On the peak radio and X-ray emission from neutron star and black hole candidate X-ray transients

We have compiled and analysed reports from the literature of (quasi-)simultaneous observations of X-ray transients at radio and X-ray wavelengths and compared them with each other and with more unusual radio-bright sources such as Cygnus X-3, GRS 1915+105 and Circinus X-1. There exists a significant (>97 per cent likelihood) positive (rank) correlation between the peak X-ray flux PX and radio flux density PR for the black hole candidate (BHC) systems, and a marginally significant positive (rank) correlation for the neutron star (NS) systems. This is further evidence for a coupling between accretion and outflows in X-ray binary systems, in this case implying a relation between peak disc-accretion rate and the number of synchrotron-emitting electrons ejected. However, we also show that the distribution of 'radio loudness', PR/PX, is significantly different for the two samples, in the sense that the BHCs generally have a higher PR/PX ratio. The origin of this discrepancy is uncertain, but probably reflects differences in the energetics and/or radiative efficiency of flows around the neutron stars and black holes; we briefly discuss some of these possibilities. Furthermore, the data for the two recently discovered 'fast transients' (FTs), XTE J0421+560/CI Cam and SAX J1819.3-2525/V4641 Sgr are entirely compatible with the distribution of BHCs. As at least three of the BHCs and both FTs have been directly resolved into mildly relativistic jets, it seems likely that such outflows are the origin of the radio emission in all BHC and FT transients, and probably also for the NS transients. We further note that the range of X-ray and radio fluxes observed from the unusual superluminal source GRS 1915+105 is also entirely compatible with the distribution for transients, implying that there is nothing special about the physics of jet formation in that system. We conclude that these data point to the formation of a mildly relativistic jet whose luminosity is a function of the accretion rate in the majority, if not all, X-ray transient outbursts, but whose relation to the observed X-ray emission is dependent on the nature of the accreting compact object.