Bulletin of Taras Shevchenko National University of Kyiv. Astronomy, no. 67, p. 23-27 (2023)


Valery ZHDANOV, DSc (Phys. & Math.), Prof.
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine


Background. Ring-like structures may appear in some images of accretion disks around relativistic compact objects, which represent a possible alternative to conventional black holes. We present a qualitative treatment of photometric microlensing effects in case of such structures.

Methods. Amplification curves are constructed for simple models of a thin accretion disk which is the source in the gravitational lens system. Two models of a source consisting of two unconnected regions are considered: (i) a circular area in the center and a ring, (ii) two rings with comparable integral brightness. We use the linear caustic approximation. The graphs show amplification curves giving the increase of the microlensed radiation flux when the caustic moves in the direction of its “tail” (the region, where there is an amplification). We first considered microlensing of a single ring. The simulation was carried out for a number of radii and thicknesses of the ring, observed in full face.

Results. As expected, when the caustic is far from the source and its tail covers the source, the amplification is approximately the same as in the case of a point source. Then, when the caustic approaches the left side of the ring, the graph reaches a maximum, which is sharper, the smaller the thickness of the ring. Thereafter, the total amplification decreases relatively slowly. When the caustic approaches the right side of the ring, there is a sharp drop almost to zero over a small interval of the order of the ring thickness. In the case of two rings the amplification curve can be obtained by a superposition of different elements, yielding a graph with two maxima, the height of which depends on the contribution of the components, and two “knees”. The latter circumstance distinguishes the case of two rings from that with one ring and a region in the center.

Conclusions. We summarize that in the presence of ring regions, characteristic features of the light curves appear, which can serve as an indication of the possible existence of these regions. However, different models can give quite similar light curves, and additional independent data are needed to unambiguously identify the source.

Key words
Gravitational lenses, fold caustic, microlensing, ring-like source, light curves.


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