Astronomy & Astrophysics

Free access article

Issue		A&A Volume 415, Number 1, February III 2004
Page(s)		1 - 7
Section		Astrophysical processes
DOI		10.1051/0004-6361:20034587

A&A 415, 1-7 (2004)
DOI: 10.1051/0004-6361:20034587

Kerr black holes as retro-MACHOs

F. De Paolis¹, A. Geralico^{1, 2}, G. Ingrosso¹, A. A. Nucita¹ and A. Qadir^{3, 4}

¹ Dipartimento di Fisica, Università di Lecce, and INFN, Sezione di Lecce, via Arnesano, CP 193, 73100 Lecce, Italy
² International Center for Relativistic Astrophysics - ICRA, University of Rome "La Sapienza", 00185 Roma, Italy
³ Department of Mathematics, Quaid-i-Azam University, Islamabad, Pakistan
⁴ Department of Mathematical Sciences, King Fahd university of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

(Received 14 May 2003 / Accepted 17 October 2003 )

Abstract
Gravitational lensing is a well known phenomenon predicted by the General Theory of Relativity. It is now a well-developed observational technique in astronomy and is considered to be a fundamental tool for acquiring information about the nature and distribution of dark matter. In particular, gravitational lensing experiments may be used to search for black holes. It has been proposed that a Schwarzschild black hole may act as a retro-lens (Holz & Wheeler 2002) which, if illuminated by a powerful light source (e.g. the Sun), deflects light ray paths to large bending angles so that the light may reach the observer. Here, by considering the strong field limit in the deflection angle and confining our analysis to the black hole equatorial plane, we extend the Holz-Wheeler results to slowly spinning Kerr black holes. By considering the Holz-Wheeler geometrical configuration for the lens, source and observer we find that the inclusion of rotation does not substantially change the brightness of the retro-lensing images with respect to the Schwarzschild case. We also discuss the possibility that the next generation space-based telescopes may detect such retro-images and eventually put limits on the rotational parameter of the black hole.

Key words: gravitation -- gravitational lensing

Offprint request: F. De Paolis, Francesco.DePaolis@le.infn.it

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