On 17 August 2017 the LIGO and Virgo detectors observed the gravitational waves emitted by two neutron stars as they spiraled into each other and merged. Seventy observatories promptly trained their instruments on the event, dubbed GW170817, to look for an afterglow. They found one in the elliptical galaxy NGC 4993. Half a day after the merger, the afterglow’s blackbody-like spectrum peaked in the UV. As the afterglow cooled and dimmed, the peak shifted through the visible; after 10 days it was in the near-IR.
The spectral evolution is consistent with the thermalization of merger remnants by radioactive nuclides created by the rapid capture sequence, or r-process. Neutrons slam into iron nuclei to produce, via neutron capture, new, successively heavier species. In 1974 James Lattimer and David Schramm proposed that the ripping apart of a neutron star in a close encounter with a black hole creates conditions favorable to the production of heavy elements via the r-process. When two neutron stars rip each other apart as they merge, similar conditions prevail. GW170817’s afterglow vindicated the notion that elements heavier than iron are produced in neutron star mergers.
Unfortunately for astronomers, the merger’s host galaxy is in the constellation Hydra, which sprawls between Virgo and Cancer. The Sun entered Virgo on 23 August. As NGC 4993 left the night sky and entered the twilight sky, it became progressively more challenging to observe. Because of their vantage above Earth’s atmosphere, satellite observatories could observe NGC 4993 for longer. One in particular, the Spitzer Space Telescope, had the sensitivity and instrumentation to observe the afterglow as its peak shifted into the mid-IR.
Mansi Kasliwal of Caltech and her collaborators used Spitzer to observe the afterglow at 43 days and 73 days after GW170817. They made a third observation after 264 days, when NGC 4993 had returned to the night sky. By using two filters—one centered on 3.6 μm, the other on 4.5 μm—Kasliwal and her colleagues found that the afterglow retained a blackbody-like spectrum up to the second observation and that it had continued to dim (by a factor of 0.16) and cool (to 430 K). The afterglow was undetectable in the third observation.
The longevity of the afterglow is significant. The distribution of heavy elements in the solar neighborhood has three peaks at mass numbers 70–88, 120–140, and 180–200. For the remnants to continue glowing two months after the merger, their source of heat—the radioactive r-process nuclides—must have half-lives of 10 days or more. Only nine meet that criterion, and they are all heavy: barium-140, praseodymium-143, neodymium-147, europium-156, osmium-191, radium-223, radium-225, protactinium-233, and thorium-234. The Spitzer observations suggest that all neutron star mergers can produce all the r-process nuclides, even the heaviest. (M. M. Kasliwal et al., Mon. Not. R. Astron. Soc., 2019, doi:10.1093/mnrasl/slz007.)
Editor’s note, 5 November: The article has been updated to list protactinium-233 as one of the nine heavy, long-lived r-process nuclides. A previous version of the article incorrectly stated the nuclide as palladium-233.