Bursts from the Cold War

In August 1963 three nuclear powers—the Soviet Union, the UK, and the US—signed the Partial Nuclear Test Ban Treaty. (France, which tested its first nuclear weapon in 1960, declined to sign; China’s first test took place the following year.)

The treaty bans the testing of nuclear weapons in three settings where radioactive fallout could reach and harm humans and other life forms: Earth’s surface, atmosphere, and oceans. Because even trace amounts of the nuclides produced in fission and fusion bombs are readily detectable, evading the ban was difficult—except, perhaps, in the very highest reaches of the atmosphere.

Worried that the Soviet Union might test nuclear weapons in space, the US developed and launched a series of satellites known as Vela (from velador, the Spanish word for “watchman” not the constellation in the southern sky, Vela, as I mistakenly thought). Each of the Vela satellites was equipped with sensors that could detect the x rays, gamma rays, and neutrons emitted by nuclear explosions. On 2 July 1967 the Vela 3 and Vela 4 satellites detected a flash of gamma rays unlike any known or conceivable signature of a nuclear weapon.

As more of the anomalous flashes were detected and as the instrumentation on subsequent Vela satellites improved, it became clear to the data-analyzing team at Los Alamos National Laboratory that the flashes came from outside the solar system. The team published its findings in a 1973 Astrophysical Journal paper entitled “Observations of gamma-ray bursts of cosmic origin.” A new field of study, gamma-ray bursts (GRBs), was born.

In the 40 years since the ApJ paper appeared, astronomers have established that GRBs originate in distant galaxies and come in two distinct flavors based on their duration: short (less than 2 seconds; median 300 ms) and long (greater than 2 s; median 40 s).

Several lines of evidence point persuasively to the collapse of massive stars as the origin of long-duration GRBs, notably the bursts’ association with galaxies undergoing star formation and the coincidence of some long-duration GRBs with supernova explosions.

The origin of short-duration bursts has been harder to pin down. But now comes a paper from Nial Tanvir of the University of Leicester in the UK and his colleagues that ties a short-duration GRB detected on 3 June by NASA’s Swift satellite with a delayed flare of near-IR light detected by the Hubble Space Telescope.

The near-IR flare is different from the afterglow that follows GRBs and spans the electromagnetic spectrum from radio to x ray. Unlike the afterglow, which wanes soon after the burst of gamma rays, the flare apparently peaked weeks later. Such behavior, dubbed a “kilonova,” is expected if the GRB is produced by the merger of two neutron stars—which turns out to be the leading contender for the origin of short-duration GRBs.

Besides providing welcome evidence in support of the neutron-star-merger hypothesis, Tanvir and his colleagues’ observation could also supply a means to identify sources of gravitational waves, which result from the rapid movement of massive objects. Neutron-star mergers could also seed the universe with heavy elements via the nucleosynthesis of their neutron-rich material.

The discovery also has an afterglow of sorts from the Cold War. Simulating what happens when two neutron stars merge entails carrying out a fully relativistic calculation that spans cosmic and nuclear length scales. The Soviet Union failed to keep up with the US in supercomputing, and Russia did not rejoin the race. Now, if Russian astrophysicists want to simulate a neutron-star merger, they have to run their code on a machine from the US, Japan, or China.