Waiting for Starlight

Davide Lazzati tries to make sense of violent bursts that shine from the far reaches of the universe

May 9, 2017

Oh, let the sun beat down upon my face

And stars fill my dreams.

I’m a traveler of both time and space

To be where I have been.

— “Kashmir,” Led Zeppelin

By Julia Rosen

Davide Lazzati’s computer screen is a blur of numbers. They race across in an endless torrent, far too fast for anyone to read. If you could, you’d be looking at the sudden swell of a cosmic explosion.

Lazzati leans back in his chair and waits, humming along to Led Zeppelin. He likes to listen to music while he codes. “It can get pretty loud,” says Lazzati, an astrophysicist at Oregon State University. He has charcoal hair that’s just beginning to gray at the temples, and a sheepish grin that he deploys often, including now. “Sometimes I close the door so I don’t bother other people,” he says.

At the moment, he’s just making sure that his code works properly. When he’s satisfied, he’ll send the code to a supercomputer at NASA’s Ames Research Center in California. These supercomputers are vastly more powerful than Lazzati’s Mac Pro, but even so, it will take weeks to complete a simulation. That’s because the phenomena Lazzati studies are far more powerful still.

Powerful jets of high-speed material are pictured in blue in this computer simulation of an exploding star that harbors a gamma-ray burst. Davide Lazzati uses three-dimensional computer simulations to shed light on the dynamics of these energetic outflows, which stream out of their progenitor star (in red and orange). The jets eventually escape from the dense and cold stellar material and convert their energy into gamma-ray radiation that can be detected with telescopes. (From Lopez-Camara, Lazzati, Morsony, & Bergelman 2013)

Gamma ray bursts — aka GRBs — are the most energetic events in the known universe. They last a few seconds to a few minutes and occur when a giant star collapses into a black hole, or, researchers think, when objects like neutron stars collide. In both cases, the result is an explosion of gamma rays — the most powerful form of electromagnetic radiation, more energetic than X-rays. A single GRB can release as much energy as the sun emits in its whole life.

GRBs are mysteries worthy of study in their own right, and they can also cast light — literally — on some of the greatest puzzles of the cosmos, from the origins of the universe to gravitational waves (see “Beacons of Light”). Some scientists study them using telescopes, and some scratch away at the physics that drive their super-charged jets. Lazzati’s work sits right at the intersection of observations and theory; he wants to understand what we see, and why.

It’s not an easy question to tackle. “We don’t build stars in the lab,” Lazzati says. So he and his students do the next best thing: They use computer models to recreate the explosive deaths of stars. Then, they explore how these blasts should look to astronomers here in our tiny corner of the universe.

Of course, it’s not always that elegant, says Lazzati, warily eyeing the computer screen for signs of trouble. “In practice, I’m fighting with a numerical code that keeps crashing on me,” he says. “And that is not poetic.”

Winks from Across the Universe

The stream of numbers on Lazzati’s monitor screeches to a halt and a terse error message appears. Something has gone wrong. It could be an obvious mistake, or a typo buried deep in the calculations. Either way, such snags are inevitable; Lazzati leans forward to start scrolling through the lines of code in search of the culprit as the music blares away in the background.

Davide Lazzati uses computer models to simulate the deaths of massive stars. (Photo: Chris Becerra)

In the foreground, a constellation of dirty coffee cups encircle Lazzati’s computer. An espresso maker sits on a side table, covered in a light dusting of coffee grounds. Children’s drawings adorn the walls. Several have space-related themes, including a colorful schematic of the solar system — complete with the asteroid belt — by his 6-year-old son, Alejandro. He and his wife, Catalina Segura, a hydrologist in the OSU College of Forestry (and a collaborator with Lazzati on modeling water flow), also have a 9-year-old daughter, Diana. “They both love astronomy and space travel,” Lazzati says. When Lazzati was their age, growing up in northern Italy, he wanted to be a truck driver. But his parents and teachers pushed him to study science, and he took an astronomy course in his final year of high school. “It was so impressive that you could know so many things about something that’s so inaccessible,” he says. “It was mind-blowing for a geek.” He was hooked, although — for the record — he still loves trucks.

Lazzati had made his way to graduate school when gamma ray bursts captured the attention of the astrophysics community. American military satellites first discovered the cosmic explosions in the 1960s. They aren’t rare; if we could monitor the whole sky, scientists think we could see a handful every day from Earth. Many more evade detection. Back then, however, scientists knew very little about GRBs, including what triggered them and whether they took place in our own galaxy or beyond it.

Then Italy launched a satellite called BeppoSAX in 1996. It was, in Lazzati’s words, one of his country’s “miracles.” BeppoSAX was an X-ray satellite that could precisely pinpoint the location of a gamma ray burst. That allowed astronomers on the ground to study its afterglow with telescopes and determine how fast it was moving away from us. Because the universe is expanding at an increasing pace, faraway objects recede faster than close ones, providing a measure of their distance. Thanks to BeppoSAX, astronomers discovered that GRBs are not local phenomena, but explosions that wink at us from remote galaxies.

A few years later, the satellite made yet another big discovery: It detected a GRB in the same spot as a supernova, which scientists knew signaled the death of a star. This helped confirm the hypothesis that GRBs were linked to the fatal collapse of massive stars. However, supernovas and GRBs don’t always go hand in hand. GRBs are only produced by stars large enough to create a black hole when they die. A GRB, Lazzati says, is “the birth cry of a black hole.”

