Radio emission and the timing properties of the hard X-ray state of GRS 1915+105

We combine a complete sample of 113 pointed observations taken with the Ros si X-Ray Timing Explorer between 1996 and 1999, monitoring observations tak en with the Ryle Telescope and the Green Bank Interferometer, and selected observations with the Very Large Array to study the radio and X-ray propert ies of GRS 1915+105 when its X-ray emission is hard and steady. We establis h that radio emission always accompanies the hard-steady state of GRS 1915105 but that the radio flux density at 15.2 GHz and the X-ray flux between 2 and 200 keV are not correlated. Therefore, we study the X-ray spectral an d timing properties of GRS 1915+105 using three approaches: first by descri bing in detail the properties of three characteristic observations, then by displaying the time evolution of the timing properties during periods of b oth faint and bright radio emission, and lastly by plotting the timing prop erties as a function of the radio flux density. We find that as the radio e mission becomes brighter and more optically thick, (1) the frequency of a u biquitous 0.5-10 Hz quasi-periodic oscillation (QPO) decreases (2) the Four ier phase lags between hard (11.5-60 keV) and soft (2-4.3 keV) in the frequ ency range of 0.01-10 Hz change sign from negative to positive, (3) the coh erence between hard and soft photons at low frequencies decreases, and (4) the relative amount of low-frequency power in hard photons compared to soft photons decreases. We discuss how these results reflect upon basic models from the literature describing the accretion flow around black holes and th e possible connection between Comptonizing electrons and compact radio jets .