IPF 2010: Imagining the universe one photon at a time

IPF 2010 “All I saw hast kept safe in a written record, here thy worth and eminent endowments come to proof.”

—Dante Alighieri, Inferno

Tuesday’s Industrial Physics Forum talk by Peter Michelson of Stanford University reminded me of the opening of the Inferno, the first part of Dante’s 14th-century epic poem the Divine Comedy. The discussion opened the IPF’s Frontiers in Physics session.

Michelson’s subject was the Fermi Gamma-Ray Space Telescope, which surveys the sky every few hours and has detected some of the most spectacularly energetic events in the known universe. Although those events recall the hellfire Dante imagined in the Inferno, I was mostly reminded of Dante because of Fermi‘s time frame. I recall reading somewhere that the original design life of the Fermi mission was five years. Because it was launched into orbit about two and a half years ago, it is just about halfway through its design life, much like the Divine Comedy’s Dante who, at 35 years old, considered himself to be halfway along his life’s path.

So where do things stand “in the midway of this our mortal life,” as Dante wrote? That was what I was hoping to find in Michelson’s talk.

First a bit of background: The Fermi Gamma-Ray Space Telescope began its active life on 11 June 2008 when its rocket lifted off from Cape Canaveral, Florida.

After a 75-minute flight, the spacecraft was safely delivered into a near-Earth orbit. From a vantage 550 kilometers high, it now surveys the entire sky every three hours, detecting gamma rays, the highest-energy light in the universe.

Fermi promises to help scientists better understand some of the most fundamental questions in astrophysics, including the origins of cosmic rays, the identity of dark matter in the universe, the workings of massive black holes at the centers of distant galaxies, and the origins of gamma-ray bursts, the most energetic explosions that are known in the universe.

One of the interesting engineering challenges that Fermi had to overcome was the tremendous background collected by its main detector, the Large Area Telescope (LAT), an instrument Michelson described as having more than 100 square meters of silicon detectors interwoven in stacks. The detectors pick up charged particles impinging on them as well as gamma rays—and far more the former than the latter.

“There are 100 000 background particles for every gamma ray we want to detect,” Michelson told his standing-room-only audience.

The telescope designers overcame this challenge by using a plastic scintillator, which lights up when a charged particle hits. If you want a more technical description of the LAT and links to some of the recent publications by the 200 or so scientists who collaborate on it, you can read more here .

What the scientists measure once they subtract this rather large background are the directions, energies, and times of arrival of high-energy gamma rays—basically energetic photons produced by interactions of very energetic particles. In principle the early star formation in the early universe can be studied by looking at these gamma rays.

In his talk, Michelson showed an image of the first photon collected by Fermi, followed by a projection of the gamma-ray sky that was taken over the first few days of the telescope’s active life. The plane of the Milky Way and several distant active galaxies could be clearly seen. From there he showed image after image collected by the LAT over the last few years.

What amazes me about Fermi is how much further it seems to be seeing than its predecessor, the 17-ton Compton Gamma Ray Observatory . The LAT was designed to study the gamma-ray sky with greater sensitivity than has ever been done before, and with all due respect to Compton, it seems to be succeeding wildly, allowing astronomers to look deeper and deeper into the high-energy universe and surveying the entire gamma-ray sky every few days. In just a week of exposure over the entire sky, Fermi‘s LAT was able to match what Compton achieved in one year. And after two years, Fermi has exceeded what Compton did over a decade.

Michelson explained the differences between the two instruments in an e-mail this week.

• 1. Fermi‘s LAT has much more effective area.
• 2. The LAT has better angular resolution, so the sensitivity to a localized source is also improved.
• 3. The LAT’s wide field of view and scanning mode of observation mean that Fermi is exposed to celestial sources of gamma rays all the time over the entire orbit.

“That opens up a huge discovery space that we’ve been exploiting for the last few years,” said Michelson. In the last two and a half years, he and his colleagues have identified numerous gamma-ray sources, including novae, super-massive black holes, globular clusters, and starburst galaxies.

One of the first discoveries they made involved a gamma-ray source that Compton had detected, but without enough precision for it to be resolved. It turned out to be a pulsar in a supernova remnant named CTA I. This was the subject of one of the first publications to come from Fermi, and in the months since, the scientists on the project have been able to directly detect numerous pulsars using only gamma rays .

They have also detected a number of millisecond pulsars, which Michelson referred to as “rather exquisite clocks spread throughout the galaxy.” To date, the furthest object the telescope has observed is an ancient galaxy some 9 billion light-years away.

One of the highlights of the talk, for me, was when Michelson detailed the 514 gamma-ray bursts that the Fermi LAT has detected so far. Those include 18 high-energy bursts that Michelson said were from outside the Milky Way.

Those 18, he added, were from two different origins. The long bursts were from the formation of black holes following the collapse of a massive star. The short bursts arose from the coalescence of two neutron stars. He showed video simulations of both types of events that were impressive, even in an innocuous and modest-sized conference room in Rochester.

“The energy scales involved are enormous,” Michelson said. (For those of you who are counting, he listed an isotropic energy equivalent of one of the bursts at 10⁵⁴ erg, indicating relativistic beaming.)

That is hellfire, no doubt, that far exceeds even what Dante could have imagined!

Jason Bardi