Research in the laboratory of Prof. Doron Chelouche aims to understand the means by which material is being funneled into giant black holes that reside at the centers of galaxies, and occasionally making their central regions shine copiously (as can be seen in the particular image on the left), in which case they are known as quasars or active galactic nuclei. To study these extreme compact environments, and resolve their structure, direct imaging techniques are insufficient, and complementary methods must be sought. In a recent study, which was published in Nature Astronomy, minute light-intensity variations were discovered from the central region of a galaxy. These were traced across the optical range of the electromagnetic spectrum (from blue to red wavelengths), and it was found that short time-delays, of order a day, exist between the light variations at different wavelengths. This phenomenon is explained by the effect of light echoing, which can be used to map the inner regions around the black hole. This effect is conceptually analogous to the tracing of echoes from a playing piano that reverberate across a concert hall, thus providing information about its size and geometry. The results pointed, for the first time, to the fact that a significant fraction of the light emitted in the optical range from the vicinity of the black hole originates not from its immediate environs (the accretion disk) but rather from larger scales, which are associated with farther away gas components (possibly the broad line region), whose emission had been significantly under-estimated. If general to other black hole systems, then the findings imply that the optical spectra of quasars cannot be directly used to study accretion physics. Further, the results hint that current estimates for the size of the accretion disk in quasars may be over-estimated, thereby potentially solving a long-standing puzzle with implications for the physics of accretion in the general astrophysical context. More information may be found here: Astrophysical research at the Faculty of Natural Sciences, Nature Astornomy, Ynet, HCTPA