Astronomy & Astrophysics

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Issue		A&A Volume 420, Number 3, June IV 2004
Page(s)		881 - 888
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20034035

A&A 420, 881-888 (2004)
DOI: 10.1051/0004-6361:20034035

On the contribution of microlensing to X-ray variability of high-redshifted QSOs

A. F. Zakharov^{1, 2, 3, 4}, L. C. Popovic^{5, 6, 7} and P. Jovanovic^{5, 7}

¹ National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, China
² Institute of Theoretical and Experimental Physics, 25 B.Cheremushkinskaya st., Moscow 117259, Russia
³ Astro Space Centre of Lebedev Physics Institute, Moscow, Russia
⁴ Isaac Newton Institute of Chile, Moscow Branch, Russia
⁵ Astronomical Observatory, Volgina 7, 11160 Beograd, Serbia
⁶ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Isaac Newton Institute of Chile, Yugoslavia Branch, Yugoslavia

(Received 2 July 2003 / Accepted 10 March 2004)

Abstract
We consider a contribution of microlensing to the X-ray variability of high-redshifted QSOs. Such an effect could be caused by stellar mass objects (SMO) located in a bulge or/and in a halo of this quasar as well as at cosmological distances between an observer and a quasar. Here, we not consider microlensing caused by deflectors in our Galaxy since it is well-known from recent MACHO, EROS and OGLE observations that the corresponding optical depth for the Galactic halo and the Galactic bulge is lower than 10^-6. Cosmologically distributed gravitational microlenses could be localized in galaxies (or even in bulge or halo of gravitational macrolenses) or could be distributed in a uniform way. We have analyzed both cases of such distributions. As a result of our analysis, we obtained that the optical depth for microlensing caused by stellar mass objects is usually small for quasar bulge and quasar halo gravitational microlens distributions ( $\tau\sim 10^$ ). On the other hand, the optical depth for gravitational microlensing caused by cosmologically distributed deflectors could be significant and could reach 10^-2 - 0.1 at $z\sim 2$ . This means that cosmologically distributed deflectors may contribute significantlly to the X-ray variability of high-redshifted QSOs ( z>2). Considering that the upper limit of the optical depth ( $\tau\sim 0.1$ ) corresponds to the case where dark matter forms cosmologically distributed deflectors, observations of the X-ray variations of unlensed QSOs can be used for the estimation of the dark matter fraction of microlenses.

Key words: accretion, accretion disks -- gravitational lensing -- galaxies: quasars: general

Offprint request: L. Popovic, lpopovic@aip.de

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