GW170817 and Multi-messenger Astronomy

The first astronomers had only their own eyes as tools, and visible light was their only source of information. Recent instruments have broadened our sight to include all types of electromagnetic radiation, from radio waves to X-rays and gamma rays. Each part of the spectrum is suited to different types of observations and gave us incredible new insight into the cosmos. However, the second decade of the 21st century saw the advent of a fundamentally new kind of astronomy: the detection of gravitational waves.

Gravitational waves, as discussed in a previous post, are the "ripples" in spacetime that propagate in response to the acceleration of massive objects (stars, black holes, and the like). All objects with mass produce these waves, but the vast majority are far too small to detect. It was only with the advent of extremely sensitive instruments that the first detection of gravitational waves was made by LIGO (the Laser Interferometer Gravitational-Wave Observatory) in 2015. This detection, and its immediate successors, were of binary black hole merger events, in which two black holes orbiting one another spiraled inwards and finally combined into a single, larger black hole. The last moments before merging brought exceptionally colossal objects (weighing perhaps dozens of solar masses) to great accelerations, the perfect recipe for producing strong gravitational waves detectible across the cosmos. However, these cataclysmic events were quite dark: little electromagnetic radiation was emitted, and no "visual" evidence for these events accompanied the wave signal. Something quite different occurred in 2017.

On August 17, 2017 at 12:41 UTC, the LIGO detectors at Hanford, Washington and Livingston, Louisiana and the Virgo gravitational wave detector in Italy simultaneously measured an event as shown above (click to enlarge). The two LIGO frequency-time diagrams clearly show a curve that increases in frequency before disappearing at time 0. This corresponds to two inspiraling objects orbiting one another faster and faster before merging finally occurs and the signal stops. In the Virgo diagram, the same line is not very visible, but further analysis of the data nevertheless expose the same signal from the noise. The gravitational wave event, designated GW170817, was genuine.

Having three detectors at different points on the Earth measure the event allowed a better triangulation of the location of the source than had occurred previously (when LIGO and Virgo were not simultaneously active).

The above figure shows a visualization of the celestial sphere (representing all possible directions in the sky from which the signal could have come) and locations from which the signal data suggest the signal originated. The green zone is the highest probability region taking all three instruments into account. This area is still 31 square degrees, quite large by astronomical standards. Fortunately, corroboration of the event came immediately from an entirely separate source.

The above figure (click to enlarge) shows at the bottom the same gravitational wave signal from before. The rest of the data come from the Fermi Gamma-ray Space Telescope and the International Gamma Ray Astrophysics Laboratory, both satellites in Earth orbit. As their names suggest, they search the cosmos for astrophysical sources of high-energy gamma rays. In particular, they monitor the cosmos for gamma-ray bursts (GRBs), especially intense flashes of radiation that typically accompany only the most explosive events, such as supernovae. As the figure shows, less than two seconds after the gravitational wave signal stopped (indicated the merger of two orbiting objects), there was an elevated count of gamma rays in each detector across the different photon energy levels. The source of this burst is indicated by a reticle in the celestial sphere figure above, lying right within the estimated location of the merger! It appeared that this merger had an electromagnetic counterpart! Further, analysis of the gravitational waves indicated that the masses of the two objects were around 1.36-2.26 and 0.86-1.36 solar masses (these were the uncertainty ranges), respectively, not heavy enough for black holes. What was going on?

The conclusion drawn from these events was that the merger was not of black holes, but of neutron stars, compact remnants of large stars that were yet not massive enough to collapse into black holes. An artist's conception of a binary neutron star black hole merger is shown above. Following the initial identification of the event, countless telescopes around the world trained on the event the very same day after a notice was released around 13:00 UTC, hoping to observe more following the merger.

And they were not disappointed. Less than a day after the initial gamma ray burst had faded, the source began to appear at other frequencies, and remained bright for several weeks before fading. The above figure shows the Hubble image of the merger's host galaxy, NGC 4993. This galaxy is at a distance of roughly 130 million light-years, and even at this distance, the collision of the neutron stars was clearly visible against the billions of other stars. Finally, the chart below demonstrates just how well documented the event was:

Many different instruments took images in X-rays as well as ultraviolet, visible, infrared, and radio waves. The horizontal axis indicates the rough timeline of events (on a logarithmic scale) in each part of the electromagnetic spectrum, stretching from less than a day to several weeks after the merger. Several representative images of NGC 4993 and the source within are shown at bottom.

Without extensive collaboration within the astronomical community, collecting this wealth of data on this binary neutron star merger would not have been possible. This marked the first time in history that a single event was measured in both gravitational waves and electromagnetic waves, not to mention how thoroughly the merger was photographed across the spectrum. This coordinated observation is known as multi-messenger astronomy, and may have profound implications on our future understanding of the universe. Some of what we learned from the binary neutron star merger is discussed in the next post.

Note: Most of the figures above are taken from the open access papers detailing the discovery and analysis of the binary neutron star merger. For further reading on the event, links to these papers may be found in the sources below.

Sources: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.119.161101, https://arxiv.org/pdf/1710.05834.pdf, http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9/pdf