Weak ripples in the fabric of space-time called gravitational waves washed over Earth on Aug. 17, 2017. Unlike earlier detected gravitational waves, these were escorted by light, permitting astronomers to pinpoint the source.
The Hubble Space Telescope from NASA turned its powerful gaze onto the new beacon, attaining both spectra and images. The data obtained will help disclose details about the titanic collision that produced the gravitational waves, and its aftermath.
At 8:41 a.m. EDT on Aug. 17, gravitational waves were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Two seconds later, a short pulse of gamma rays known as a gamma-ray burst was measured by NASA’s Fermi Gamma-ray Space Telescope. A number of observatories, including space telescopes, explored the suspected location of the source, and within just 12 hours several spotted their quarry.
In a distant galaxy known NGC 4993, around 130 million light-years from Earth, a point of light shone where nothing had been before. It was almost a thousand times brighter than a type of stellar flare known as a nova, placing it in a class of objects astronomers refer to as “kilonovae.” It also faded prominently over six days of Hubble observations.
This appears to be the trifecta for which the astronomical community has been waiting: Gravitational waves, a gamma-ray burst and a kilonova all happening together.
Ori Fox, The Space Telescope Science Institute, Baltimore
The collision of two neutron stars, the aged remains of a binary star system, was the source of all three. A neutron star develops when the core of a dying massive star collapses, a process that is extremely violent such that it crushes electrons and protons together to produce subatomic particles known as neutrons. The result is just like a giant atomic nucleus, cramming several Suns' worth of material into one ball only a few miles across.
Two neutron stars once spiraled around each other at blinding speed in NGC 4993. As they came closer together, they resulted in whirling even faster, spinning as fast as a blender close to the end. Powerful tidal forces ripped off large chunks while the remainder collided and then merged, developing a huge neutron star or maybe a black hole. Leftovers were discharged into space. Neutrons, freed from the crushing pressure, turned back into electrons and protons, forming a wide range of chemical elements that are heavier than iron.
We think neutron star collisions are a source of all kinds of heavy elements, from the gold in our jewelry to the plutonium that powers spacecraft, power plants and bombs.
Andy Fruchter, The Space Telescope Science Institute, Baltimore
Several teams of scientists are employing Hubble's suite of spectrographs and cameras in order to study the gravitational wave source. Fruchter, Fox and their colleagues made use of Hubble in order to obtain a spectrum of the object in infrared light. Astronomers split the light of the source into a rainbow spectrum in order to probe the chemical elements that are present. The spectrum displayed several broad bumps and wiggles that indicate the formation of some of the heaviest elements in nature.
“The spectrum looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear. It tied this object to the gravitational wave source beyond all reasonable doubt,” said Andrew Levan of the University of Warwick in Coventry, England, who headed one of the proposals for Hubble spectral observations. NialTanvir of the University of Leicester, England, headed additional spectral observations.
It is possible to use spectral lines as fingerprints in order to identify individual elements. However, this spectrum is rather challenging to interpret.
“Beyond the fact that two neutron stars flung a lot of matter out into space, we’re not yet sure what else the spectrum is telling us,” explained Fruchter. “Because the material is moving so fast, the spectral lines are smeared out. Also, there are all kinds of unusual isotopes, many of which are short-lived and undergo radioactive decay. The good news is that it’s an exquisite spectrum, so we have a lot of data to work with and analyze.”
Hubble also picked up visible light from the event that slowly faded over the course of several days. Astronomers assume that this light resulted from a powerful “wind” of material speeding outward. These observations suggest that astronomers viewed the collision from above the orbital plane of the neutron stars. If observed from the side (along the orbital plane), matter ejected during the merger would have concealed the visible light and only infrared light would be have been visible.
“What we see from a kilonova might depend on our viewing angle. The same type of event would appear different depending on whether we’re looking at it face-on or edge-on, which came as a total surprise to us,” said Eleonora Troja of the University of Maryland, College Park, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Troja is also a key investigator of a team employing Hubble observations to study the object.
Now, the gravitational wave source is extremely close to the Sun on the sky for Hubble and other observatories to study. In November, it will come back into view. Until then, astronomers will be working meticulously on learning all they can about this unique event.
Additionally, the launch of NASA’s James Webb Space Telescope will offer an opportunity to study the infrared light from the source, if that glow remains detectable in the years and even months to come.
The Hubble Space Telescope is a project of global cooperation between NASA and the European Space Agency (ESA). The telescope is managed by NASA’s Goddard Space Flight Center. The Space Telescope Science Institute (STScI) in Baltimore executes Hubble science operations. The Association of Universities for Research in Astronomy Inc., in Washington, D.C., operates STScI for NASA.