Letter to the Editor

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

¹ 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

Corresponding author: P. Reig, pablo@physics.uoc.gr

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.