Together, the BeppoSAX discoveries launched a new era of GRB research and drew many young researchers, including Lazzati, to the field. Much of Lazzati’s research today focuses on how the protons and electrons that get spewed out of a dying star produce radiation. He wants to know why these explosions emit mostly gamma rays — not X-rays or other kinds of radiation — and why they sometimes pulse on and off like strobe lights. Or why GRB radiation forms narrow beams instead of emanating out in all directions. He and others suspect that magnetic fields may play an important role. It all starts when a massive star implodes and begins to form a black hole with the mass of several suns packed into an object 10 kilometers in diameter. It’s a messy process, and it leaves behind a spinning disk of matter laced with magnetic-field lines. Think of these field lines as spaghetti, Lazzati says. (Many of his analogies revolve around Italian food.) The inside of the disk spins faster than the outside, so the spaghetti gets twisted up as if some invisible diner had wound it around a cosmic fork.

“Particles cannot travel across the spaghetti,” Lazzati says. “They have to follow the spaghetti.” And in doing so, they form jets of matter traveling at the speed of light.

With funding from the National Aeronautics and Space Administration (NASA), Lazzati studies how these jets give off high-energy photons. His research suggests that these photons are produced by particles that fly along curved magnetic field lines. The trick to testing this idea, he says, is to marry calculations about how matter behaves in a GRB and how radiation behaves. “We are trying to get the whole picture out of a single calculation, rather than doing one and then doing the second part,” he says. Or, put into gustatory terms, “we are cooking the cheese on the pizza instead of cooking the pizza and putting the cheese on top.”

Getting a handle on this transformation is important because astronomers only get to study the radiation from a GRB; the protons and electrons never make it to our detectors. Lazzati’s work helps researchers understand what this radiation can tell us about these distant explosions, says Rosalba Perna, an astrophysicist at Stony Brook University and one of Lazzati’s frequent collaborators. “It’s really addressing the very basic question of where the radiation that we see comes from.”

When Black Holes Collide

At long last, Lazzati is ready to send his code to the supercomputer at Ames. He has teased out all the errors and run a suite of tests to confirm — as best he can — that the model is doing what it should. So Lazzati takes out his phone and taps open an app. It generates a secret password that grants him access to NASA’s computers; he has just 20 seconds to login before it expires.

He makes it in time and enters his code into a queue of jobs. It will be months before the simulation is finished. But even then, the wait won’t be over. That’s because the thing Lazzati is modeling hasn’t technically been observed yet. He is trying to predict what astronomers might see if two massive objects collide, producing both a GRB and gravitational waves at the same time. “That would be extremely exciting,” Lazzati says.

Researchers only recently developed the ability to measure gravitational waves, confirming a central prediction of Einstein’s general theory of relativity. In 2015, newly operational detectors measured subtle disturbances in space-time when two black holes spiraled into each other, sending ripples of excitement through the scientific community. They are now waiting for more mergers to generate new sets of waves, and scientists hope some of these collisions will also produce GRBs.

Most astronomers think a GRB will occur if at least one of the colliding objects is a neutron star, which can provide matter to fuel the jets. (A black hole — by definition — does not give up its contents.) Researchers suspect such mergers are responsible for a flavor of GRB known as a short gamma ray burst, which lasts less than a few seconds. At the moment, they lack “iron-clad proof,” Perna says, but spotting a short GRB associated with gravitational waves would be just the kind of evidence they need. And Lazzati wants to increase the odds that they don’t miss their chance.

Though GRBs are tremendously bright, they are like lighthouses, sending out narrow beams in two directions. So, for every GRB aimed at Earth, Lazzati says, there are 100 others that we don’t see because they point away from us (roughly 20 percent of all GRBs are short GRBs). Thus, there’s a good chance that even if a merger does occur and produces both gravitational waves and a GRB, we might not catch the latter.

The code Lazzati is currently running at Ames could broaden astronomers’ net by giving them a sense of what GRBs might look like from the side. There wouldn’t be any gamma rays to observe, but there could be X-rays produced when the jet blasts through the debris left behind by the collision. Lazzati thinks astronomers might be able to distinguish such a signal from other X-ray emissions beaming across the cosmos because of its short duration and unique fingerprint.

Lazzati also thinks the fingerprint of radiation from a short GRB might contain clues about the properties of the merger that produced it. How elliptical were the orbits of the circling objects? How strong was the magnetic field? What happened in the final moments before the collision? He hopes that by simulating a range of mergers between different objects under different conditions, he can give astronomers a key to interpret the radiation they measure. “What we provide is the framework for understanding what we see,” Lazzati says.

Scientists who study gravitational waves expect to detect another merger sometime in the next few years. But no one knows whether it will be the type of collision that could produce a GRB, and whether that GRB will be aimed at Earth. “It might take us 10 to 15 years to get lucky,” Lazzati says. Either way, scientists will learn something. If researchers do spot a flash of radiation, he says, “it would tell us what is really producing short gamma ray bursts.” If they don’t, he adds, “it’s, ‘geez, we were wrong’.” Then it’s back to square one.

For now, though, Lazzati just needs to let the code run and wait to see what the universe has to offer. All the time, light from ancient explosions is streaming across the cosmos like the numbers on Lazzati’s screen. It’s just a matter of time before something hits Earth and gives scientists an error message — or an answer.

Editors note: Julia Rosen received a Ph.D. from OSU in geological sciences in 2014. Her stories have appeared in Nature, Science, High Country News and other publications. Contact her on Twitter @ScienceJulia.


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