the moment during the run that discovered PTF 11kly – and very little of Carver was involved, to boot. A typical daily PTF workload uses about 60 of Carver's cores, but "The night we were doing it," Nugent told us, "we were trying to catch up, so we were on about 120 cores throughout the night.

"That's the nice thing about NERSC having all these cores available," he said. "We could just increase the load with just one change in one line in a piece of code and, boom, it just goes off and grabs more processors."

The method that the PTF projects uses to track down supernovae is straightforward, if maddeningly detailed. Images taken by the Samuel Oschin Telescope are compared with enhanced images taken at the beginning of the survey – if there are differences, they're flagged and logged in a database.

That may sound simple, but as Nugent explains, "There are lots of artifacts on the images, and of course there'll be some new things – there'll be asteroids, there'll be variable stars, and occasionally there'll be a supernova."

When Nugent says "a lot" of artifacts, he's not exaggerating. "Every night we typically get about one million candidates that pass some sort of threshold, and of those, only about a few hundred are actually real, and of those few hundred, only about two are interesting new supernova."

The PTF project has discovered over one thousand supernovae since it started up in 2008, but PTF 11kly is not simply another interesting new supernova, it's an exceptionally valuable source of scientific data.

Not just any supernova...

First, it's a Type 1a supernova, especially interesting to astronomers because these supernovae were used in the 1990's to determine that the expansion of the universe is accelerating, and are used in the search for dark matter. Unfortunately, more needs to be learned about Type 1a supernovae before they can be used to help better understand accelerating universal expansion and dark matter, and PTF 11kly should provide excellent data.

Type 1a supernovae are believed to be the death throes of white dwarves, stars with masses of about 1.4 times that of our sun. As Nugent explains the supernova process, the white dwarf is "accreting material from its companion, and when it gets massive enough, the temperatures on the inside can ignite the carbon in a runaway thermonuclear explosion."

Images taken of PTF 11kly's location by the Hubble Space Telescope in 2002 show what appear to be two red giants in the vicinity. If that's indeed what Hubble's pixels picture, that'd fit well into the accretion scenario that Nugent paints – one or both of those giants could have provided the meal that led to PTF 11kly's explosion.

This type of supernova is unique. "We know that these [white dwarves] are pretty old stars," Nugent says, "because we see them in both young galaxies and old galaxies, and all the other types of supernovae we only see in very young galaxies."

Another huge reason that PTF 11kly is such great candidate for study is that it's exploding almost next door – only around 21 million light-years away in the Pinwheel Galaxy, more properly known as M101, which can be found in the Big Dipper, more properly known as Ursa Major. It's the closest supernova of its type discovered since 1986 – and that one, Nugent says, was obscured by dust. "Before that, you'd have to go back to 1972, 1937, and 1572 to find more nearby Type 1a supernovae."

Finally, PTF 11kly was discovered mere hours after it first exploded – well, a few hours plus 21 million years, but let's not quibble.

Enter the humans

The baby supernova was picked out of the reams of PTF data by a combination of computer power and human intelligence. The filtering process that Carver uses to trim those one million anomalies down to a few hundred was developed by Josh Bloom, an associate professor of astronomy at the University of California, Berkeley.

To develop the machine-learning algorithm that he created, a large group of experts went through prefiltered anomaly data, and classified each anomaly as to whether it was of interest or not. Bloom then took those observations and developed what Nugent calls a "boosted decision tree" to filter and categorize the millions of nightly anomalies into a manageable number of reasonable possibilities.

After each pass through the PTF data, those few hundred possibilities are distributed to a group of "citizen scientist" supernova seekers – yes, actual humans – known as the Galaxy Zoo team (which describes itself as "very small and very busy"), who eyeball the possibilities to attempt to discover actual supernovae, along with a bot that Bloom developed that also roams through the data.

Nugent is one of those final filterers – and he was lucky enough to be in the right place at the right time. "Wednesday at noon, California time, was when I looked and said, 'Okay, database, tell me what the best one we have from last night is', and – boom – it was there," he said. And it hadn't been there in the previous night's readings.

Type 1a supernova PTF 11kly expanding

Monday (left), nothing. Tuesday (center), maybe. Wednesday (right), yup – supernova (click to enlarge)

"At first I was completely skeptical. I thought it was an asteroid passing in front of the galaxy, but then I searched the asteroid database and, nope, there was nothing there. Then because it was so faint, I said 'Well, this could be a nova' – which is another type of outburst, but not as interesting to us. And, no, that wasn't quite right."

Nugent then took advantage of a computer far less powerful than Carver. "Fortunately, I happened to be online and chatting with Mark Sullivan in England, and he said, 'Well, the sun is setting in the Canary Islands right now. We can have them take a spectrum of this object and let us know.' And so that happened three hours after I found it – they took a spectrum, and confirmed it was a supernova."

And not just any supernova, but "a very young, Type 1a supernova" as identified by the strong silicon lines that the Liverpool Telescope on the Canary island of LaPalma, Spain, identified in its spectrum.

After the Liverpool Telescope's confirmation, Sullivan emailed that team about its quick response: "Wow, what fun! From an email alert to data in <1 hour."

That "fun" is now being shared worldwide, thanks to the PTF network. Within 12 hours, PTF 11kly's Type 1a designation had been confirmed by California's Lick Observatory and Hawaii's powerful Keck Observatory, and many more telescopes around the world are making observations and gathering data. Thanks to an urgent request to NASA, the Hubble will take a look this weekend, as well.

"When you catch them this early, mixed in with the explosion you can actually see unburned bits from star that exploded!" effused Andrew Howell of UC Santa Barbara, exemplifying the excitement of astronomers worldwide. "Despite looking at thousands of supernovae, I've never seen anything like this before," he said.

It's possible that you may not even need a telescope to see PTF 11kly. "The best time to see this exploding star will be just after evening twilight in the Northern hemisphere in a week or so," says Sullivan. "You'll need dark skies and a good pair of binoculars, although a small telescope would be even better."

Just look for the Big Dipper and track down that Pinwheel with the help of Stargazers Lounge. And if you still can't find it, you might ask your friendly neighborhood supercomputer for assistance. ®