摘要
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.
基金
supported by the National Natural Science Foundation of China(Grant Nos.12105191,11947225,and 11875196)
supported by the International Visiting Program for Excellent Young Scholars of Sichuan University。
