When massive stars exhaust their nuclear fuel, they undergo a gravitational collapse and explode as supernovae. If a massive companion exists nearby, it may merge with the compact remnant left by the explosion, forming a binary black hole system. The interaction and final merger of the binary components further release enormous amounts of energy in the form of gravitational waves, the ripples in spacetime predicted by Einstein's theory of general relativity.
The presence of a rapidly rotating, or spinning, black hole in the binary system would significantly affect the gravitational waveforms. However, due to the complexity of astrophysics involved in the formation and evolution of binary black holes, there is still no consensus on the formation efficiency of rapidly spinning black holes.
By performing extensive computer simulations, the researchers found that the orbital motion and disk's precession in a post-supernova binary black hole system are significantly altered because of the companion black hole's rapid spin. The precession effect makes the accretion disk around the companion black hole display time-dependent variability.
"This variability, imprinted in the X-ray light curves observed from our line of sight, opens up a new way to probe the astrophysical properties of the companion black hole and even constrain the poorly known natal kick velocity distribution," said Prof. Tong Liu from Shanghai Jiao Tong University, the lead author of the study.
The research, published in The Astrophysical Journal Letters, suggests future space missions like Einstein Probe, Lynx, Athena, and future Large Observatory For X-ray Timing (LOFT), which are designed to provide X-ray timing data with high sensitivity and broad energy coverage, will have the potential to unveil these hidden black holes through the discovery and characterization of the predicted variable signals.
