Hints of gravitational waves found in the stars

Energetic events, such as this artist’s rendition of a binary-star merger, are thought to create gravitational waves that cause ripples in space and time. 

 Credit: NASA

Scientists have shown how gravitational waves, invisible ripples in the fabric of space and time that propagate through the universe, might be "seen" by looking at the stars.

The new model proposes that a star that oscillates at the same frequency as a gravitational wave will absorb energy from that wave and brighten, an overlooked prediction of Einstein's 1916 theory of general relativity.

The study, which was published today in the Monthly Notices of the Royal Astronomical Society: Letters, contradicts previous assumptions about the behavior of gravitational waves.

"It's pretty cool that a hundred years after Einstein proposed this theory, we're still finding hidden gems," said Barry McKernan, a research associate in the Museum's Department of Astrophysics, who is also a professor at CUNY's Borough of Manhattan Community College; a faculty member at CUNY's Graduate Center; and a Kavli Scholar at the Kavli Institute for Theoretical Physics.

Gravitational waves can be thought of like the sound waves emitted after an earthquake, but the source of the "tremors" in space are energetic events like supernovae (exploding stars), binary neutron stars (pairs of burned-out cores left behind when stars explode), or the mergers of black holes and neutron stars.

Although scientists have long known about the existence of gravitational waves, they've never made direct observations but are attempting to do so through experiments on the ground and in space.

Part of the reason why detection is difficult is because the waves interact so weakly with matter but McKernan and his colleagues from CUNY, the Harvard-Smithsonian Center for Astrophysics, the Institute for Advanced Study, and Columbia University, suggest that gravitational waves could have more of an effect on matter than previously thought.

The new model shows that stars with oscillations, vibrations, that match the frequency of gravitational waves passing through them can resonate and absorb a large amount of energy from the ripples.

"It's like if you have a spring that's vibrating at a particular frequency and you hit it at the same frequency, you'll make the oscillation stronger," McKernan said. "The same thing applies with gravitational waves."

If these stars absorb a large pulse of energy, they can be "pumped up" temporarily and made brighter than normal while they discharge the energy over time.

This could provide scientists with another way to detect gravitational waves indirectly.

"You can think of stars as bars on a xylophone, they all have a different natural oscillation frequency," said co-author Saavik Ford, who is a research associate in the Museum's Department of Astrophysics as well as a professor at the Borough of Manhattan Community College, CUNY; a faculty member at CUNY's Graduate Center; and a Kavli Scholar at the Kavli Institute for Theoretical Physics.

"If you have two black holes merging with each other and emitting gravitational waves at a certain frequency, you're only going to hit one of the bars on the xylophone at a time but because the black holes decay as they come closer together, the frequency of the gravitational waves changes and you'll hit a sequence of notes. So you'll likely see the big stars lighting up first followed by smaller and smaller ones."

The work also presents a different way to indirectly detect gravitational waves. From the perspective of a gravitational wave detector on Earth or in space, when a star at the right frequency passes in front of an energetic source such as merging black holes, the detector will see a drop in the intensity of gravitational waves measured."

"In other words, stars, including our own Sun, can eclipse background sources of gravitational waves.

"You usually think of stars as being eclipsed by something, not the other way around," McKernan said.

The researchers will continue to study these predictions and try to determine how long it would take to observe these effects from a telescope or detector.

More Information:
B. McKernan, K.E.S. Ford, B. Kocsis, Z. Haiman. "Stars as resonant absorbers of gravitational waves." Monthly Notices of the Royal Astronomical Society: Letters, 2014 - arxiv.org/abs/1405.1414

Hubble Image: Spiral galaxy NGC 1084 holds many Supernovae

Credit: NASA, ESA, and S. Smartt (Queen's University Belfast), Acknowledgement: Brian Campbell

In this Hubble image, we can see an almost face-on view of the galaxy NGC 1084.

At first glance, this galaxy is pretty unoriginal.

Like the majority of galaxies that we observe it is a spiral galaxy, and, as with about half of all spirals, it has no bar running through its loosely wound arms.

However, although it may seem unremarkable on paper, NGC 1084 is actually a near-perfect example of this type of galaxy, and Hubble has a near-perfect view of it.

NGC 1084 has hosted several violent events known as supernovae, explosions that occur when massive stars, many times more massive than the sun, approach their twilight years.

As the fusion processes in their cores run out of fuel and come to an end, these stellar giants collapse, blowing off their outer layers in a violent explosion.

Supernovae can often briefly outshine an entire galaxy, before then fading away over several weeks or months.

Although directly observing one of these explosions is hard to do, in galaxies like NGC 1084 astronomers can find and study the remnants left behind.

Astronomers have noted five supernova explosions within NGC 1084 over the past half century.

These remnants are named after the year in which they took place—1963P, 1996an, 1998dl, 2009H, and 2012ec.

Supernova Legacy Survey: Powerful ancient explosions explain new class of supernovae

A small portion of one of the fields from the Supernova Legacy Survey showing SNLS-06D4eu and its host galaxy (arrow). 

The supernova and its host galaxy are so far away that both are a tiny point of light that cannot be clearly differentiated in this image. 

The large, bright objects with spikes are stars in our own galaxy. 

Every other point of light is a distant galaxy. 

Credit: UCSB

Astronomers affiliated with the Supernova Legacy Survey (SNLS) have discovered two of the brightest and most distant supernovae ever recorded, 10 billion light-years away and a hundred times more luminous than a normal supernova. Their findings appear in the Dec. 20 issue of the Astrophysical Journal.

These newly discovered supernovae are especially puzzling because the mechanism that powers most of them—the collapse of a giant star to a black hole or normal neutron star—cannot explain their extreme luminosity.

Discovered in 2006 and 2007, the supernovae were so unusual that astronomers initially could not figure out what they were or even determine their distances from Earth.

"At first, we had no idea what these things were, even whether they were supernovae or whether they were in our galaxy or a distant one," said lead author D. Andrew Howell, a staff scientist at Las Cumbres Observatory Global Telescope Network (LCOGT) and adjunct faculty at UC Santa Barbara.

"I showed the observations at a conference, and everyone was baffled. Nobody guessed they were distant supernovae because it would have made the energies mind-bogglingly large. We thought it was impossible."

One of the newly discovered supernovae, named SNLS-06D4eu, is the most distant and possibly the most luminous member of an emerging class of explosions called superluminous supernovae.

These new discoveries belong to a special subclass of superluminous supernovae that have no hydrogen.

The new study finds that the supernovae are likely powered by the creation of a magnetar, an extraordinarily magnetized neutron star spinning hundreds of times per second.

Magnetars have the mass of the sun packed into a star the size of a city and have magnetic fields a hundred trillion times that of the Earth.

While a handful of these superluminous supernovae have been seen since they were first announced in 2009, and the creation of a magnetar had been postulated as a possible energy source, the work of Howell and his colleagues is the first to match detailed observations to models of what such an explosion might look like.

Co-author Daniel Kasen from UC Berkeley and Lawrence Berkeley National Lab created models of the supernova that explained the data as the explosion of a star only a few times the size of the sun and rich in carbon and oxygen.

The star likely was initially much bigger but apparently shed its outer layers long before exploding, leaving only a smallish, naked core.

More information: dx.doi.org/10.1088/0004-637X/779/2/98