On the origin of radio emission in the X-ray states of XTE J1650-500 during the 2001-2002 outburst

We report on simultaneous radio and X-ray observations of the black hole candidate XTE J1650-500 during the course of its 2001-2002 outburst. The scheduling of the observations allowed us to sample the properties of XTE J1650-500 in different X-ray spectral states, namely, the hard state, the steep power-law state, and the thermal dominant state, according to the recent spectral classification of McClintock & Remillard. The hard state is consistent with a compact jet dominating the spectral energy distribution at radio frequencies; however, the current data suggest that its contribution as direct synchrotron emission at higher energies may not be significant. In that case, XTE J1650-500 may be dominated by Compton processes (either inverse Comptonization of thermal disk photons and/or synchrotron self-Compton radiation from the base of the compact jet) in the X-ray regime. We surprisingly detect a faint level of radio emission in the thermal dominant state that may be consistent with the emission of previously ejected material interacting with the interstellar medium, similar (but on a smaller angular scale) to what was observed in XTE J1550-564 by Corbel and coworkers. Based on the properties of radio emission in the steep power-law state of XTE J1650-500 and taking into account the behavior of other black hole candidates (namely, GX 339-4, XTE J1550-564, and XTE J1859+226) while in the intermediate and steep power-law states, we are able to present a general pattern of behavior for the origin of radio emission in these two states that could be important for understanding the accretion-ejection coupling very close to the black hole event horizon.