Neutron star mergers create powerful kilonovae. Astrophysicists are watching the x-rays to learn more about these cosmic calamities.
The Expanding Debris Cloud From the Kilonova Tells the Story of What Happens When Neutron Stars Collide. The event causes both gravitational waves and emissions of electromagnetic energy. In 2017 the LIGO-Virgo gravitational-wave observatories detected a merger of two neutron stars about 130 million light-years away in the galaxy NGC 4993. The merger is called GW170817, and it remains the only cosmic event observed in both gravitational waves and electromagnetic radiation.
When two neutron stars merge, it produces a cloud of debris and a burst of light called a kilonova. A pair of astronomers introduced thein 1998. They said that the mergers synthesized radioactive nuclei that provide a long-term heat source for the expanding debris envelope. The optical and infrared light in the kilonova comes from the decay of elements like platinum and gold created during the merger.
The first explanation is that a kind of shock is involved, akin to a sonic boom. When the cloud of debris from the kilonova slams into gas around GW170817, the material is heated. The temperature is enough to produce x-rays and can account for the steady kilonova afterglow Chandra detected. Further observations should determine the cause of the x-ray afterglow. Astronomers will continue to observe GW170817 in both x-rays and radio waves. If the glow comes from the kilonova, the radio emissions should brighten in the coming months and years. But if the glow comes from material falling into a black hole, then the x-rays should stay steady or decline rapidly, but there’ll be no radio emissions over time.
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