Two neutron stars, ultra-dense remnants of massive stars, merge into one, and the space around them shudders: a gravitational wave is born, a ripple in the very fabric of spacetime. Almost simultaneously, a narrow stream of matter, the jet, erupts from the scene. It moves at nearly the speed of light and, punching through the outer shell of ejected material, triggers a flash.
The authors took an existing computer model of the merger and launched a jet in it, varying the start time and power. They tracked how the jet's shock wave breaks through the shell to the layer where light can escape (the photosphere). Then they calculated the apparent brightness of the flash for different viewing angles and two shell transparency options. It turned out that the delay between the gravitational signal and the light hardly depends on either the viewing angle or the shell's opacity. The difference is no more than a tenth of a second.
This result was compared with a real event — GRB 170817A, detected in 2017. The best match was the model where the jet almost got stuck in the shell, failing to break through completely. That scenario gave the correct brightness and duration of the signal.
The delay between the gravitational wave and the light from a neutron star merger is a cosmic stopwatch that doesn't lose time even when we view the explosion at an angle.
In Carl Sagan's novel 'Contact,' a signal from space also arrives through different channels with a delay, and deciphering that delay becomes the key to understanding the message.