16 Oct 2017 in Research & Technology
Author: Andrew Grant
Seconds after LIGO and Virgo detected gravitational waves from the merger of neutron stars, the Fermitelescope spotted a gamma-ray burst. Nearly six dozen other observatories have joined the fun.
On 17 August the Laser Interferometer Gravitational-Wave Observatory (LIGO), along with sister observatory Virgo, detected a swell of gravitational waves. Less than two seconds after that signal ceased, the Fermi Gamma-Ray Space Telescope identified a flash in the southern sky. Though it would take several hours to verify, researchers had spotted a gamma-ray burst (GRB) triggered by the collision of two neutron stars.
In the ensuing weeks, 70 telescopes in space and on the ground collected data across the electromagnetic spectrum on the GRB, which occurred about 130 million light-years away in the elliptical galaxy NGC 4993, located in the constellation Hydra. The discovery, announced Monday at a Washington, DC, press conference and in some 50 scientific papers, has implications that stretch far beyond gravitational-wave astronomy. It proves that at least some short-duration GRBs are triggered by crashing neutron stars. It offers evidence of the tidal forces that rip the ultradense orbs apart and that the subsequent explosion creates heavy metals such as gold, platinum, and uranium. It even provides a novel means of measuring the universe’s expansion rate. Overall, the discovery sets the standard for how gravitational-wave and electromagnetic observatories can work in tandem to probe the universe’s densest and most energetic objects.
Astronomers have puzzled over the origins of GRBs since a US satellite scanning for Soviet nuclear tests spotted one 50 years ago. A distinct class of GRBs, known as short because the primary burst lasts less than two seconds, has been thought to be triggered by the merger of compact stellar objects (see Physics Today, November 2005, page 17). Spotting such a union ahead of time was impossible until LIGO and Virgo came along. Though astronomers found no electromagnetic counterpart to any of LIGO’s first four detections, those were of coalescing black holes, which presumably emit little radiation.
The 17 August gravitational-wave signal immediately stood out. The two LIGO detectors, in Louisiana and Washington State, registered about 3300 oscillations over more than a minute and a half, some 500 times longer than each of the four chirps LIGO had previously detected. The duration and amplitude of the signal implied the orbital dance of neutron stars, with a combined mass of about 2.7 times that of the Sun, that ultimately merged to form either a larger neutron star or, more likely, a black hole. Due to the event’s relative proximity, the strength of the gravitational radiation was remarkably high: The signal-to-noise ratio of roughly 32 exceeded by a third the already impressive one for LIGO’s first black hole–black hole merger (see Physics Today, April 2016, page 14).
Because Virgo in Italy had begun operating just weeks earlier, researchers had the benefit of a third detector to narrow down the origin of the signal to a 28-square-degree slice of sky. That target area fit within the bounds of the source region suggested by Fermi’s Gamma-Ray Burst Monitor. Between the location and timing, researchers from Fermi and the LIGO–Virgo collaboration were confident that they had observed the same event. They were incredibly lucky. Neutron-star mergers emit gravitational waves in all directions, but gamma rays are thought to be confined to a narrow jet. A paper released earlier this year warned that “we should not expect the first—or even the first several dozen—GW chirps” from neutron-star binaries to be aligned favorably enough to produce detectable GRBs.
A tilted, heavy-metal burst
About 10 hours after the Fermi-observed flash, the Swope Telescope in Chile spotted an optical counterpart to the burst; five other observatories chimed in within the next hour. Over the next several days, optical, IR, and UV observations captured emissions from an event associated with short GRBs: a kilonova. In the final moments of the fatal orbital tango, tidal forces tore neutron-rich matter from the surfaces of the compact stellar cores. Atomic nuclei gobbled up free neutrons faster than they could undergo radioactive decay. On the order of seconds, that rapid neutron-capture mechanism, or r-process, forged about 10 000 Earth masses of elements heavier than iron. Subsequently, the unstable products broke down via fission and alpha and beta decay, emitting heat and leading to a thermally glowing mass of ejecta.
The kilonova observation provides the best evidence of the r-process in action. Based on this single detection, scientists can now confidently say that much of the universe’s gold and platinum and nearly all its uranium are produced in neutron-star mergers, says LIGO executive director David Reitze. In the months ahead, theorists will be contemplating whether there’s room for any other cataclysmic events, particularly core-collapse supernovae, to play a role in the r-process (see the article by John Cowan and Friedrich-Karl Thielemann, Physics Today, October 2004, page 47).
Not every observation of the collision aftermath proved as textbook as that of the kilonova. The intensity of the gamma rays was several orders of magnitude less than expected from a short GRB at that distance. In addition, x-ray astronomers had to wait nine days to glean a signal from the event; radio astronomers’ searches came up empty until 2 September. Those factors led researchers to conclude that the gamma-ray jet, which shoots out perpendicular to the orbital plane of the progenitor neutron stars, wasn’t aligned directly with Earth. Astronomers have been hunting for such an off-axis GRB for 20 years, says Alessandra Corsi, a Texas Tech University astronomer who studies relativistic transients. Studying the jet from an angle may provide a unique suite of information about the jet and its interaction with the interstellar medium, Corsi says.
Tricks of the trade
In addition to creating the conditions for a kilonova, tidal forces also leave their mark on gravitational-wave emissions. They ramp up quickly once the orbital frequency reaches about 50 Hz, sapping the binary system of energy and accelerating the final merger. Astrophysicists would love to pin down the strength of those tides to help expose the composition and density profile of neutron stars’ tightly packed nuclear matter (see the article by Nanda Rea, Physics Today, October 2015, page 62). Analyzing the gravitational waveforms, however, is time-consuming and computationally demanding—running numerical relativity simulations for about a quarter-second’s worth of orbits takes about three weeks. In the meantime, LIGO–Virgo researchers relied on a faster but less precise relativity analysis; the maximum tidal parameter they obtained corresponds to neutron-star radii of about 14 km. They plan to improve that estimate in the coming months.
Beyond exploring explosions and their progenitors, the researchers probed bigger-picture questions. By combining the distance to the source indicated by the gravitational-wave signal with the redshift of the host galaxy, they estimated the universe’s expansion rate, or Hubble constant. Though the initial estimate is rough—between 62 km/s/Mpc and 82 km/s/Mpc—it is consistent with the values derived by other techniques (see the article by Mario Livio and Adam Riess, Physics Today, October 2013, page 41), and the precision will improve with future detections.
Unless another merger is hidden in the LIGO–Virgo data, the next detection won’t come until this time next year at the earliest—both observatories are shut down for upgrades. Yet the single detection will keep scientists occupied for that time and longer. The burst continues to glow, Corsi says, and it’s even getting brighter in the radio band.