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We report an anticorrelation between continuum luminosity and the equivalent width (EW) of the Ha emission line in X-ray binary systems. The effect is evident both in a universal monotonic increase in Ha EW with time following outbursts, as systems fade, and in a comparison between measured EWs and contemporaneous X-ray measurements. The effect is most clear for black hole binaries in the low/hard X-ray state, which is prevalent at X-ray luminosities below similar to 1 per cent of the Eddington luminosity. We do not find strong evidence for significant changes in line profiles across accretion state changes, but this is hampered by a lack of good data at such times. The observed anticorrelation, highly significant for black hole binaries, is only marginally so for neutron star systems, for which there are far less data. Comparison with...

The optical/near-infrared (OIR) region of the spectra of low-mass X-ray binaries (XBs) appears to lie at the intersection of a variety of different emission processes. In this paper we present quasi-simultaneous OIR-X-ray observations of 33 XBs in an attempt to estimate the contributions of various emission processes in these sources, as a function of X-ray state and luminosity. A global correlation is found between OIR and X-ray luminosity for low-mass black hole candidate XBs (BHXBs) in the hard X-ray state, of the form L-OIR proportional to L-X(0.6). This correlation holds over eight orders of magnitude in L-X and includes data from BHXBs in quiescence and at large distances [Large Magellanic Cloud (LMC) and M31]. A similar correlation is found in low-mass neutron star XBs (NSXBs) in the hard state. For BHXBs in the soft state, all ...

We present a compilation of spectroscopic observations of the sgB[e] star CI Cam, the optical counterpart of XTE J0421+560. This includes data from before, during, and after its 1998 outburst, with quantitative results spanning 37 years. The object shows a rich emission line spectrum originating from circumstellar material, rendering it difficult to determine the nature of either star involved or the cause of the outburst. We collate all available pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with outburst and post-outburst data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with Fe II and [N II] lines still a factor of simi...