On the relation of hard X-ray peak flux and outburst waiting time in the black hole transient GX 339-4

Aims. We reinvestigate the empirical relation between the hard X-ray peak flux and the outburst waiting time found previously in the black hole transient GX 339-4. We verify the relation using the observed hard X-ray peak flux of the 2007 outburst of GX 339-4, we clarify the nature of faint flares, and we estimate the lower limit to hard X-ray peak flux of the next outburst. Methods. We analyze Swift/BAT data obtained in the past four years. Combined with the CGRO/BATSE and RXTE/HEXTE light curves, the observational data analyzed cover a period of 18 years. Results. The observation of the 2007 outburst confirms the empirical relation discovered before, strengthening the apparent link between the mass in the accretion disk and the peak luminosity of the brightest hard state that the black hole transient can reach. We also show that faint flares with peak fluxes lower than about 0.12 crab do not affect the empirical relation. We predict that the hard X-ray peak flux of the next outburst should be greater than 0.65 crab, which will make it at least the second brightest in hard X-rays since 1991.