Penn State Scientists Help Discover Neutron Star Collision

Oct 16, 2017

Illustration of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the burst of gamma rays that are shot out just seconds after the gravitational waves. Swirling clouds of material ejected from the merging stars are also depicted—these clouds glow with visible and other wavelengths of light.
Credit Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet

NASA’s Swift satellite, controlled by scientists at Penn State, took part in a groundbreaking scientific discovery announced Monday: the first observation of two colliding neutron stars.

First, some background.

Last year, scientists at LIGO (the Laser Interferometer Gravitational-Wave Observatory) first detected gravitational waves, or ripples in the fabric of space-time. Gravitational waves are caused by high-energy events in the cosmos, and fan out in all directions like a stone thrown into a pond.

LIGO was built to detect these waves, and in 20016 made their first discovery of gravitational waves caused by two black holes colliding. That first-ever detection of gravitational waves, confirmed Einstein’s General Theory of Relativity. Three scientists from LIGO won this year’s Nobel Prize in Physics for that discovery. 

Since then, there have been a few more black hole collisions detected. But now, for the first time, the LIGO group and other scientists, have announced they’ve found a NEW source of gravitational waves. Jamie Kennea is associate research professor and head of the Science Operations Team at Penn State that controls NASA’s Swift satellite, tells us what they observed.

“The big news is that LIGO has finally detected a neutron star-neutron star merger,” Kenna said.

Neutron stars are the dense remnants of exploded stars.  Observing them collide for the first time confirmed a couple of theories: that these explosive collisions create gold, platinum and other heavy metals; and that this type of collision causes short gamma ray bursts, which are the brightest electromagnetic events in the universe.

Aaron Tohuvavohu, science operations and research assistant for the Penn State Swift mission, says scientists learned these things in part because they were able to detect the event in-progress.

“With the LIGO data, we’re actually able to probe the system up to a hundred seconds before merger,” Tohuvavohu says. “So they actually hear, with gravitational waves, the two separate neutron stars in the in-spiral and merging phase.  So we finally now know for sure that short gamma ray bursts are caused by the merging of two neutron stars.”

And here’s the big news for the future: the age of multi-messenger astronomy is here.  Think of a doctor, who can learn much more about a patient with both a physical exam and an MRI than she can from either one alone.  Well, now scientists can examine events like neutron star collisions through gravitational wave detectors and optical telescopes. 

“What’s happening is we’re opening up a new branch of science,” says Michael Siegel, associate research professor at Penn State and head of the ultraviolet optical telescope in NASA’s Swift mission. “For a long time, the only way you could study the stars was through light. And now, gravitational waves are a new way of looking at the universe. So it’s not only in this case revealing a door that we knew was there. But hopefully in the future will open doors that we didn’t even know were there – discover new phenomena going off in the cosmos that I didn’t even suspect existed.”

The discovery happened on the morning of August 17th, 2017,  as the LIGO observatory alerted scientists around the world.

“We were informed of LIGO’s detection via text message,” said Kennea. 

And Tohuvavohu said he knew “immediately” that he had something very exciting. “This was a really unique detection insofar as the merger event was detected both by LIGO and by two other space telescopes,” he said. “So we immediately knew that this was very special.  No one really expected a joint, coincident detection.”

Kennea says the text from LIGO came during the Penn State Swift team’s daily planning meeting.

“We have a meeting every morning at 9am to decide what Swift is going to look at for the day. And our phones went off, and we’re like ‘Oh, LIGO has found something exciting!’ So at that point, we have to go away to our desks and plan what we’re going to do.”

“Thankfully, this occurred during the daytime, Kennea adds. “Usually the universe is not so kind and will wake us up at 3 in the morning.” 

Kennea hopes Swift will catch many more neutron star collisions .

“Right now, we only have one event,’ He says. “So that means in terms of theories and understanding populations and everything, one event’s not enough.  We need more.”

Kennea explains that while this first observation of a neutron star collision confirms some theories, it raises questions about another.

“And the one event that we have looks weird,” he says.
And what we saw was it was relatively bright in ultraviolet.  Now previously, no one had suspected that we’d see ultraviolet light.  Everyone said, ‘These things are going to look bright in infrared. The ultraviolet light’s going to be obscured.’ We saw it in ultraviolet, and we saw it fade away rapidly in ultraviolet.  So theorists kind of have to now go away and figure it out why this is.”

But Kenna says he welcomes a discovery that comes with a new mystery.

“Yes, absolutely.  As always is the case with science. Whenever we think we know what’s going to happen, something comes up that doesn’t look exactly as we expected. And that’s what we love.”

Kennea says the NASA Swift team at Penn State will be ready to help LIGO find more of these collisions, and solve the new mysteries they may bring.

And in the future, LIGO plans to make it’s gravitational wave alerts public, so that even amateur astronomers can get out their telescopes to help look for the next pair of neutron stars swirling their way toward a collision.  So you might say things are looking up.