Ja n 19 99 Hard X-ray lags in GRO J 1719 − 24

We have used the Fourier cross spectra of GRO J1719−24, as obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20–50 and 50–100 keV energy bands as a function of Fourier frequency in the interval 0.002–0.488 Hz. Our analysis covers the entire ∼ 80 day X-ray outburst of this black-hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50–100 keV band lag those in the 20–50 keV energy band by an approximately constant phase difference of 0.072± 0.010 rad in the frequency interval 0.02–0.20 Hz. The peak phase lags in the interval 0.02–0.20 Hz are about twice those of Cyg X-1 and GRO J0422+32. These results are consistent with models for Comptonization regions composed of extended non-uniform clouds around the central source. Subject headings: accretion — stars: binaries: close — stars: individual: GRO J1719−24 — X-rays: stars