Gravitational waves and emitted light reveal merger of two neutron stars – and a kilonova
Neutron stars are the extremely dense, burnt-out remnants of massive stars, and the merger of two such neutron stars (or one neutron star and a black hole) has been considered a primary target for gravitational wave observations. For the first time ever, on 17 August 2017, the LIGO and Virgo observatories measured a distinctive gravitational-wave signal from such a merger – and what is more, at the same time the merger was also detected by the Fermi and INTEGRAL satellites as a short Gamma-Ray Burst (GRB), i.e. one of the most energetic explosions in the Universe. Follow-up observations across the electromagnetic spectrum provided evidence that the transient was indeed powered by the radioactive decay of heavy elements formed during the cataclysmic event.
“The physical parameters of the transient event match the theoretical predictions for a so-called kilonova from a neutron star merger remarkably well,” states Anders Jerkstrand from the Max Planck Institute for Astrophysics (MPA), who performed the theoretical interpretation of the event. “Especially the rate at which the light from the source gets dimmer over the 10 days or so following the merger is exactly as predicted if the ejecta are dominated by radioactive elements much heavier than iron.”
Enormous amounts of energy are released during a merger of two neutron stars, which produces a GRB. At the same time, dense matter is being expelled at high velocities. Since these ejecta contain high concentrations of free neutrons, the heaviest elements in the Universe can be assembled in a process called rapid neutron capture, or r-process for short (see Cosmic Crashes Forging Gold, press release from 2011).
“The origin of the really heaviest chemical elements in the Universe has baffled the scientific community for quite a long time,” says Hans-Thomas Janka, senior scientist at the MPA, who contributed to models for neutron star mergers for more than two decades (see e.g. Cosmic Vibrations from Neutron Stars, Highlight February 2012). “Now we have the first observational proof for neutron star mergers as sources; in fact, they could well be the main source of the r-process elements.” While most of the matter of the two neutron stars finally ends up in a black hole, the scientists calculate that 1 to 2 percent of the total system mass get ejected and could be converted into heavy elements.
Most of the newly formed elements are radioactive and decay over several days following the merger. The resulting emission heats up the surrounding matter, which can then be observed across different wavelengths. The optical and infrared counterpart for the GRB was found near the core of galaxy NGC 4993, which is only about 130 million light-years away – at the distance predicted from the gravitational wave signal. No transient or astrophysical variability had been seen at this position for more than a year before, making it highly unlikely that this was a chance coincidence.
“While we did have strong hints that neutron star mergers are indeed the progenitors for short GRBs, we did not have proof. The simultaneous observation of this two-second Gamma-Ray Burst by INTEGRAL/Fermi and by gravitational wave detectors is the first conclusive evidence that at least some GRBs are indeed powered by neutron star mergers,” says Rashid Sunyaev, director at the MPA and involved in the analysis of the INTEGRAL observations. “INTEGRAL detects about 20 GRBs per year, and this was quite a faint one, relatively close to us, which makes it a very interesting object to study.”
The relative dimness of this GRB suggests either an off-axis observation or event-dependent variations in the intrinsic brightness distribution. Further study is also needed to settle the question of colours: the kilonova was blue at early times but became redder over the following days. This indicates different contributions to the matter expelled in the neutron star mergers, which was predicted by theoretical models.
Future kilonova detections will allow the astronomers to study both the progenitors and the populations of mergers in more detail. “Actually, the detection of this neutron star merger came surprisingly early, it is only the fifth gravitational wave signal detected ever,” says Anders Jerkstrand. “This could indicate that these events are quite common and we expect quite a few more in the coming years.”