Mon. Not. R. Astron. Soc., 351, 1359-1364 (2004/July-2)
Radio and X-ray observations during the outburst decay of the black hole candidate XTE J1908+094.
JONKER P.G., GALLO E., DHAWAN V., RUPEN M., FENDER R.P. and DUBUS G.
Abstract (from CDS):
Obtaining simultaneous radio and X-ray data during the outburst decay of soft X-ray transients is a potentially important tool to study the disc-jet connection. Here we report results of the analysis of (nearly) simultaneous radio [Very Large Array (VLA) or Westerbork Synthesis Radio Telescopes (WSRT)] and Chandra X-ray observations of XTE J1908+094 during the last part of the decay of the source after an outburst. The limit on the index of a radio-X-ray correlation that we find is consistent with the value of ∼0.7 found for other black hole candidates in the low/hard state. Interestingly, the limit that we find seems more consistent with a value of 1.4, which was recently shown to be typical for radiatively efficient accretion flow models. We further show that, when the correlation index is the same for two sources, one can use the differences in normalization in the radio-X-ray flux correlation to estimate the distance towards the sources if the distance of one of them is accurately known (assuming black hole spin and mass and jet Lorentz factor differences are unimportant or minimal). Finally, we observed a strong increase in the rate of decay of the X-ray flux. Between 2003 March 23 and April 19, the X-ray flux decayed by a factor ∼5, whereas between 2003 April 19 and May 13, the X-ray flux decreased by a factor ∼750. The source (0.5-10 keV) luminosity at the last Chandra observation was L~ 3 x 1032(d/8.5 kpc)2/ergs.