Appearance of an Infalling Star in Black Holes with Multiple Photon Spheres

Abstract

Photon spheres play a pivotal role in the imaging of luminous objects near black holes.In this paper, we examine observational appearances of a star freely falling in hairy black holes, which can possess one or two photon spheres outside the event horizon. When there exists a single photon sphere, the total luminosity measured by distant observers decreases exponentially with time at late times. Due to successive arrivals of photons orbiting around the photon sphere different times, a specific observer would see a series of light flashes with decreasing intensity, which share a similar frequency content. Whereas in the case with two photon spheres, photons temporarily trapped between the photon spheres can cause a peak of the total luminosity, which is followed by a slow exponential decay, at late times. In addition, these photons lead to one more series of light flashes seen by the specific observer.

Quasinormal modes and greybody factor of a Lorentz-violating black hole

“…The detection of the gravitational waves by Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo [32] and the first picture of the black hole by Event Horizon Telescope (EHT) [33] label that we have entered the multi-messenger astronomy. The study on the gravitational waves and black hole shadows have been attracted a lot of attentions [34][35][36][37][38][39][40][41]. For the gravitational waves of a binary black hole merger system, there are three parts: inspiral, merger, and ringdown.…”