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Issue A&A Volume 403, Number 1, May III 2003 Page(s) L15 - L18 Section Letters DOI 10.1051/0004-6361:20030449

A&A 403, L15-L18 (2003)
DOI: 10.1051/0004-6361:20030449

Letter

Energy and time-lag spectra of galactic black-hole X-ray sources in the low/hard state

P. Reig^{1, 2}, N. D. Kylafis^{3, 2} and D. Giannios<sup>3

1</sup>  G.A.C.E, ICMUV, Universitat de Valencia, 46071 Paterna-Valencia, Spain
²  Foundation for Research and Technology-Hellas, 711 10 Heraklion, Crete, Greece
³  University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece

(Received 29 November 2002 / Accepted 25 March 2003)

Abstract
Most, probably all, accreting binaries that are believed to contain a black-hole emit radio waves when they are in the low/hard state. Whenever this radio emission has been resolved, a jet-like structure has become apparent. We propose that Compton upscattering of low-energy photons in the jet can explain both the energy spectra and the time lags versus Fourier frequency observed in the low/hard state of black-hole systems. The soft photons originate in the inner part of the accretion disk. We have performed Monte Carlo simulations of Compton upscattering in a jet and have found that for a rather wide range of values of the parameters we can obtain power-law high-energy X-ray spectra with photon-number index in the range 1.5-2 and power-law time lags versus Fourier frequency with index ~0.7. The black-hole source Cyg X-1 in the low/hard state is well described by our model.

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

Offprint request: P. Reig, pablo@physics.uoc.gr

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