August 17, 2017 was a watershed moment in the nascent field of gravitational-wave astronomy. On this date, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a ripple in space-time, hereafter known as GW170817, originating near the constellation Hydra. A short while later, NASA’s Fermi Gamma-ray Burst Monitor detected a faint pulse of gamma rays in this same location. When the full might of the world’s telescopes, both ground- and space-based, trained their sights on this location, a new source was detected.
For the first time, astronomers captured the electromagnetic signature of a gravitational wave event. It was a signal whose most likely origin was the merger of two neutron stars—something known as a kilonova. A kilonova is an extremely* energetic event, a thousand times brighter than an exploding star. To put this into perspective, a kilonova releases more energy in a fraction of a second than the Sun will produce in a million years—and the Sun is no slouch in energy production: it produces enough energy in one second to meet the electricity needs of the entire world for over 600,000 years! The extreme environment produced in a kilonova also seeds the surrounding space with heavy elements like gold and platinum. Indeed, the metals that we consider precious and place high value on, fully embrace their “precious” qualifier. They were literally forged in some of the most energetic events in the universe.
* superlatives fail to really convey the energy of this event!
The LIGO detection of GW170817 also begs the question, if two neutron stars merge, and no one is there to hear it, do they make a sound? The answer is a resounding yes!
The Hubble Perspective
Soon after the LIGO team announced its detection of GW170817, a target of opportunity was triggered and Hubble began its study of the optical counterpart. Over the course of a week, Hubble snapped a series of photos of NGC 4993 using the near-infrared filters on its Wide Field Camera 3 detector (WFC3/IR). We were able to compile a color image using these new data sets, combined with a slightly older image taken in the optical light with the Advanced Camera for Surveys (ACS). The source appeared in this image as a bright yellow dot since its signal was strongest in the near-infrared. This makes sense when you remember that we’re seeing the afterglow of an extremely powerful explosion as it cools off and decays over time. The source was actually visible across nearly the entire spectrum from radio to X-rays, peaking in the ultraviolet at first and then settling into an infrared-dominated state before fading out. The Hubble observations, in particular the near-infrared spectrum of the source, precisely matched up with the theoretical physics of a neutron star merger—removing any doubts that this was, in fact, the electromagnetic afterglow of a gravitational-wave source.
From a public outreach and news perspective, we had our beautiful image to go with the story, but the image alone would not convey the significance of this event. We also needed to show the source changing over time. Fortunately, enough data were acquired at each visit to be able to make color images at each step of the source’s evolution. Cosmic events evolving on a human time-scale never cease to amaze!
A Ripple of Insight
You may have noticed that the main image for this article contains a pullout graphic showing GW170817 diminishing in brightness in three black-and-white panels spaced over six days. You may be wondering why black-and-white images were used for those pullouts instead of color images. To answer this question, we must start with the fact that the source was only observed with two near-infrared filters at each visit over the span of a week.
We can and do routinely make three-color (RGB) images from observations taken in two filters using a technique known as “pseudo-green” which combines half the light of each filter to create the green channel of an image. There is less color information than a true three-color image, but the results can still be just as fantastic. For example, an image taken with the F110W filter (near-infrared, 1100 nanometers in wavelength) can be combined with an image taken with the F160W filter (1600 nanometers). The F110W image is a shorter wavelength compared with F160W so we’ll assign them the colors of blue and red respectively, following the chromatic ordering of light. The green channel is derived from a combination of both filters. Below you’ll see how the three snapshots appear in color using this technique. After compiling the color images, we noticed two artifacts with implications towards a clean, understandable graphic for the release.
 Due to the lack of the optical filter used in the main color image, the source now appears differently than in the main image. The source which is yellow in the main image is now white in the pullout.
 In the second frame of the pullout the source appears to become more blue! The source was expected to dim over time, but it was not expected to change color in the near-infrared. The two observations making up the second panel were tagged as belonging to the second set of observations, post gravitational-wave discovery. A change in color would imply some other physical mechanism was at play in the afterglow of the merger. It turned out that due to scheduling constraints, the longer wavelength filter (F160W) observation was actually taken two days after the shorter wavelength observation (F110W). The delay was enough that the source had faded significantly during that time and when combined, this meant that the red channel had faded slightly more than the blue channel, giving a blue bump to the resulting color image.
Ultimately, displaying the color of the source in these images was not as important as showing the intensity fading over time and would have created an unnecessary distraction in the graphic. For simplicity, we decided to use the black-and-white frames for the pullout and reserved the color for the main image.
An editorial decision like this is a very important part of the process of crafting the content of press-release imagery and story. Just as image processing is a delicate balance of aesthetics and science, telling an engaging and informative story through that imagery requires a balance of information and presentation. For more information on this source and links to more material, check out the Hubble press release.