EDP Sciences
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Volume 382, Number 3, February II 2002
Page(s) L13 - L16
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20011776

A&A 382, L13-L16 (2002)
DOI: 10.1051/0004-6361:20011776


Population X: Are the super-Eddington X-ray sources beamed jets in microblazars or intermediate mass black holes?

E. Körding, H. Falcke and S. Markoff

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 27 August 2001 / Accepted 12 December 2001)

Recent X-ray observations reveal an increasing number of X-ray sources in nearby galaxies exceeding luminosities of $L_{\rm
x}\ga2 \times 10^{39}~\mbox{erg}\,\mbox{s}^{-1}$ . Assuming isotropic emission, the Eddington limit suggests a population of intermediate-mass black holes of $M_\bullet\gg10~M_\odot$. However, Markoff et al. (2001a) proposed that jets may be contributing to the X-ray emission from X-ray binaries (XRBs), implying that some X-ray sources may be relativistically beamed. This could reduce the required black hole masses to standard values. To test this hypothesis, we investigate a simple X-ray population synthesis model for X-ray point sources in galaxies with relativistic beaming and compare it with an isotropic emission model. The model is used to explain a combined data set of X-ray point sources in nearby galaxies. We show that the current distributions are consistent with black hole masses $M_\odot\la10$ and bulk Lorentz factors for jets in microquasars of $\gamma_{\rm j}\sim5$. Alternatively, intermediate mass black holes up to 1000  $M_\odot$ are required which are distributed in a powerlaw with roughly $\frac{{\rm d}N}{{\rm d}M} \sim M^{-2}$.

Key words: X-rays: binaries -- accretion, accretion disks -- black hole physics -- radiation mechanisms: non-thermal

Offprint request: E. Körding, körding@mpifr-bonn.mpg.de

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